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Time-sliced velocity map ion imaging (VMI) experiments were performed to investigate the photodissociation of H2S+X2B1, via the excitation to the A2A1 (0, 11, 0) state. Experimental images of the S+ (4Su) products were recorded near 349.60 nm for the K = 1 band, and near 344.30 nm for the K = 2 band. The derived product total kinetic energy release (TKER) spectra exhibit partially rotationally resolved structures corresponding to the H 2 (X 1 Σ g +) co-products. The observed product state and angular distributions both exhibit sensitive dependence on the photolysis wavelength at near 349.60 nm and 344.30 nm. These phenomena indicate the underlying rich dynamic details and the role of the rotational excitation of H2S+. [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]

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]

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]

The duration of laser pulses and the carrier-envelope phase (CEP) play a crucial role in shaping kinetic energy release (KER) spectra. In this study, we performed theoretical calculations on pulse duration-dependent KER spectra, ranging from hundreds to sub-femtoseconds, focusing on the $\mathrm{MgH^+}$ scenario. Our findings reveal a distinct shift in KER peaks from sub-cycle pulses, deviating from the resonance energy. Utilizing two-level perturbation theory, we identify that this shift is attributable to the energy-dependent transition matrix elements. Moreover, our investigation uncovers a notable CEP effect in KER from sub-cycle pulses, arising from interference between counter-rotating and rotating terms within a single ultraviolet photon transition. To leverage this insight, we propose a novel pump-probe methodology for precise CEP characterization of ultra-short laser pulses. We hope this method would promise advancements in understanding and manipulating ultrafast processes.

A new diabatic potential energy matrix (PEM) of the coupled 1ππ* and 1πσ* states for the 1πσ*-mediated photodissociation of thiophenol was constructed using a neural network (NN) approach. The diabatization of the PEM was specifically achieved by our recent method [Chin. J. Chem. Phys. 34, 825 (2021)], which was based on adiabatic energies without the associated costly derivative couplings. The equation of motion coupled cluster with single and double excitations (EOM-CCSD) method was employed to compute adiabatic energies of two excited states in this work due to its high accuracy, simplicity, and efficiency. The PEM includes three dimensionalities, namely the S−H stretch, C−S−H bend, and C−C−S−H torsional coordinates. The root mean square errors of the NN fitting for the S1 and S2 states are 0.89 and 1.33 meV, respectively, suggesting the high accuracy of the NN method as expected. The calculated lifetimes of the S1 vibronic 00 and 31 states are found to be in reasonably good agreement with available theoretical and experimental results, which validates the new EOM-CCSD-based PEM fitted by the NN approach. The combination of the diabatization scheme solely based on the adiabatic energies and the use of EOM-CCSD method makes the construction of reliable diabatic PEM quite simple and efficient. [ABSTRACT FROM AUTHOR]

