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Free molecules undergo processes with photons; in particular, they can undergo photoionization and photodissociation, which are relevant processes in nature and laboratory. Recently, it has been shown that in a cavity, the reverse process of photoionization, namely, electron capture becomes highly probable. The underlying mechanism is the formation of a hybrid resonance state. In this work, we demonstrate that the idea of enhanced reverse processes is more general. We discuss the case of the reverse process of photodissociation, namely, making a molecule out of separate atoms in a cavity. For bound electronic states, the interaction of atoms and molecules with quantum light as realized in cavities is known to give rise to the formation of hybrid light–matter states (usually called polaritons). In the scenarios discussed here, the hybrid light–matter states are resonance (metastable) states, which decay into the continuum of either electrons or of the fragments of a molecule. Resonances can substantially enhance the outcome of processes. In addition to the new resonant mechanism of molecule formation, the impact of the hybrid resonances on the scattering cross section of the atoms can be dramatic. [ABSTRACT FROM AUTHOR]

We investigate roaming in the photodissociation of acetaldehyde (CH3CHO), providing insights into the contrasting roaming dynamics observed for this molecule compared to formaldehyde. We carry out trajectory studies for full-dimensional acetaldehyde, supplemented with an analysis of a two-degree-of-freedom restricted model and obtain evidence for two distinct roaming pathways. Trajectories exhibit roaming at both shorter (9–11.5 au) and larger (14.5–22.9 au) maximum CH3–HCO separations, characterized by differing amounts of HCO rotation. No roaming trajectories were found in the intervening gap region. The roaming dynamics near 14.5–22.9 au are well-reproduced by the restricted model and involve passage through a centrifugal barrier, analogous to formaldehyde roaming. However, the shorter-range 9–11.5 au roaming appears unique to acetaldehyde and is likely facilitated by repulsive interactions absent in the simplified models. Phase space analysis reveals that this additional roaming pathway is inaccessible in the reduced dimensionality system. The findings suggest that acetaldehyde's increased propensity for roaming compared to formaldehyde may arise from the presence of multiple distinct roaming mechanisms rather than solely the higher roaming fragment mass. [ABSTRACT FROM AUTHOR]

This study was focused on the photochemistry of OAlOH and three possible pathways, which were studied with high-level multireference configuration interaction ab initio calculations. We computed cuts of the six-dimensional potential energy surfaces for the ground, the lowest singlet and triplet excited states, and probed the photodissociation mechanisms and the stabilities. The OAlOH electronic spectrum, with an energy reaching 7.15 eV, contained four prominent peaks. Photodissociation to AlO, OH, and AlOH constituted a plausible mechanism within the deep-UV range (λ = 250.4 nm). Our data indicated the photostability of OAlOH in the near-UV‒Vis region, so detection with laser-induced fluorescence is possible. Fluorescence and phosphorescence may occur upon excitation at 363.5 nm. The roles of OAlOH in the photochemical reactions of Al-bearing molecules in the upper atmosphere and VY Canis Majoris are discussed. [ABSTRACT FROM AUTHOR]

C H 3 + , a cornerstone intermediate in interstellar chemistry, has recently been detected for the first time by using the James Webb Space Telescope. The photodissociation of this ion is studied here. Accurate explicitly correlated multi-reference configuration interaction ab initio calculations are done, and full-dimensional potential energy surfaces are developed for the three lower electronic states, with a fundamental invariant neural network method. The photodissociation cross section is calculated using a full-dimensional quantum wave packet method in heliocentric Radau coordinates. The wave packet is represented in angular and radial grids, allowing us to reduce the number of points physically accessible, requiring to push up the spurious states appearing when evaluating the angular kinetic terms, through projection technique. The photodissociation spectra, when employed in astrochemical models to simulate the conditions of the Orion bar, result in a lesser destruction of C H 3 + compared to that obtained when utilizing the recommended values in the kinetic database for astrochemistry. [ABSTRACT FROM AUTHOR]

There is a longstanding difficulty that time-dependent density functional theory relying on adiabatic local density approximation is not applicable to the electron dynamics, for example, for an initially excited state, such as in photochemical reactions. To overcome this, we develop non-adiabatic excited-state time-dependent GW molecular dynamics (TDGW) on the basis of the extended quasiparticle theory. Replacing Kohn–Sham orbitals/energies with correlated, interacting quasiparticle orbitals/energies allows the full correspondence to the excited-state surfaces and corresponding total energies, with satisfying extended Koopmans' theorem. We demonstrate the power of TDGW using methane photolysis, CH 4 → CH 3 • + H , an important initiation reaction for combustion/pyrolysis and hydrogen production of methane. We successfully explore several possible pathways and show how this reaction dynamics is captured accurately through simultaneously time-tracing all quasiparticle levels. TDGW scales as O ( N B 3 - 4 ) , where NB is the number of basis functions, which is distinctly advantageous to performing dynamics using configuration interaction and coupled cluster methods. [ABSTRACT FROM AUTHOR]

In response to a community prediction challenge, we simulate the nonadiabatic dynamics of cyclobutanone using the mapping approach to surface hopping (MASH). We consider the first 500 fs of relaxation following photoexcitation to the S2 state and predict the corresponding time-resolved electron-diffraction signal that will be measured by the planned experiment. 397 ab initio trajectories were obtained on the fly with state-averaged complete active space self-consistent field using a (12,11) active space. To obtain an estimate of the potential systematic error, 198 of the trajectories were calculated using an aug-cc-pVDZ basis set and 199 with a 6-31+G* basis set. MASH is a recently proposed independent trajectory method for simulating nonadiabatic dynamics, originally derived for two-state problems. As there are three relevant electronic states in this system, we used a newly developed multi-state generalization of MASH for the simulation: the uncoupled spheres multi-state MASH method (unSMASH). This study, therefore, serves both as an investigation of the photodissociation dynamics of cyclobutanone, and also as a demonstration of the applicability of unSMASH to ab initio simulations. In line with previous experimental studies, we observe that the simulated dynamics is dominated by three sets of dissociation products, C3H6 + CO, C2H4 + C2H2O, and C2H4 + CH2 + CO, and we interpret our predicted electron-diffraction signal in terms of the key features of the associated dissociation pathways. [ABSTRACT FROM AUTHOR]

Produced by both nature and human activities, sulfur dioxide (SO2) is an important species in the earth's atmosphere. SO2 has also been found in the atmospheres of other planets and satellites in the solar system. The photoabsorption cross sections and photodissociation of SO2 have been studied for several decades. In this paper, we reported the experimental results for photodissociation dynamics of SO2 via the G ̃ 1B1 state. By analyzing the images from the time-sliced velocity map ion imaging method, the vibrational state population distributions and anisotropy parameters were obtained for the O(1D2) + SO(X3Σ−, a1Δ, b1Σ+) and O(1S0) + SO(X3Σ−) channels, and the branching ratios for the channels O(1D2) + SO(X3Σ−), O(1D2) + SO(a1Δ), and O(1D2) + SO(b1Σ+) were determined to be ∼0.3, ∼0.6, and ∼0.1, respectively. The SO products were dominant in electronically and rovibrationally excited states, which may have yet unrecognized roles in the upper planetary atmosphere. [ABSTRACT FROM AUTHOR]

The increasing presence of 1,1,1,2-tetrafluoroethane (CF3CH2F) in the atmosphere has prompted detailed studies into its complex photodissociation behavior. Experiments focusing on CF3CH2F irradiation have unveiled an array of ions, with the persistent observation of the rearrangement product CHF2+ not yet fully understood. In this work, we combine density functional theory, coupled-cluster calculations with a complete basis set formalism, and atom-centered density matrix propagation molecular dynamics to investigate the energetics and dynamics of different potential pathways leading to CHF2+. We found that the two-body dissociation pathway involving an HF rearrangement, which was previously considered complex for CHF2+ formation, is actually straightforward but not likely due to the facile loss of HF. In contrast, our calculations reveal that the H elimination pathway, once thought of as a potential route to CHF2+, is not only comparably disadvantageous from both thermodynamic and kinetic points of view but also does not align with experimental data, particularly the lack of a rebound peak at m/z 101–102. We establish that the formation of CHF2+ is predominantly via the HF elimination channel, a conclusion experimentally corroborated by studies involving the trifluoroethylene cation CF2CHF+, a key intermediate in this process. [ABSTRACT FROM AUTHOR]

The identification and analysis of quantum state-specific effects can significantly deepen our understanding of detailed photodissociation dynamics. Here, we report an experimental investigation on the vibrational state-mediated photodissociation of the OCS+ cation via the A2Π1/2 (ν1 0 ν3) states by using the velocity map ion imaging technique over the photolysis wavelength range of 263–294 nm. It was found that the electronically excited S+ product channel S+(2Du) + CO (X1Σ+) was significantly enhanced when the ν1 and ν3 vibrational modes were excited. Clear deviations in the branching ratios of the electronically excited S+ channel were observed when the vibrational modes ν1 and ν3 were selectively excited. The results reveal that vibrationally excited states play a vital role in influencing the nonadiabatic couplings in the photodissociation process. [ABSTRACT FROM AUTHOR]

The N–H photodissociation dynamics of the pyridinyl radical upon continuous excitation to the optically bright, first excited ππ* electronic state by an ultra-violet (UV) laser pulse has been investigated within the mathematical framework of optimal control theory. The genetic algorithm (GA) is employed as the optimization protocol. We considered a three-state and three-mode model Hamiltonian, which includes the reaction coordinate, R (a1 symmetry); the coupling coordinates (namely, out-of-plane bending coordinate of the hydrogen atom of azine group), Θ (b1 symmetry); and the wagging mode, Q9 (a2 symmetry). The three electronic states are the ground, ππ*, and πσ* states. The πσ* state crosses both the ground state and the ππ* state, and it is a repulsive state on which N–H dissociation occurs upon photoexcitation. Different vibrational wave functions along the coupling coordinates, Θ and Q9, of the ground electronic state are used as the initial condition for solving the time-dependent Schrödinger equation. The optimal UV laser pulse is designed by applying the GA, which maximizes the dissociation yield. We obtained over 95% dissociation yield through the πσ* asymptote using the optimal pulse of a time duration of ∼30 000 a.u. (∼725.66 fs). [ABSTRACT FROM AUTHOR]

Bromine atom (Br) reactions lead to ozone depletion in the troposphere and stratosphere. Photodegradation of bromocarbons is one of the main sources of bromine atoms in the atmosphere. Here, we use high-level ab initio methods, including spin–orbit effects, to study the photodissociation of the CH2Br radical. All possible fragmentation pathways, namely CH2Br + hν → CH2 + Br, HCBr + H, and CBr + H2, have been analyzed. Potential-energy curves of the ground and several excited electronic states along the corresponding dissociating bond distance of each pathway have been calculated. Considering the actinic fluxes of solar irradiation in the troposphere and in the stratosphere in the relevant range of frequencies, it is found that the first five excited states of CH2Br can be accessed from the ground state. Analysis of the potential curves shows that the pathways producing CH2 + Br and HCBr + H can proceed through a fast direct dissociation mechanism, while the pathway leading to CBr + H2 involves much slower dissociation mechanisms like internal conversion between electronic states, predissociation, or tunneling through exit barriers. The main implications are that the two faster channels are predicted to be dominant, and the slower pathway is expected to be less relevant. The tropospheric and stratospheric solar actinic fluxes also allow for further dissociation of the HCBr and CBr fragments, generating additional Br atoms, provided that they survive possible collisions with other atmospheric reagents. Finally, we discuss the possible effect of each of the three CH2Br dissociation pathways on the depletion of atmospheric ozone. [ABSTRACT FROM AUTHOR]

The bond dissociation energy of FeCr+ is measured using resonance enhanced photodissociation spectroscopy in a cryogenic ion trap. The onset for FeCr+ → Fe + Cr+ photodissociation occurs well above the lowest Cr+(6S, 3d5) + Fe(5D, 3d64s2) dissociation limit. In contrast, the higher energy FeCr+ → Fe+ + Cr photodissociation process exhibits an abrupt onset at the energy of the Cr(7S, 3d54s1) + Fe+(6D, 3d64s1) limit, enabling accurate dissociation energies to be extracted: D(Fe–Cr+) = 1.655 ± 0.006 eV and D(Fe+–Cr) = 2.791 ± 0.006 eV. The measured D(Fe–Cr+) bond energy is 10%–20% larger than predictions from accompanying CAM (Coulomb Attenuated Method)-B3LYP and NEVPT2 and coupled cluster singles, doubles, and perturbative triples electronic structure calculations, which give D(Fe–Cr+) = 1.48, 1.40, and 1.35 eV, respectively. The study emphasizes that an abrupt increase in the photodissociation yield at threshold requires that the molecule possesses a dense manifold of optically accessible, coupled electronic states adjacent to the dissociation asymptote. This condition is not met for the lowest Cr+(6S, 3d5) + Fe(5D, 3d64s2) dissociation limit of FeCr+ but is satisfied for the higher energy Cr(7S, 3d54s1) + Fe+(6D, 3d64s1) dissociation limit. [ABSTRACT FROM AUTHOR]

The precise determination of the bond dissociation energy of N2 is crucial for thermochemistry database and theoretical calculations. However, there has been ongoing debate regarding its exact value. In this study, we used the velocity map imaging method combined with an extreme ultraviolet laser to measure the threshold fragment yield (TFY) spectra of N2 in the N(2D) + N(2D) photodissociation channels. By integrating the signals within a small circular area on the fragment velocity map images, we were able to obtain TFY spectra at nine different dissociation thresholds. These spectra are rotational state-resolved for the N2(J″) molecules and spin–orbit state-resolved for the dissociation channels involving N(2D) fragments. By employing the Wigner threshold law to simulate the TFY spectra and conducting statistical analysis on the comprehensive dataset, we determined the N2 bond dissociation energy to be 78 691.09 ± 0.15 cm−1. This work now places N2 among the few diatomic molecules with bond dissociation energies measured at sub-wavenumber precision. [ABSTRACT FROM AUTHOR]

Photodissociation of CO2 via the 1Πg state is investigated using a time-sliced velocity-mapped ion imaging apparatus combined with a tunable vacuum ultraviolet photolysis source. The main O(1D2) + CO(X1Σ+) channel is directly observed from the measured images of O(1D2) photoproducts at 129.08–134.76 nm. The total kinetic energy release spectra determined based on these images show that the energetic thresholds for the O(1D2) + CO(X1Σ+) photoproducts correspond to the thermochemical thresholds for the photodissociation of CO2(v2 = 0) and CO2(v2 = 1). One significant difference among the CO(X1Σ+, v) vibrational distributions for the predominant CO2(v2 = 0) dissociation is that the population of CO(v = 0) becomes favorable at 130.23–133.45 nm compared to the Boltzmann-like component (v > 0) that always exists at 129.08–134.76 nm. The wavelength dependences of the overall β are found to follow the variation trend of the CO(v = 0) abnormal intensity. The vibrational state-specific β values present a roughly decreasing trend with an increase in v, whereas β(v = 0) appears to be significantly larger than β(v = 1) at 130.23–133.45 nm compared to 134.76 and 129.08 nm. The non-statistical CO(v = 0) with larger β values at 130.23–133.45 nm implies that an additional pathway may open through the conical intersection coupling to the dissociative 21A′ state, except for the ever-existing pathway that yields the Boltzmann-like component. In contrast, at 129.08 nm, the restoration of the statistical equilibrium in the CO(X1Σ+, v) vibrational distribution may be caused by the emergence of novel dissociation pathways arising from the participation of the 31A″ state. [ABSTRACT FROM AUTHOR]