The product branching ratio between different products in multichannel reactions is as important as the over-all rate of reaction, both in terms of practical applications (e.g. models of combustion or atmosphere chemistry) in understanding the fundamental mechanisms of such chemical reactions. A global ground state potential energy surface for the dissociation reaction of deuterated alkyl halide CD3CH2F was computed at the CCSD(T)/CBS//B3LYP/aug-cc-pVDZ level of theory for all species. The decomposition of CD3CH2F is controversial concerning C−F bond dissociation reaction and molecular (HF, DF, H2, D2, HD) elimination reaction. Rice-Ramsperger-Kassel-Marcus (RRKM) calculations were applied to compute the rate constants for individual reaction steps and the relative product branching ratios for the dissociation products were calculated using the steady-state approach. At the different energies studied, the RRKM method predicts that the main channel for DF or HF elimination from 1,2-elimination of CD3CH2F is through a four-center transition state, whereas D2 or H2 elimination from 1,1-elimination of CD3CH2F occurs through a direct three-center elimination. At 266, 248, and 193 nm photodissociation, the main product CD2CH2+DF branching ratios are computed to be 96.57%, 91.47%, and 48.52%, respectively; however, at 157 nm photodissociation, the product branching ratio is computed to be 16.11%. Based on these transition state structures and energies, the following photodissociation mechanisms are suggested: at 266, 248, 193 nm, CD3CH2F→absorption of a photon→TS5→the formation of the major product CD2CH2+DF; at 157 nm, CD3CH2F→absorption of a photon→D/F interchange of TS1→CDH2CDF→H/F interchange of TS2→CHD2CHDF→the formation of the major product CHD2+CHDF. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of Br − C bond cleavage for BrCN in the wavelength region from 225 nm to 260 nm has been studied by our homebuilt time-slice velocity-map imaging setup. The images for both of the ground state Br(2P3/2) and spin-orbit excited Br*(2P1/2) channels are obtained at several photodissociation wavelengths. From the analysis of the translational energy release spectra, the detailed vibrational and rotational distributions of CN products have been measured for both of the Br and Br* channels. It is found that the internal excitation of the CN products for the Br* channel is colder than that for the Br channel. The most populated vibrational levels of the CN products are v=0 and 1 for the Br and Br* channels, respectively. For the Br channel, the photodissociation dynamics at longer wavelengths are found to be different from those at shorter wavelengths, as revealed by their dramatically different vibrational and rotational excitations of the CN products. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of oxalyl bromide is investigated near 265 and 234 nm using the velocity map ion imaging technique coupled with a state selective [2+1] resonance-enhanced multiphoton ionization scheme. The translational energy distribution and 2PJ, J = 3/2, 1/2 parameters of the Br atom formation process are extracted from two-dimensional ion images. The trimodal energy distribution of Br(2P3/2) atoms is attributed to the primary C–Br bond fission as well as the secondary dissociation of bromine-containing C2O2Br radicals produced from the primary Br and Br* pathways. The conclusions are confirmed by the recoil anisotropy and the distribution width corresponding to the individual components. The branching ratio Br(2P3/2)/Br(2P1/2) confirms the excitation energy dependence. [ABSTRACT FROM AUTHOR]

The recently constructed cryogenic cylindrical ion trap velocity map imaging spectrometer (CIT-VMI) has been upgraded for coincidence imaging of both ionic and neutral photofragments from photodissociation of ionic species. The prepared ions are cooled down in a home-made cryogenic cylindrical ion trap and then extracted for photodissociation experiments. With the newly designed electric fields for extraction and acceleration, the ion beam can be accelerated to more than 4500 eV, which is necessary for velocity imaging of the neutral photofragments by using the position-sensitive imaging detector. The setup has been tested by the 355 nm photodissociation dynamics of the argon dimer cation (Ar2+). From the recorded experimental images of both neutral Ar and ionic Ar+ fragments, we interpret velocity resolutions of Δv/v≈4.6% for neutral fragments, and Δv/v≈1.5% for ionic fragments, respectively. [ABSTRACT FROM AUTHOR]

The D atom Rydberg tagging time-of-flight technique was applied to investigate the photodissociation dynamics of DNCO in the vacuum ultraviolet 133–137 nm region. The D atom product was detected and the corresponding product translational energy distributions and angular distributions were obtained. Three product channels, D + NCO(X2Π), D + NCO(A2Σ+) and D + NCO(B2Π), which arise from different dissociation pathways have been observed. It is found that the D + NCO(X2Π) channel involves two different dissociation pathways; one is via internal conversion from the excited state to the S0 state, and the other is via internal conversion from the excited state to the S1 state. The D + NCO(A2Σ+) channel was ascribed to be from dissociation on S2 surface, and the D + NCO(B2Π) channel possibly come from a relatively slow dissociation process. Vibrational structures observed in the D + NCO(A2Σ+) channel shows that there is dominant bending with a little stretching vibrational excitation of NCO(A2Σ+). This is consistent with what has been found in the previous study following VUV photodissociation of HNCO in a similar wavelength region. [ABSTRACT FROM AUTHOR]

By using the direct current slice velocity map imaging technique, the polarization experiment for S(1D2) product from the ultraviolet photodissociation of carbonyl sulfide at 207 nm was studied. The angular momentum polarization character of the photofragment S(1D2) was detected via two different resonance enhanced multiphoton ionization intermediate states, 1F3 and 1P1, and four different pump-probe laser polarization geometries. The angular distribution of the corresponding CO(X1Σ+) coproducts was extracted and analyzed using the molecular-frame polarization and the laboratory-frame anisotropy models. The observed total kinetic energy release spectrum indicates that there are three dissociation channels, corresponding to the low, medium, and high kinetic energy. The sources of the low and medium kinetic energy channels are consistent with those of bimodal translational energy distribution at longer photolysis wavelengths. The high kinetic energy channel is a new dissociation channel arising from the direct dissociation from the single repulsive A(21A′) state. [ABSTRACT FROM AUTHOR]

Photodissociation of HOBr is an important step in the reaction network of the depletion of ozone in stratosphere. Here, we report the first three-dimensional potential energy surfaces for the lowest three singlet states for HOBr, based on high level multi reference configuration interaction calculations. Quantum dynamics calculations are performed with a real wavepacket method, yielding not only absorption spectra but also internal state and angular distributions of the photodissociation fragments. Our results agree quantitatively with the measured total absorption cross sections of HOBr in the ultraviolet region and reproduce well the observed vibrationally cold and rotationally hot OH/OD fragments via photodissociation of HOBr/DOBr at 266 nm. In addition, we predict that the recoil anisotropy parameters for OH/OD are close to the limiting value of a parallel transition, suggesting a rapid dissociation process at 266 nm following an in-plane transition from the ground state (11A′) to the 21A′ state. This is consistent with the experimental conclusion derived from the measured rotational alignment. However, spin and electronic angular momenta need to be taken into account in the future to achieve a more quantitative agreement with experiment. Our work is expected to motivate further experimental investigations for this benchmark system. [ABSTRACT FROM AUTHOR]

The [1+1] two-photon dissociation dynamics of mass-selected 79Br2+ has been studied in a cold ion beam using a cryogenic cylindrical ion trap velocity map imaging spectrometer. The quartet 1 4 Σ u , 3 / 2 − state of 79Br2+ is employed as an intermediate state to initiate resonance enhanced two-photon excitation to high-lying dissociative states in the 4.0–5.0 eV energy region above the ground rovibronic state. Total kinetic energy release (TKER) and the two-dimensional recoiling velocity distributions of fragmented 79Br+ ions are measured using the technique of DC-slice velocity map imaging. Branching ratios for individual state-resolved product channels are determined from the TKER spectra. The measured photofragment angular distributions indicate that the dissociation of 79Br2+ occurs in dissociative Ω=3/2 state via ΔΩ=0 parallel transition from the 1 4 Σ u , 3 / 2 − intermediate state. Due to the considerable spin-orbit coupling effects in the excited states of 79Br2+, higher-lying dissociative quartet states are likely responsible for the observed photodissociation processes. [ABSTRACT FROM AUTHOR]

The dynamics of chlorine atom (2Pj) formation in the photodissociation process of halogen substituted pyrimidines, namely, 2,4,6-trichloropyrimidine and 5-chloro-2,4,6-trifluoropyrimidine have been studied around 235 nm using Resonance Enhanced Multiphoton Ionisation Time-of-Flight Mass Spectrometry technique. For the chlorine atom dissociation channel, we have determined the translational energy distribution, the recoil anisotropy parameter, β, and the spin–orbit branching ratio. In both the molecules, the TOF profiles for Cl (2P3/2) and Cl* (2P1/2) are found to be independent of laser polarisation suggesting a zero value for β, within the experimental uncertainties. For 2,4,6-trichloropyrimidine, the average translational energies for Cl and Cl* elimination channels are determined to be 6.0 ± 1.2 and 7.0 ± 1.5 kcal/mol, respectively. Similarly, for 5-chloro-2,4,6-trifluoropyrimidine, the average translational energies for Cl and Cl* elimination channels are determined to be 6.5 ± 1.2 and 7.9 ± 1.6 kcal/mol, respectively. Computational calculations are performed to generate the potential energy curves along the dissociating C-Cl bond using equation of motion coupled cluster with single and double excitations (EOM-CCSD) method. Computational studies suggest the role of triplet states in the photodissociation process forming the Cl atom. [ABSTRACT FROM AUTHOR]