The H atom tunneling dissociation dynamics of the S1 state of meta- or para-cresol has been investigated by using the picosecond time-resolved pump-probe spectroscopy in a state-specific manner. The S1 state lifetime (mainly due to the H atom tunneling reaction) is found to be mode-dependent whereas it quickly converges and remains constant as the rapid intramolecular vibrational energy redistribution (IVR) starts to participate in the S1 state relaxation with the increase of the S1 internal energy (Eint). The IVR rate and its change with increasing Eint have been reflected in the parent ion transients taken by tuning the total energy (hνpump + hνprobe) just above the adiabatic ionization threshold (so that the dissipation of the initial mode-character could be monitored as a function of the reaction time), indicating that the mode randomization rate into the S1 isoenergetic manifolds exceeds the tunneling rate quite early in terms of Eint for m-cresol (≤∼1200 cm−1) or p-cresol (≤∼800 cm−1) compared to the case of phenol (≤∼1800 cm−1). Though the H atom tunneling dynamics of phenol (S1) seems to be little influenced by the methyl substitution on the either m- or p-position, the IVR rate has been found to be strongly accelerated due to the sharply-increasing (S1) density of states with increasing Eint due to the pivotal role of the low-frequency CH3 torsional mode. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of jet-cooled ethyl radical (C2H5) via the A ̃ 2A′(3s) states are studied in the wavelength region of 230–260 nm using the high-n Rydberg H-atom time-of-flight (TOF) technique. The H + C2H4 product channels are reexamined using the H-atom TOF spectra and photofragment translational spectroscopy. A prompt H + C2H4( X ̃ 1Ag) product channel is characterized by a repulsive translational energy release, anisotropic product angular distribution, and partially resolved vibrational state distribution of the C2H4( X ̃ 1Ag) product. This fast dissociation is initiated from the 3s Rydberg state and proceeds via a H-bridged configuration directly to the H + C2H4( X ̃ 1Ag) products. A statistical-like H + C2H4( X ̃ 1Ag) product channel via unimolecular dissociation of the hot electronic ground-state ethyl ( X ̃ 2A′) after internal conversion from the 3s Rydberg state is also examined, showing a modest translational energy release and isotropic angular distribution. An adiabatic H + excited triplet C2H4(ã3B1u) product channel (a minor channel) is identified by energy-dependent product angular distribution, showing a small translational energy release, anisotropic angular distribution, and significant internal excitation in the C2H4(ã3B1u) product. The dissociation times of the different product channels are evaluated using energy-dependent product angular distribution and pump–probe delay measurements. The prompt H + C2H4( X ̃ 1Ag) product channel has a dissociation time scale of <10 ps, and the upper bound of the dissociation time scale of the statistical-like H + C2H4( X ̃ 1Ag) product channel is <5 ns. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics leading to the C–N bond cleavage in methylamine (CH3NH2) are investigated upon photoexcitation in the blue edge of the first absorption A-band, in the 198–204 nm range. Velocity map images of the generated methyl (CH3) fragment detected in specific vibrational modes, i.e., ν = 0, ν1 = 1, and ν2 = 1, through resonance enhanced multiphoton ionization, are presented along with the corresponding translational energy distributions and the angular analysis. The experimental results are complemented by high-level ab initio calculations of potential energy curves as a function of the C–N bond distance. While a similar single Boltzmann-type contribution is observed in all the translational energy distributions measured, the speed-dependent anisotropy parameter obtained through the angular analysis reveals the presence of two different mechanisms. Prompt dissociation through the conical intersection between the A ̃ 1 A ′ first excited state and the ground state located in the exit channel is, indeed, revealed as a minor channel. In contrast, slow dissociation on the ground state, presumably from frustrated N–H bond cleavage trajectories, constitutes the major reaction pathway leading to the methyl formation. [ABSTRACT FROM AUTHOR]

The nonadiabatic interactions between the C ̃ state and neighboring electronic states of SO2 have attracted much attention; however, the predissociation mechanisms are not yet completely understood. In this work, the predissociation dynamics of SO2 via its C ̃ state have been investigated at λ = 193–201 nm by using the time-sliced velocity map ion imaging technique. The translational energy distributions and the branching ratios of the O(3PJ=2,1,0) spin–orbit products at six photolysis wavelengths have been acquired. The SO(3Σ−) product population gradually decreases in v = 0 and increases in v = 2 as the photolysis wavelength decreases. The branching ratios of O(3P J=2,1,0) products are almost similar at most wavelengths, except at 194.8 nm. Our data suggest that the predissociation between 193 and 201 nm is via an avoided crossing between the C ̃ state and the repulsive triplet 23A′ state. The state-to-state dynamical pictures shown in this work provide a rigorous test of the potential energy surfaces (PESs) of the SO2 and the nonadiabatic couplings between these PESs. [ABSTRACT FROM AUTHOR]

Understanding and controlling the chemical behavior of iron and iron oxide clusters requires accurate thermochemical data, which, because of the complex electronic structure of transition metal clusters, can be difficult to calculate reliably. Here, dissociation energies for Fe 2 + , Fe2O+, and Fe 2 O 2 + are measured using resonance enhanced photodissociation of clusters contained in a cryogenically cooled ion trap. The photodissociation action spectrum of each species exhibits an abrupt onset for the production of Fe+ photofragments from which bond dissociation energies are deduced for Fe 2 + (2.529 ± 0.006 eV), Fe2O+ (3.503 ± 0.006 eV), and Fe 2 O 2 + (4.104 ± 0.006 eV). Using previously measured ionization potentials and electron affinities for Fe and Fe2, bond dissociation energies are determined for Fe2 (0.93 ± 0.01 eV) and Fe 2 − (1.68 ± 0.01 eV). Measured dissociation energies are used to derive heats of formation ΔfH0( Fe 2 + ) = 1344 ± 2 kJ/mol, ΔfH0(Fe2) = 737 ± 2 kJ/mol, ΔfH0( Fe 2 − ) = 649 ± 2 kJ/mol, ΔfH0(Fe2O+) = 1094 ± 2 kJ/mol, and ΔfH0( Fe 2 O 2 + ) = 853 ± 21 kJ/mol. The Fe 2 O 2 + ions studied here are determined to have a ring structure based on drift tube ion mobility measurements prior to their confinement in the cryogenic ion trap. The photodissociation measurements significantly improve the accuracy of basic thermochemical data for these small, fundamental iron and iron oxide clusters. [ABSTRACT FROM AUTHOR]

We studied the physical conditions in the local interstellar medium (ISM) and at high redshift by fitting observed column densities of CO and H2 rotational levels and C  i fine-structure levels using photodissociation models calculated with the Meudon PDR code. We analysed CO absorption systems in 28 sight lines in the local ISM and seven damped Lyman-α absorption systems at high redshift, covering |$N({\rm H_2})=10^{19}\!-\!10^{21.5}\,{\rm cm}^{-2}$| and |$N({\rm CO})=10^{13}\!-\!10^{18}\,{\rm cm}^{-2}$|⁠. We constructed a method to accurately calculate CO excitation, incorporating the effects of photon trapping. Our findings indicate that in the local ISM, CO excitation is primarily driven by collisions and excitation due to the CMB radiation. We demonstrated that an increase in CO excitation observed near |$N({\rm CO})\simeq 10^{15}~{\rm cm^{-2}}$|⁠ , is attributed to an increase in gas densities from |${\simeq}100\,{\rm cm}^{-3}$| to |${\simeq}300\!-\!1000\,{\rm cm}^{-3}$|⁠. CO absorption systems in the local ISM are characterized by a gas number density of about 10–1000 cm−3, a kinetic temperature of 10–100 K, and an intensity of external UV field ranging from 0.1 to 10 units of Mathis field. Compared to the average gas probed by C  i absorption, the gas detected in CO is denser and colder, while the external UV field remains nearly constant. We observed a negative correlation between the kinetic temperature and both N (CO) and N (H2), with power-law slopes of |$-0.21\pm 0.02$| and |$-0.65\pm 0.05$|⁠ , respectively. At the same time, the gas number density exhibits a positive correlation with N (CO) and N (H2) with slopes of |$0.38\pm 0.02$| and |$1.15\pm 0.05$|⁠ , respectively. [ABSTRACT FROM AUTHOR]