Time‐resolved pump–probe gas‐phase X‐ray scattering signals, extrapolated to zero momentum transfer, provide a measure of the number of electrons in a system, an effect that arises from the coherent addition of elastic scattering from the electrons. This allows to identify reactive transients and determine the chemical reaction kinetics without the need for extensive scattering simulations or complicated inversion of scattering data. We examine the photodissociation reaction of trimethylamine and identify two reaction paths upon excitation to the 3p state at 200 nm: a fast dissociation path out of the 3p state to the dimethyl amine radical (16.6±1.2 %) and a slower dissociation via internal conversion to the 3s state (83.4±1.2 %). The time constants for the two reactions are 640±130 fs and 74±6 ps, respectively. Additionally, it is found that the transient dimethyl amine radical has a N−C bond length of 1.45±0.02 Å and a C−N−C bond angle of 118°±4°. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of isocyanic acid (HNCO) has been studied by the timesliced velocity map ion imaging technique at 193 nm. The NH(α¹Δ) products were measured via (2+1) resonance enhanced multiphoton ionization. Images have been accumulated for the NH(α¹Δ) rotational states in the ground and vibrational excited state (v=0 and 1). The center-of-mass translational energy distribution derived from the NH(α¹Δ) images implies that the CO vibrational distributions are inverted for most of the measured ¹NH(v|j) internal states. The anisotropic product angular distribution observed indicates a rapid dissociation process for the N--C bond cleavage. A bimodal rotational state distribution of CO(v) has been observed, this result implies that isocyanic acid dissociates in the S1 state in two different pathways. [ABSTRACT FROM AUTHOR]

Copyright of Chinese Journal of Chemical Physics (1674-0068) is the property of American Institute of Physics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The photodissociation dynamics of 4-bromo-2,3,5,6-tetrafluoropyridine (BTFP) have been studied in a supersonic molecular beam around 234 nm, which prepares the molecule in its ππ* state. The dynamics of the C Br bond dissociation have been investigated using resonance-enhanced multiphoton ionization coupled with time-of-flight mass spectrometer (REMPI-TOFMS) by detecting the nascent spin orbit states of the primary bromine atoms, namely, Br ( 2 P 3/2 ) and Br*( 2 P 1/2 ). State specific polarization dependent TOF profiles were obtained, from which the translational energy distributions and recoil anisotropy parameters, β i , were extracted using forward convolution method. A strong polarization dependence of TOF profiles suggests anisotropic distributions of the Br and Br* fragments. Two components, namely, the fast and the slow, are observed in the translational energy distribution of Br and Br* atoms, formed from different potential energy surfaces. The average translational energies released into the Br and Br* channels for the fast component are 17.3 ± 2.0 and 11.1 ± 2.0 kcal/mol, respectively. Similarly, for the slow component, the average translational energies imparted into the Br and Br* channels are 2.6 ± 1.0 and 1.5 ± 1.0 kcal/mol, respectively. The relative quantum yields of Br and Br* are 0.87 ± 0.15 and 0.13 ± 0.06. The anisotropy parameters for Br and Br* are characterized by a similar value of 0.60 ± 0.05. The energy partitioning into the translational modes is interpreted with the help of various models. The experimental studies revealing the nature of translational energies distribution for Br and Br* and its average value along with the theoretical calculations employing Time-Dependent Density Functional Theory (TD-DFT) and Multi Configuration Quasi Degenerate second order Perturbation Theory (MCQDPT2) suggest that the initially prepared ππ* state crosses over to a nearby σ* repulsive state along the C Br bond, mainly, πσ* state from where the dissociation takes place. The results also indicate the process of fast internal conversion to ground state from the initially prepared ππ* state, which eventually forms slow Br atoms after C-Br bond dissociation process on the ground state. During the course of experiments, the absorption spectrum was also obtained with its absolute absorption cross section. Non-adiabatic curve crossing plays an important role in the C Br bond dissociation of BTFP. [ABSTRACT FROM AUTHOR]

The dissociation potential energy surfaces of acetyl fluoride (CH 3 C(O)F) in the lowest three electronic states (S 0 , S 1 , and T 1 ) have been calculated at the CASPT2-f12/VTZ-f12 level of theory. Combining with the surface intersection points, four mechanistic channels have been elucidated, namely, intersystem crossing (ISC) to the T 1 state, internal conversion (IC) to the ground state, H-atom transfer, and direct dissociation along the S 1 pathway. Unlike other acetyl halides, it is found that the α-C C bond cleavage in T 1 surface after ISC to yield ground-state products CH 3 ( 2 A′) + COF( 2 A′) is the most probable mechanism upon the excitation at 248 nm, which agrees well with the experimental observation. Both the S 1 and T 1 potential energy surfaces are reported systematically for the first time. A comparison of the reactivity among CH 3 C(O)F, CH 3 C(O)Cl, CH 3 C(O)Br, CH 3 C(O)I and CH 3 C(O)CH 3 has been made. [ABSTRACT FROM AUTHOR]