Oxalyl chloride, (ClCO)2$\text{(ClCO)}_2$, is widely used as a photolytic source of Cl atoms in reaction kinetics studies. (ClCO)2$\text{(ClCO)}_2$ photolysis is typically assumed to produce Cl atoms with an overall yield of 2 via three‐body dissociation, (ClCO)2+hν→Cl+CO+ClCO∗$\text{(ClCO)}_2 + h\nu \rightarrow \text{Cl} + \text{CO} + \text{ClCO}^*$, followed by fast subsequent ClCO unimolecular decomposition of either the energetically excited ClCO∗${\rm ClCO}^*$ fragment, ClCO∗→Cl+CO$\text{ClCO}^* \rightarrow \text{Cl} + \text{CO}$, or the thermalized ClCO radical, ClCO+M→Cl+CO+M$\text{ClCO} + \text{M} \rightarrow \text{Cl} + \text{CO} + \text{M}$. However, a study by Huang et al. (J. Phys. Chem. A 121 (2017) 2888–2895) found that UV photolysis of (ClCO)2$\text{(ClCO)}_2$ at 248nm$248\ {\rm nm}$ directly yields Cl2${\rm Cl}_2$ with a photolysis quantum yield of ϕ(Cl2)>14%$\phi (\text{Cl}_2) > 14\%$. This new product pathway may complicate the use of (ClCO)2$\text{(ClCO)}_2$ as a clean source of Cl atoms and challenges the previously accepted photodissociation scheme. The purpose of the present work was 2‐fold. Firstly, the unimolecular decomposition of ClCO∗${\rm ClCO}^*$ and ClCO radicals has been investigated in (ClCO)2$\text{(ClCO)}_2$/C2H6/Ar$\text{C}_{2}\text{H}_6{\rm {/Ar}}$ gas mixtures after UV photolysis at 266$266\ {\rm }$ and 355nm$355\ {\rm nm}$. Cl atoms were captured by C2H6$\text{C}_{2}\text{H}_6$ added in excess such that concentration‐time profiles of HCl measured by means of mid‐infrared frequency modulation spectroscopy reflect the temporally separated Cl formation pathways. The low‐pressure thermal ClCO decomposition rate constant was determined to be k=(1.79±0.17)×10−14cm3molecule−1s−1$k = (1.79 \pm 0.17)\times 10^{-14}\ \text{cm}^{3}\ \text{molecule}^{-1}\ \text{s}^{-1}$ at 295K$295\ {\rm K}$, which is in very good agreement with previously reported literature values. Secondly, the photolysis quantum yield of direct Cl2${\rm Cl}_2$ formation from (ClCO)2$\text{(ClCO)}_2$ photofragmentation was studied with time‐of‐flight mass spectrometry using a photoelectron photoion coincidence setup. Calibrated Cl2${\rm Cl}_2$ concentration‐time profiles were recorded and analyzed using kinetic simulations accounting for both direct and secondary formation of Cl2${\rm Cl}_2$ from photolysis and reactions involving Cl, ClCO, and (ClCO)2$\text{(ClCO)}_2$, respectively. Direct Cl2${\rm Cl}_2$ formation could be confirmed, where wavelength‐dependent quantum yields of ϕ(Cl2)=(5.0±1.6)%$\phi (\text{Cl}_2) = (5.0 \pm 1.6)\%$, (10.0±3.3)%$(10.0 \pm 3.3)\%$, and (5.6±2.0)%$(5.6 \pm 2.0)\%$ at 213nm$213\ {\rm nm}$, 266nm$266\ {\rm nm}$, and 355nm$355\ {\rm nm}$ were determined. Complementary quantum‐chemical calculations of the potential energy diagram for ground‐state photodissociation reveal a low‐lying energy barrier for the formation of phosgene, Cl2CO${\rm Cl}_2{\rm CO}$. We suggest that subsequent Cl2${\rm Cl}_2$ and Cl formation from energetically excited Cl2CO${\rm Cl}_2{\rm CO}$ may actually play a role for the overall photodissociation of (ClCO)2$\text{(ClCO)}_2$. [ABSTRACT FROM AUTHOR]

Optical emission line diagnostics, which are a common tool for constraining the properties of the interstellar medium (ISM) of galaxies, become progressively inaccessible at higher redshifts for ground-based facilities. Far-infrared (FIR) emission lines, which are redshifted into atmospheric windows that are accessible for ground-based submillimetre facilities, could provide ISM diagnostics alternative to optical emission lines. We investigated FIR line ratios involving [CII] λ158 μm, [OIII] λ88 μm, [OIII] λ52 μm, [NII] λ122 μm, and [NIII] λ57 μm using synthetic emission lines applied to a high-resolution (mgas = 883.4 M⊙) cosmological zoom-in simulation, including radiative transfer post-processing with the code KRAMSES-RT at z = 6.5. We find that the [CII]/[NII]122 ratio is sensitive to the temperature and density of photodissociation regions. It might therefore be a useful tool for tracing the properties of this gas phase in galaxies. We also find that [NII]/[NIII] is a good tracer of the temperature and that [OIII]52/[OIII]88 is a good tracer of the gas density of HII regions. Emission line ratios containing the [OIII] λ88 μm line are sensitive to high-velocity outflowing gas. [ABSTRACT FROM AUTHOR]

The origin and evolution of CO gas in debris discs has been debated since its initial detection. The gas could have a primordial origin, as a remnant of the protoplanetary disc or a secondary exocometary origin. This paper investigates the origin of gas in two debris discs, HD110058 and HD131488, using Hubble Space Telescope (HST) observations of CI and CO, which play critical roles in the gas evolution. We fitted several electronic transitions of CI and CO rovibronic bands to derive column densities and temperatures for each system, revealing high CO column densities (∼3–4 orders of magnitude higher than β Pictoris), and low CI/CO ratios in both. Using the exogas model, we simulated the radial evolution of the gas in the debris disc assuming a secondary gas origin. We explored a wide range of CO exocometary release rates and α viscosities, which are the key parameters of the model. Additionally, we incorporated photodissociation due to stellar ultraviolet (UV) to the exogas model and found that it is negligible for typical CO-rich discs and host stars, even at a few au due to the high radial optical depths in the Extreme ultraviolet radiation (EUV). We find that the current steady-state secondary release model cannot simultaneously reproduce the CO and CI HST -derived column densities, as it predicts larger CI/CO ratios than observed. Our direct UV measurement of low CI/CO ratios agrees with results derived from recent Atacama Large Millimetre/submillimetre Array findings and may point to vertical layering of CI, additional CI removal, CO shielding processes, or different gas origin scenarios. [ABSTRACT FROM AUTHOR]