Copyright of Chinese Journal of Chemical Physics (1674-0068) is the property of American Institute of Physics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The photodissociation dynamics of halogen substituted pyridines, namely, 3-chloropyridine (ClPy) and 3-chloro-2,4,5,6-tetrafluoropyridine (ClFPy), has been studied around 235 nm by detecting chlorine atoms in their spin orbit states Cl( 2 P 3/2 ) and Cl ∗ ( 2 P 1/2 ) using the REMPI-TOF technique. We have determined the translational energy distribution, the recoil anisotropy parameter, β , and the spin–orbit branching ratio, for chlorine atom elimination channels. The TOF profiles for Cl and Cl ∗ are found to be independent of laser polarization suggesting a zero value for β , within the experimental uncertainties. For 3-chloropyridine, the average translational energies for Cl and Cl ∗ elimination channels are determined to be 3.7 ± 1.0 and 7.0 ± 1.5 kcal/mol, respectively. Similarly, for 3-chloro-2,4,5,6-tetrafluoropyridine, the average translational energies for Cl and Cl ∗ elimination channels are determined to be 8.0 ± 1.5 and 9.0 ± 1.5 kcal/mol, respectively. The theoretical calculation suggests that the fluorine substitution increases the possibility of cross over to the π – σ ∗ state from the initially prepared π – π ∗ state. [ABSTRACT FROM AUTHOR]

The ultraviolet photodissociation dynamics of 2‑bromohexane around 234 nm has been investigated using the DC slice velocity map imaging technique. Relative quantum yield of the ground state bromine atom is measured to be 0.961. By analyzing the slice images, we can obtain the speed and angular distributions of the ground state and excited state bromine atoms. The speed distribution contains two Gaussian components, which are attributed to two dissociation channels. The high translational energy component is formed via the direct dissociation along the C–Br bond. Nevertheless, the low translational energy component is formed via the multidimensional dissociation along the C–Br bond coupling with the bending modes. The alkyl radical branching promotes the multidimensional dissociation. What's more, the contribution of the electric-dipole allowed 3 Q 1 , 3 Q 0 and 1 Q 1 states to the product formation is also discussed. The nonadiabatic 1 Q 1 ← 3 Q 0 transition plays a significant role in the dissociation process. [ABSTRACT FROM AUTHOR]

[Display omitted] • Photodissociation dynamics of astrophysically and combustion relevant xylene isomers C 6 H 4 (CH 3) 2 at 157 nm. • Formation of 'hot' tolyl radicals via C-C bond rupture of xylene reactants. • The presence of tolyl radicals in the interstellar medium as a precursor to methylated polyreactions with vinylacetylene (C 4 H 4). We investigated the photodissociation dynamics of three xylene isomers C 6 H 4 (CH 3) 2 at 157 nm. The center-of-mass translational energy distributions of C 7 H 7 radicals were found to peak at 26 kJ mol−1. Although the ionization energy of the C 7 H 7 tolyl fragment exceeds the energy of a 157 nm photon, C 7 H 7 + was observed as a result of the photoionization of vibrationally 'hot' tolyl (C 7 H 7) radicals and/or two-photon ionization. The formation of rovibrationally excited tolyl fragments was discussed. Our experiments suggest the presence of tolyl radicals in the interstellar medium as a precursor to methylated polycyclic aromatic hydrocarbons upon reaction with vinylacetylene (C 4 H 4). [ABSTRACT FROM AUTHOR]