As a result of studies of the relaxation mechanisms of electronic excitation in triazide molecules: triazidopyrimidine (TAP) and triazido-S-triazine (TAT), it was shown that triplet and singlet excited states are states of the ππ* and πσ* type. Both molecules have a flat shape and the same structure of π-molecular orbitals (π-MO). However, they differ significantly in terms of symmetry, which determines the properties of excited states and the relaxation of electrons from excited states. The lowest excited state of both molecules is the binding T1(ππ*) state. It is followed in TAP by the dissociative T2(πσ*)-state. In TAT, the T5(πσ*) state is dissociative. Both molecules are excited to the singlet S4(ππ*) state, from which the molecule relaxes non-equilibrium through intermediate states to the triplet dissociative state. From this state, triazidopyrimidine dissociates with a quantum yield of h = 1.0, while the quantum yield of triazido-S-triazine dissociation will be significantly reduced due to relaxation to the lower triplet states of the same symmetry. Among the singlet states in both molecules, there is a dissociative state, but on the way to dissociation there is an energy barrier capable of slowing down the corresponding process. When the N-N bond in both molecules is extended to 1.75 Å, the dissociative state crosses the main S0 state of the molecule, as a result of which the dissociation product (nitrene) appears in the main triplet state. It is shown that the excitation of a solution of TAP in ethanol increases the quantum yield of photodissociation up to 3.0 due to secondary processes. Such a process is absent in the case of TAT solution. [ABSTRACT FROM AUTHOR]

The correlation discrete variable representation (CDVR) enables (multilayer) multi-configurational time-dependent Hartree (MCTDH) calculations with general potentials. The CDVR employs a set of grids corresponding to single-particle functions to efficiently evaluate all potential matrix elements appearing in the MCTDH equations of motion. In standard CDVR approaches, the number of grid points employed is tied to the number of corresponding single-particle functions. This limits the accuracy of the quadrature, which can be achieved for a given single-particle function basis. In this work, an extended CDVR approach that facilitates a numerically exact quadrature of all potential matrix elements is introduced. The number of grid points employed can be increased independent of the number of corresponding single-particle function to achieve any desired quadrature accuracy. The properties of the new scheme are illustrated by numerical calculations studying the photodissociation of NOCl and the vibrational states of CH3. Fast convergence with respect to the number of additional quadrature points is observed: Employing a single additional point in each physical or logical coordinate already ensures negligible quadrature errors. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of methylamine (CH3NH2) upon excitation in the blue edge of the first absorption A-band, in the 198–203 nm range, are investigated by means of nanosecond pump–probe laser pulses and velocity map imaging combined with H(2S)-atom detection through resonance enhanced multiphoton ionization. The images and corresponding translational energy distributions for the H-atoms produced show three different contributions associated with three reaction pathways. The experimental results are complemented by high-level ab initio calculations. The potential energy curves computed as a function of the N–H and C–H bond distances allow us to draw a picture of the different mechanisms. Major dissociation occurs through N–H bond cleavage and it is triggered by an initial geometrical change, i.e., from a pyramidal configuration of the C–NH2 with respect to the N atom to a planar geometry. The molecule is then driven into a conical intersection (CI) seam where three outcomes can take place: first, threshold dissociation into the second dissociation limit, associated with the formation of CH3NH ( A ̃) , is observed; second, direct dissociation after passage through the CI leading to the formation of ground state products; and third, internal conversion into the ground state well in advance to dissociation. While the two last pathways were previously reported at a variety of wavelengths in the 203–240 nm range, the former had not been observed before to the best of our knowledge. The role of the CI and the presence of an exit barrier in the excited state, which modify the dynamics leading the two last mechanisms, are discussed considering the different excitation energies used. [ABSTRACT FROM AUTHOR]

In this work, we have studied the photodissociation of the protonated derivatives of N-nitrosodimethylamine [(CH3)2N–NO] with the CASPT2 method. It is found that only one of the four possible protonated species of the dialkylnitrosamine compound absorbs in the visible region at 453 nm, that is, N-nitrosoammonium ion [(CH3)2NH-NO]+. This species is also the only one whose first singlet excited state is dissociative to directly yield the aminium radical cation [(CH3)2NHN·]+ and nitric oxide. In addition, we have studied the intramolecular proton migration reaction {[(CH3)2N–NOH]+ → [(CH3)2NH–NO]+} both in the ground and excited state (ESIPT/GSIPT); our results indicate that this process is not accessible neither in the ground nor in the first excited state. Furthermore, as a first approximation, MP2/HF calculations on the nitrosamine–acid complex indicate that in acidic solutions of aprotic solvents, only [(CH3)2NH–NO]+ is formed. [ABSTRACT FROM AUTHOR]

Understanding vacuum ultraviolet photodissociation dynamics of Carbonyl sulfide (OCS) is of considerable importance in the study of atmospheric chemistry. Yet, photodissociation dynamics of the CS(X1Σ+) + O(3Pj=2,1,0) channels following excitation to the 21Σ+(ν1′,1,0) state has not been clearly understood so far. Here, we investigate the O(3Pj=2,1,0) elimination dissociation processes in the resonance-state selective photodissociation of OCS between 147.24 and 156.48 nm by using the time-sliced velocity-mapped ion imaging technique. The total kinetic energy release spectra are found to exhibit highly structured profiles, indicative of the formation of a broad range of vibrational states of CS(1Σ+). The fitted CS(1Σ+) vibrational state distributions differ for the three 3Pj spin–orbit states, but a general trend of the inverted characteristics is observed. Additionally, the wavelength-dependent behaviors are also observed in the vibrational populations for CS(1Σ+, v). The CS(X1Σ+, v = 0) has a significantly strong population at several shorter wavelengths, and the most populated CS(X1Σ+, v) is gradually transferred to a higher vibrational state with the decrease in the photolysis wavelength. The measured overall β-values for the three 3Pj spin–orbit channels slightly increase and then abruptly decrease as the photolysis wavelength increases, while the vibrational dependences of β-values show an irregularly decreasing trend with increasing CS(1Σ+) vibrational excitation at all studied photolysis wavelengths. The comparison of the experimental observations for this titled channel and the S(3Pj) channel reveals that two different intersystem crossing mechanisms may be involved in the formation of the CS(X1Σ+) + O(3Pj=2,1,0) photoproducts via the 21Σ+ state. [ABSTRACT FROM AUTHOR]

We study the vacuum ultraviolet (VUV) photodissociation dynamics of carbonyl sulfide (OCS) by using the time sliced velocity map ion imaging technique. Experimental images of the dissociative O (3PJ=0,1,2) products were acquired at five VUV photolysis wavelengths from 133.26 to 139.96 nm that correspond to the F Rydberg state of OCS. High vibrational states of the carbon monosulfide (CS) co-products are partially resolved in the images. The product total kinetic energy releases, angular distributions, and the product state branching ratios were derived from the experimental images. Notably, it is found that the anisotropic parameters change systematically with the photolysis wavelength. The anisotropic parameters and the product state branching ratios are significantly sensitive to the J quantum number of the O (3PJ) products. The phenomenon indicates that multiple nonadiabatic pathways are strongly involved in the photodissociation processes. [ABSTRACT FROM AUTHOR]

Multi-state electronic dynamics at higher excitation energies is needed for the understanding of a variety of energy rich situations, including chemistry under extreme conditions, vacuum ultraviolet (VUV) induced astrochemistry, and attochemistry. It calls for an understanding of three stages, energy acquisition, dynamical propagation, and disposal. It is typically not possible to identify a basis of uncoupled quantum states that is sufficient for the three stages. The handicap is the large number of coupled quantum states that is needed to describe the system. Progress in quantum chemistry provides the necessary background to the energetics and the coupling. Progress in quantum dynamics takes this as input for the propagation in time. Right now, it seems that we have come of age with potential detailed applications. We here report a demonstration to a coupled electron-nuclear quantum dynamics through a maze of 47 electronic states and with attention to the order in perturbation theory that is indicated using propensity rules for couplings. Close agreement with experimental results for the VUV photodissociation of 14N2 and its isotopomer 14N15N is achieved. We pay special attention to the coupling between two dissociative continua and an optically accessible bound domain. The computations reproduce and interpret the non-monotonic branching between the two exit channels producing N(2D) and N(2P) atoms as a function of excitation energy and its variation with the mass. [ABSTRACT FROM AUTHOR]