Photo dissociation dynamics of linear triatomic molecule, cyanogen chloride (ClCN) in polar medium have been explored using Fourier grid Hamiltonian (FGH) method based two-dimensional mean field methodology. Ground electronic state potential of the molecule is calibrated with coupled cluster singles and doubles (CCSD) level ab initio theory using Gaussian 03 package taking 6-311++G(3df,2pd) as the basis set and fitted with two-dimensional coupled Morse potential and this potential is used for numerical calculation. Time-integrated probability current density is used as the measure of dissociation probability. Monochromatic and bichromatic laser fields are used as the external field and dissociation probability is calculated as the function of field frequency, dipolar position and pulse shape. [ABSTRACT FROM AUTHOR]

Photodissociation of 2-chloro-6-nitrotoluene (ClNT) at 193, 248 and 266 nm and nitrocyclopentane (NCP) at 193 nm leads to the formation of OH, as detected by laser-induced fluorescence (LIF). The nascent OH produced from the photolysis of ClNT at all the wavelengths is vibrationally cold, with the Boltzmann type rotational state distributions. However, the nascent OH product from NCP is in the ground and vibrationally excited states with the measured average relative population in ν ″ = 1 to that in ν ″ = 0 of 0.12 ± 0.03, and these levels are characterized by rotational temperatures of 650 ± 180 K and 1570 ± 90 K, respectively. The translational energy partitioned in the OH fragment has been measured for photodissociation of both ClNT and NCP. On the basis of both the experimental results and the ground state molecular orbital (MO) calculations, a plausible mechanism for the OH formation has been proposed. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of acetaldehyde in the radical channel CH3+HCO has been reinvestigated using time-sliced velocity map imaging technique in the photolysis wavelength range of 275 321 nm. The CH3 fragments have been probed via (2+1) resonance-enhanced - multiphoton ionization. Images are measured for CH3 formed in the ground and excited states (v2=0 and 1) of the umbrella vibrational mode. For acetaldehyde dissociation on T1 state after intersystem crossing from S 1state, the products are formed with high translational energy release and low internal excitation. The rotational and vibrational energy of both fragments increases with increasing photodissociation energy. The triplet barrier height is estimated at 3.881±0.006 eV above the ground state of acetaldehyde. [ABSTRACT FROM AUTHOR]

[Display omitted] • The ion-imaging study was performed for photodissociation of dimethylamine. • Two kinetic energy components of the methyl radical product were measured. • The calculation revealed the potential energy curves for N–CH 3 and N–H dissociations. State-resolved ion-imaging of the CH 3 product of dimethylamine photolysis in the 3s and 3p Rydberg bands (200–235 nm) revealed bimodal CH 3 velocity distributions with a fast portion disappearing at the longer photolysis wavelength. The fast component is assigned to direct dissociation based on the calculated potential energy curves. The slow component is assigned to indirect dissociation via N–H channel conical intersection with subsequent CH 3 dissociation. The origin of the threshold wavelength near 225 nm was attributed to the N–CH 3 bond rupture energy barrier on the S 1 (3s) state, implying an increasing contribution of direct dissociation at the shorter photolysis wavelengths. [ABSTRACT FROM AUTHOR]

In one-color REMPI-TOF experiment, the photodissociation dynamics of tetrachloroethylene has been studied by probing the chlorine atom photofragments, namely, Cl (2P3/2) and Cl∗ (2P1/2), using 2+1 REMPI scheme in the 234–236nm region. We have determined the centre-of-mass photofragment speed distribution, recoil anisotropy parameter (β), and the spin–orbit branching ratio for chlorine atom elimination channels. The β is well characterized by a value of ∼0.0, within the experimental uncertainties. Two components, namely, the fast and the slow, are observed in the translational energy distributions of Cl and Cl∗. The average translational energies for the Cl and Cl∗ channels for the fast components are 17.6±1.9 and 14.0±1.7kcal/mol, while, that for the slow components are 2.2±1.0 and 3.2±1.0kcal/mol, respectively. To understand the nature of the dissociative potential energy surface involved in the chlorine atom formation channel, detailed theoretical calculations are performed using Time-dependent Density Functional Theory (TD-DFT) method. [ABSTRACT FROM AUTHOR]