Multi-state electronic dynamics at higher excitation energies is needed for the understanding of a variety of energy rich situations, including chemistry under extreme conditions, vacuum ultraviolet (VUV) induced astrochemistry, and attochemistry. It calls for an understanding of three stages, energy acquisition, dynamical propagation, and disposal. It is typically not possible to identify a basis of uncoupled quantum states that is sufficient for the three stages. The handicap is the large number of coupled quantum states that is needed to describe the system. Progress in quantum chemistry provides the necessary background to the energetics and the coupling. Progress in quantum dynamics takes this as input for the propagation in time. Right now, it seems that we have come of age with potential detailed applications. We here report a demonstration to a coupled electron-nuclear quantum dynamics through a maze of 47 electronic states and with attention to the order in perturbation theory that is indicated using propensity rules for couplings. Close agreement with experimental results for the VUV photodissociation of 14N2 and its isotopomer 14N15N is achieved. We pay special attention to the coupling between two dissociative continua and an optically accessible bound domain. The computations reproduce and interpret the non-monotonic branching between the two exit channels producing N(2D) and N(2P) atoms as a function of excitation energy and its variation with the mass. [ABSTRACT FROM AUTHOR]

We report the resonance-enhanced two-photon ionization combined with various detection approaches and quantum chemical calculations of biologically relevant neurotransmitter prototypes, the most stable conformer of 2-phenylethylamine (PEA), and its monohydrate, PEA-H2O, to reveal the possible interactions between the phenyl ring and amino group in the neutral and ionic species. Extracting the ionization energies (IEs) and appearance energy was achieved by measuring the photoionization and photodissociation efficiency curves of the PEA parent and photofragment ions, together with velocity and kinetic energy-broadened spatial map images of photoelectrons. We obtained coinciding upper bounds for the IEs for PEA and PEA-H2O of 8.63 ± 0.03 and 8.62 ± 0.04 eV, within the range predicted by quantum calculations. The computed electrostatic potential maps show charge separation, corresponding to a negative charge on phenyl and a positive charge on the ethylamino side chain in the neutral PEA and its monohydrate; in the cations, the charge distributions naturally become positive. The significant changes in geometries upon ionization include switching of the amino group orientation from pyramidal to nearly planar in the monomer but not in the monohydrate, lengthening of the N–H⋯π hydrogen bond (HB) in both species, Cα–Cβ bond in the side chain of the PEA+ monomer, and the intermolecular O–H⋯N HB in PEA-H2O cations, leading to distinct exit channels. [ABSTRACT FROM AUTHOR]

The spin-forbidden O(3P2) + CO(X1Σ+, v) channel formed from the photodissociation of CO2 in the low energy band centered at 148 nm is investigated by using the time-sliced velocity-mapped ion imaging technique. The vibrational-resolved images of the O(3P2) photoproducts measured in the photolysis wavelength range of 144.62–150.45 nm are analyzed to give the total kinetic energy releases (TKER) spectra, CO(X1Σ+) vibrational state distributions, and anisotropy parameters (β). The TKER spectra reveal the formation of correlated CO(X1Σ+) with well resolved v = 0–10 (or 11) vibrational bands. Several high vibrational bands that were observed in the low TKER region for each studied photolysis wavelength exhibit a bimodal structure. The CO(X1Σ+, v) vibrational distributions all present inverted characteristics, and the most populated vibrational state changes from a low vibrational state to a relatively higher vibrational state with a change in the photolysis wavelength from 150.45 to 144.62 nm. However, the vibrational-state specific β-values for different photolysis wavelengths present a similar variation trend. The measured β-values show a significant bulge at the higher vibrational levels, in addition to the overall slow decreasing trend. The observed bimodal structures with mutational β-values for the high vibrational excited state CO(1Σ+) photoproducts suggest the existence of more than one nonadiabatic pathway with different anisotropies in the formation of O(3P2) + CO(X1Σ+, v) photoproducts across the low energy band. [ABSTRACT FROM AUTHOR]

We consider an ensemble of diatomic molecules resonantly coupled to an optical cavity under strong coupling conditions at normal incidence. Photodissociation dynamics is examined via direct numerical integration of the coupled Maxwell–Schrödinger equations with molecular rovibrational degrees of freedom explicitly taken into account. It is shown that the dissociation is significantly affected (slowed down) when the system is driven at its polaritonic frequencies. The observed effect is demonstrated to be of transient nature and has no classical analog. An intuitive explanation of the dissociation slowdown at polaritonic frequencies is proposed. [ABSTRACT FROM AUTHOR]

The bond strength and photodissociation dynamics of MgI+ are determined by a combination of theory, photodissociation spectroscopy, and photofragment velocity map imaging. From 17 000 to 21 500 cm−1, the photodissociation spectrum of MgI+ is broad and unstructured; photofragment images in this region show perpendicular anisotropy, which is consistent with absorption to the repulsive wall of the (1) Ω = 1 or (2) Ω = 1 states followed by direct dissociation to ground state products Mg+ (2S) + I (2P3/2). Analysis of photofragment images taken at photon energies near the threshold gives a bond dissociation energy D0(Mg+-I) = 203.0 ± 1.8 kJ/mol (2.10 ± 0.02 eV; 17 000 ± 150 cm−1). At photon energies of 33 000–41 000 cm−1, exclusively I+ fragments are formed. Over most of this region, the formation of I+ is not energetically allowed via one-photon absorption from the ground state of MgI+. Images show the observed product is due to resonance enhanced two-photon dissociation. The photodissociation spectrum from 33 000 to 38 500 cm−1 shows vibrational structure, giving an average excited state vibrational spacing of 227 cm−1. This is consistent with absorption to the (3) Ω = 0+ state from ν = 0, 1 of the (1) Ω = 0+ ground state; from the (3) Ω = 0+ state, absorption of a second photon results in dissociation to Mg* (3P°J) + I+ (3PJ). From 38 500 to 41 000 cm−1, the spectrum is broad and unstructured. We attribute this region of the spectrum to one-photon dissociation of vibrationally hot MgI+ at low energy and ground state MgI+ at higher energy to form Mg (1S) + I+ (3PJ) products. [ABSTRACT FROM AUTHOR]

The bond strength and photodissociation dynamics of MgI+ are determined by a combination of theory, photodissociation spectroscopy, and photofragment velocity map imaging. From 17 000 to 21 500 cm−1, the photodissociation spectrum of MgI+ is broad and unstructured; photofragment images in this region show perpendicular anisotropy, which is consistent with absorption to the repulsive wall of the (1) Ω = 1 or (2) Ω = 1 states followed by direct dissociation to ground state products Mg+ (2S) + I (2P3/2). Analysis of photofragment images taken at photon energies near the threshold gives a bond dissociation energy D0(Mg+-I) = 203.0 ± 1.8 kJ/mol (2.10 ± 0.02 eV; 17 000 ± 150 cm−1). At photon energies of 33 000–41 000 cm−1, exclusively I+ fragments are formed. Over most of this region, the formation of I+ is not energetically allowed via one-photon absorption from the ground state of MgI+. Images show the observed product is due to resonance enhanced two-photon dissociation. The photodissociation spectrum from 33 000 to 38 500 cm−1 shows vibrational structure, giving an average excited state vibrational spacing of 227 cm−1. This is consistent with absorption to the (3) Ω = 0+ state from ν = 0, 1 of the (1) Ω = 0+ ground state; from the (3) Ω = 0+ state, absorption of a second photon results in dissociation to Mg* (3P°J) + I+ (3PJ). From 38 500 to 41 000 cm−1, the spectrum is broad and unstructured. We attribute this region of the spectrum to one-photon dissociation of vibrationally hot MgI+ at low energy and ground state MgI+ at higher energy to form Mg (1S) + I+ (3PJ) products. [ABSTRACT FROM AUTHOR]