We present a first principles study of the carbon dioxide (CO2) photodissociation process in the 150- to 210-nm wavelength range, with emphasis on photolysis below the carbon monoxide + O(1D) singlet channel threshold at ~167 nm. The calculations reproduce experimental absorption cross-sections at a resolution of ~0.5 nm without scaling the intensity. The observed structure in the 150- to 210-nm range is caused by excitation of bending motion supported by the deep wells at bent geometries in the 21A' and 11A“ potential energy surfaces. Predissociation below the singlet channel threshold occurs via spin-orbit coupling to nearby repulsive triplet states. Carbon monoxide vibrational and rotational state distributions in the singlet channel as well as the triplet channel for excitation at 157 nm satisfactorily reproduce experimental data. The cross-sections of individual CO2 isotopologues (12C16O2, 12C17O16O, 12C18O16O, 13C16O2, and 13C18O16O) are calculated, demonstrating that strong isotopic fractionation will occur as a function of wavelength. The calculations provide accurate, detailed insight into CO2 photoabsorption and dissociation dynamics, and greatly extend knowledge of the temperature dependence of the cross-section to cover the range from 0 to 400 K that is useful for calculations of propagation of stellar light in planetary atmospheres. The model is also relevant for the interpretation of laboratory experiments on mass-independent isotopic fractionation. Finally, the model shows that the mass-independent fractionation observed in a series of Hg lamp experiments is not a result of hyperfine interactions making predissociation of 17O containing CO2 more efficient. [ABSTRACT FROM AUTHOR]

Abstract: The photodissociation dynamics of 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane) have been studied near 234nm, in a molecular beam environment, employing resonance-enhanced multiphoton ionization with time-of-flight mass spectrometer (REMPI-TOF-MS). Both bromine and chlorine atoms are detected in both the ground and spin–orbit excited states. The C–Br bond scission is observed predominantly due to direct σ∗(C–Br)←n(Br) transition, with translational energy distribution described with a soft impulsive model. The recoil anisotropy parameter (β) of 0.6±0.2 for this channel vindicates the impulsive nature of dissociation. The diabatic crossing from the nσ∗(C–Br) surface to nσ∗(C–Cl) diabatic surface is responsible for chlorine formation via the C–Cl bond fission, which is supported by the excited state molecular orbital calculations. The velocity distribution of bromine atom has one component, however, that of chlorine atom is bimodal. Both fast and slow chlorine atom channels, with the β value of 0.3±0.1, are produced impulsively from the nσ∗(C–Cl) repulsive surface. [Copyright &y& Elsevier]

Resonance Raman spectra (RRs) and quantum chemical calculations were used to investigate the photodissociation dynamics of diuron in S2 state. The RRs indicate that the photorelaxation dynamics for the S0 → S2 excited state is predominantly along nine motions: the ring C = C stretch vibration ν12 (1593 cm−1), Ph-N-H wag ν14 (1517 cm−1), CO-N(CH3)2 stretch ν23 (1365 cm−1), CCH wag in plane/ring C = C stretch ν24 (1297 cm−1), ring CH rock in plane/ring deformation ν27 (1233 cm−1), CCH wag in plane ν29 (1151 cm−1), Ph-Cl (para) stretch ν35 (1028 cm−1), Ph-N-H wag ν37 (913 cm−1) and ring breath ν44 (685 cm−1). Dissociation by Ph-Cl (para) cleavage at S2 state directly or relaxation to T2 state by internal conversion (S2 → S1) and intersystem crossing (S1/T2) is expected by ~250 nm irradiation based on the RRS, complete active space self-consistent field, configuration interaction singles and time-dependent density functional theory calculations. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: In one-color REMPI-TOF experiment, the photodissociation dynamics of POCl3 has been studied by photolyzing POCl3 and probing the chlorine atom photofragments, namely, Cl(2P3/2) and Cl∗(2P1/2) using 2+1 REMPI scheme, in the 234–236nm region. We have determined the centre-of-mass photofragment speed distribution, recoil anisotropy parameter, and the spin–orbit branching ratio for chlorine atom elimination channels. The anisotropy parameters for Cl and Cl∗ are the same, and characterized by a value of 0.0±0.05. Two components, namely, the fast and the slow, are observed in the translational energy distributions of Cl and Cl∗. The average translational energies for the Cl and Cl∗ channels for the fast components are 12.5±1.5 and 16.8±1.5kcal/mol, while, for the slow components, the average translational energies are 1.5±1.0 and 2.5±1.0kcal/mol, respectively. Apart from the chlorine atom elimination channel, Cl2 elimination is also observed in the photodissociation of POCl3. [Copyright &y& Elsevier]