As the development of new biotherapeutics advances, increasingly sophisticated tandem mass spectrometry methods are needed to characterize the most complex molecules, including antibody drug conjugates (ADCs). Lysine-linked ADCs, such as trastuzumab-emtansine (T-DM1), are among the most heterogeneous biotherapeutics. Here, we implement a workflow that combines limited proteolysis with HCD-triggered EThcD and UVPD mass spectrometry for the characterization of the resulting middle-down large-sized peptides of T-DM1. Fifty-three payload-containing peptides were identified, ranging in mass from 1.8 to 16.9 kDa, and leading to the unambiguous identification of 46 out of 92 possible conjugation sites. In addition, seven peptides were identified containing multiple payloads. The characterization of these types of heterogeneous peptides represents an important step in unraveling the combinatorial nature of lysine-conjugated ADCs. [ABSTRACT FROM AUTHOR]

Polypyridyl Ru(II) complexes have attracted much attention due to their potential as light-activatable anticancer agents in photoactivated chemotherapy (PACT). The action of ruthenium-based PACT compounds relies on the breaking of a coordination bond between the metal center and an organic ligand via a photosubstitution reaction. Here, a detailed computational investigation of the photophysical properties of a novel trisheteroleptic ruthenium complex, [Ru(dpp)(bpy)(mtmp)]2+ (dpp = 4,7-diphenyl-1,10-phenanthroline, bpy = 2,2′-bipyridine and mtmp = 2-methylthiomethylpyridine), has been carried out by means of DFT and its time-dependent extension. All the aspects of the mechanism by which, upon light irradiation, the mtmp protecting group is released and the corresponding aquated complex, able to bind to DNA inducing cell death, is formed have been explored in detail. All the involved singlet and triplet states have been fully described, providing the calculation of the corresponding energy barriers. The involvement of solvent molecules in photosubstitution and the role played by pyridyl–thioether chelates as caging groups have been elucidated. [ABSTRACT FROM AUTHOR]

This article explores the use of light-controlled catalysis triggered by ligand photodissociation from cobalt complexes. The researchers developed a method that allows for the switching between two different reaction outcomes based on the presence or absence of visible light. They demonstrated this approach in hydroboration reactions and achieved selective catalysis for 1,2- or 1,4-addition depending on the reaction conditions. The researchers also investigated the mechanism behind these reactions and used the method to access enolborates and syn-aldol products. This research has the potential to expand the possibilities for catalytic reactions and could lead to further advancements in the field. [Extracted from the article]

Aims. A series of high-precision calculations for the electronic structure of MgH have been reported in the past two decades; however, most of them d not include the core-valence correlation and still exhibit distinct differences. Furthermore, the latest high-precision results have not been applied to the studies of photodissociation dynamics. The primary motivations of this paper are to calculate a more precise electronic structure of MgH consering a core-valence correlation and to prove the photodissociation cross-sections. Methods. The electronic structure of MgH is investigated by multi-reference configuration interaction calculations with Davson correction (MRCI+Q). We performed two different sets of calculations to investigate the core-valence correlation and, as a result, obtained accurate potential energy curves (PECs) and transition dipole moments (TDMs). An extrapolation procedure was also employed to eliminate the error of basis set. Then, the photodissociation cross-sections were calculated using high-precision PECs and TDMs. Results. The PECs and TDMs of the five lowest doublet electronic states, X2Σ+, B′2Σ+, E2Σ+, A2Π, and C2Π, are obtained from calculations including core-valence correlation, termed as CV-MRCI, while PECs of the ten lowest doublet states and three quartet states are also obtained from NCV-MRCI calculations without core-valence correlation. The spectroscopic constants and band oscillator strengths are also proved with high precision levels. The equilibrium Re and vertical excitation energy Te are only 0.1% different from the measurements. Based on the CV-MRCI results, the rovibrationally resolved photodissociation cross-sections for transitions from X2Σ+ to the other four states, as well as the total local thermodynamic equilibrium cross-sections for temperatures up to 10000 K, are calculated. [ABSTRACT FROM AUTHOR]

The ionospheric response to solar 27‐day signatures has complex temporal and spatial variations, which are driven by short‐ and long‐term changes of the solar activity among other processes (e.g., geomagnetic activity). A delay between these solar and ionospheric signatures occurs due to accumulation of plasma, which depends on the rates of photoionization, photodissociation and recombination in the upper atmosphere. The balance of these processes changes significantly at different solar activity levels. Therefore the present study investigates the delayed ionospheric response during low and high solar activity. For that purpose, global as well as latitude‐ and longitude‐dependent total electron content (TEC) is analyzed via superposed epoch analysis (SEA). Different results based on the solar proxy F10.7 and solar extreme ultraviolet (EUV) irradiance measurements are presented to discuss findings of preceding studies and to provide a first wavelength‐dependent analysis of the spatial features. The SEA shows a good correlation between solar activity level and delayed ionospheric response with particular strong enhancements of the delay at mid‐latitudes. Increased delays are also observed for morning and evening hours. Thus, the present study confirms the correlation of delayed ionospheric response and solar activity level as indicated in preceding studies, and also quantifies the temporal and spatial variations with commonly used data sets. This in turn allows a better understanding of the processes that cause the delayed ionospheric response. Key Points: The global delayed ionospheric response to solar EUV increases with the solar activity levelLatitude‐ and longitude‐dependent changes of the delayed ionospheric response are observed between low and high solar activityThe delayed ionospheric response varies significantly for selected EUV bands [ABSTRACT FROM AUTHOR]

We report the detection of a rich spectrum of more than one hundred optical emission lines of vibrationally hot molecular hydrogen (H2) from the photodissociation region (PDR) around the mini-starburst cluster NGC 346 in the Small Magellanic Cloud. The lines are concentrated in the spectral range |$6000 \, {{\mathring{\rm A}}}$| to |$9300 \, {{\mathring{\rm A}}}$| and have observed brightnesses ranging from 0.01 per cent to 0.4 per cent times that of the H β λ4861 hydrogen recombination line. Analysis of the spatial distribution of the H2 lines shows that they originate from a range of depths in the PDR, intermediate between the shallow layers probed by fluorescent lines of neutral nitrogen and oxygen, and the more shielded layers probed by neutral carbon recombination lines. Comparison with other PDRs shows that the relative strength of the H2 lines with respect to the [ |$\mathrm{C\, \scriptstyle I}$| ] λ8727 line increases rapidly with decreasing metallicity, being at least 40 times larger in NGC 346 than in the prototypical PDR of the Orion Bar. The internal PDR dust extinction is also found to be anomalously low in NGC 346. A separate result is the discovery of a high-ionization bow shock around the O2 star Walborn 3. [ABSTRACT FROM AUTHOR]

The early-type star gamma Cas illuminates the reflection nebulae IC 59 and IC 63, creating two photodissociation regions (PDRs). Uncertainties about the distances to the nebulae and the resulting uncertainty about the density of the radiation fields incident on their surfaces have hampered the study of these PDRs during the past three decades. We employed far-ultraviolet (UV) – optical nebula – star colour differences of dust-scattered light to infer the locations of the nebulae relative to the plane of the sky containing gamma Cas, finding IC 63 to be positioned behind the star and IC 59 in front of the star. To obtain the linear distances of the nebulae relative to gamma Cas, we fit far-infrared archival Herschel flux data for IC 59 and IC 63 with modified blackbody curves and relate the resulting dust temperatures with the luminosity of gamma Cas, yielding approximate distances of 4.15 pc for IC 59 and 2.3 pc for IC 63. With these distances, using updated far-UV flux data in the 6–13.6 eV range for gamma Cas with two recent determinations of the interstellar extinction for gamma Cas, we estimate that the far-UV radiation density at the surface of IC 63 takes on values of G 0 = 58 or G 0 = 38 with respective values for E (B − V) for gamma Cas of 0.08 and 0.04 mag. This is a substantial reduction from the range 150 ≤ G 0 ≤ 650 used for IC 63 during the past three decades. The corresponding, even lower new values for IC 59 are G 0 = 18 and G 0 = 12. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of small (H2O)n+ (n=2–5) clusters have been studied at 308 nm using a high resolution cryogenic cylindrical ion trap velocity map imaging spectrometer. Time-of-flight mass spectra and images of ionic photofragments are recorded. (H2O)2+ clusters dissociate to yield H3O+ and H2O+ photofragments, indicating the presence of both proton-transferred (H3O+-OH) and hemibonded (H2O-OH2)+ structures for the dimer cluster. (H2O)n+ (n=3–5) clusters prevailingly dissociate to the H+(H2O)n–2, ...,1 photofragments by losing both of OH and H2O components, and the (H2O)5+ cluster shows an additional channel to produce H+(H2O)4 by only losing OH. The former suggests the (H2O)n–2H3O+OH structures for the (H2O)n+ (n=3–5) clusters, while the latter suggests in (H2O)5+ that, the H3O+ core and OH are separated by H2O. The results elucidate the structure progresses of small (H2O)n+ clusters. The experimental images yield negative and small values for the anisotropy parameters of photofragments, indicating that (H2O)n+ (n=2–5) clusters undergo vertical electronic transitions upon photon absorption followed by slow dissociation, and lead to highly internally excited photofragments. [ABSTRACT FROM AUTHOR]

Photodissociation of nitrous oxide (N2O) in the ultraviolet region has been revisited by using the time-sliced velocity map ion imaging technique, due to its importance in atmospheric chemistry. The images of O(1D) photofragments are recorded at 203.814 nm and 205.472 nm in one-color experiments and at eight photolysis wavelengths between 200 and 220 nm in two-color experiments. The rotational state distributions and angular anisotropy parameters of the N 2 (X 1 Σ g +) co-products are derived from the images. The results indicate that the rotational state distributions are inverted with the maximum around J≈70. The anisotropy parameter β mainly shows two declines as the N2 rotational quantum number Jincreases. According to theoretical calculations [J. Chem. Phys. 136, 044314 (2012)], the variations in β with J are caused by changes in the extent of axial recoil from a linear initial configuration. In the high-J region, however, additional torque exists on the ground state potential energy surface following nonadiabatic transitions, inducing both the additional rotational excitation and the lower β values. Compared to previous works, the two-color experiments combined with single vacuum ultraviolet photonionization of O(1D) allow us to acquire both the rotational state distribution and angular distribution accurately. This work deepens our understanding of triatomic molecule decomposition. [ABSTRACT FROM AUTHOR]

Excitation of vibration plays an important role in the photodissociation dynamics of molecules and ions. Experiments on the photodissociation dynamics of molecular ions via well-defined excited vibrational states present a formidable challenge, particularly when it comes to the selective preparation of ions at the vibrational combination level. Here, using time-sliced velocity map ion imaging, the photodissociation of H2S+ via A2A1(v1=1, v2=8, v3=0, K=1) state leading to S+(4S) and H2 products was investigated. The excited H2S+ cations were prepared by multiphoton ionization of H2S, followed by resonant excitation. Images of S+ ions were captured at six wavelengths ranging from 357.02 nm to 358.38 nm. From the ion images, the total kinetic energy release distributions and rotational state-specific anisotropy parameters were derived. Notably, repeatedly reversed branching ratios of the H2 (J=1) and H2 (J=3) rotational states were observed in a narrow photolysis energy region. This behavior was totally different from the results in recent studies on the photodissociation of HS+ where only a single vibrational mode v2 was excited for parent ions in A2A1(v1, v2, v3) states. The present study indicates that potential vibrational synergy effect was observed in the photodissociation dynamics of H2S+ when the parent ions were excited in a combinational vibrational mode. [ABSTRACT FROM AUTHOR]

Accurate measurements of the product spin-orbit finne-structure branching ratios are important for understanding the detailed photodissociation dynamics of small molecules. In this study, the atomic spin-orbit fine-structure branching rar tio N(2D5/2)/N(2D3/2) to the dissociation channel N(2D5/2,3/2)+N(2D5/2,3/2) is measured for the c 4 ′ 1 Σ u + (υ ′ = 6) and b ′ 1 Σ u + (υ ′ = 21) states of 14N2, and the b ′ 1 Σ u + (υ ′ = 20) and b ′ 1 Σ u + (υ ′ = 21) states of 15N2 by using the vacuum ultraviolet (VUV)pump VUV-probe time-sliced velocity-mapped ion imaging setup. The measurements show that the fine-structure branching ratio N(2D5/2)/N(2D3/2) is independent of the rotational level of the parent molecule (14N2 or 15N2) in each vibronic state, while it does show dependence on the vibronic characteristics. It is ~1.35 for the c 4 ′ 1 Σ u + (υ ′ = 6) state of 14N2 and b ′ 1 Σ u + (υ ′ = 20) state of 15N2, which are both close to the dissociation threshold N(2D5/2,3/2)+N(2D5/2,3/2); while it becomes ~1.00 for the b ′ 1 Σ u + (υ ′ = 21) state of both 14N2 and 15N2, which are relatively far above the dissociation threshold. A possible change from a statistical process near the threshold to a diabatic process far above the threshold might have occurred to be responsible for the observed vibronic dependence of the branching ratio. Detailed informations on the potential energy curves and their mutual couplings near the dissociation threshold are highly desired for understanding the present experimental measurement. [ABSTRACT FROM AUTHOR]

Ultraviolet photodissociation dynamics of 2-methylallyl radical from the 3p Rydberg state were investigated in the wavelength region of 226–244 nm using the high-n Rydberg atom time-of-flight (HRTOF) technique. The 2-methylallyl radicals were generated by 193 nm photolysis of 3-chloro-2-methyl-1-propene precursors. The photofragment yield spectrum of H-atom products increases in intensity with decreasing wavelengths in 226–244 nm. The TOF spectra of H-atom products show a bimodal structure. The predominant product channel (with ∼98% branching ratio) has a kinetic energy release peaking at ∼7 kcal/mol, with an average ratio of ET in the total available energy, (fT), of ∼0.18 in 226–244 nm and an isotropic product angular distribution. At the low ET, isotropic component is from statistical unimolecular decomposition of highly vibrationally excited hot 2-methylallyl to the methylenecyclopropane+H products, following internal conversion from the excited electronic state. The minor product channel (with ∼2% branching ratio) has a large kinetic energy peaking at ∼50 kcal/mol, with (fT)≈0.63 and an anisotropic angular distribution (β≈−0.2). At the high ET, anisotropic component is non-statistical and is postulated to be from direct loss of H atom via the 3p Rydberg state or repulsive part of the ground state to the 1,3-butadiene+H products. [ABSTRACT FROM AUTHOR]

The photodissociation quantum dynamics for the first absorption band of H2S in different initial vibrational states have been investigated using Chebyshev real wave packet method. Because of the difference of the wave functions for the initial vibrational states, the calculated absorption spectra and the distributions of vibrational and rotational state of the products display different dynamic characteristics. The width and peak position of the absorption spectra for initial stretching excited states (1,0,0) and (0,0,1) are different from that of the vibrational ground state, while the (0,1,0) vibrational state has two almost equally high peaks in its absorption spectrum because of the change of wave function in angular coordinate. The product vibrational state distribution for (0,1,0) initial state weakly depends on the excitation energy and is dominated by the products of v=0, but SH(v=1) fragment is dominant at lower energy for (1,0,0) and (0,0,1) vibrational states. The rotational state distributions of products are very cold with the peak at j=l for these four states and weakly depend on the total energy. Besides, the rotational state distribution from (0,1,0) vibrational state displays strong oscillation, and its anisotropic parameter with rotational quantum numbers is also different from that of the other three vibrational states. [ABSTRACT FROM AUTHOR]