Abstract: The photodissociation dynamics of 2-furoyl chloride at 235nm has been investigated, employing resonance-enhanced multiphoton ionization technique and time-of-flight mass spectrometry. Both the Cl fragments, Cl(2 PJ= 3/2, relative quantum yield 0.85±0.11) and Cl∗(2 PJ =1/2), have the recoil anisotropy parameter (β) value close to zero, and show bimodal translational energy distributions. The branching ratio of the high kinetic energy Ccission to the low energy Con is 0.78/0.22. The dominant high kinetic energy channel arises mainly because of electronic pre-dissociation. But, the low energy channel results from the ground electronic state of 2-furoyl chloride, formed subsequent to non-radiative relaxation of the initially prepared excited state. [Copyright &y& Elsevier]

Abstract: The recent classical dynamical approach of photodissociations with Bohr quantization [L. Bonnet, J. Chem. Phys. 133 (2010) 174108] is applied for the first time to a realistic process, the photofragmentation of the van der Waals cluster NeBr2. We illustrate the fact that this approach, formally equivalent to the standard one, may be numerically much more efficient. [Copyright &y& Elsevier]

Roaming is a recently verified unusual pathway to molecular products from unimolecular dissociation of an energized molecule. Here we present the evidence for this pathway for H2CO and CH3CHO. Theoretical analysis shows that this path visits the plateau region of the potential energy surface near dissociation to radical products. It is not clear whether roaming is a distinct isolated pathway, in addition to the conventional one via the well-known molecular saddle-point transition state. Evidence is presented to suggest that the two pathways may originate from a single, but highly complicated, dividing surface. Other examples of unusual reaction dynamics are also reviewed. [ABSTRACT FROM AUTHOR]

The contribution of the first excited 11A'' state potential energy surface (PES) to the reactivity of the C(1D)+H2 system has been examined by comparing the simulations obtained with the theoretical state-to-state differential cross sections (DCSs) with experimental data from cross molecular beam experiments. The calculations were carried out using the quantum scattering and quasiclassical trajectory calculations on the ground and first excited PESs, and the DCSs were convoluted with the various apparatus functions in order to simulate the experimental laboratory angular distributions and time-of-flight distributions. It was expected that the participation of the excited PES could solve the relatively minor discrepancies resulting from the comparison between the experimental data and the simulations using the DCSs obtained on the ground state PES. Nevertheless, the addition of the contribution of the excited 11A'' PES worsens the agreement with the experimental results. [ABSTRACT FROM AUTHOR]

An ab initio study of the role of electronic curve crossing of benzene S1 state in the photo dissociation dynamics of the iodobenzene and effect of fluorination is presented. Two dissociative life times observed in iodobenzene is attributed to the coupled repulsive potential energy curves of the low-lying n-σ*, π-σ*, π-π* states. The direct channel is attributed to the alkyl like transition and the indirect channel is attributed to the mixing of the alkyl like transitions with the low lying benzene π-π* transitions. Fluorination of iodobenzene results in a substantial increase in the direct channel product. To analyze the possible role of electronic curve crossing of these transitions, potential energy curves of low-lying n-σ*, π-σ*, π-π* states were studied including spin-orbit and relativistic effects using the Restricted Active Space state interaction multistate complete active space perturbation theory (RASSI-MS-CASPT2) method. Crossing behavior of spin-free and spin-orbit potential energy curves was analyzed for the role of the benzene S1 state. Our results indicate the curve crossing region to be around 2.00–2.35 Å for both C6H5I and C6F5I. Analysis of effect of fluorination on the energies of states corresponding to benzene π-π* and n-σ* transitions suggests an increase in the energy of benzene π-π* states and a decrease in the energy of the states corresponding to n-σ* transitions. Increased spin-orbit gap, increased separation of the benzene S1(π-π*) state and n-σ* states in the region of curve crossing, lesser mixing of the π-π* and n-σ* states, an order of magnitude decrease in the transition strength to the benzene singlet transition all contributed to the observed substantial increase in the quantum yield of the direct channel product on fluorination of aryl halides. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [ABSTRACT FROM AUTHOR]

Control of the ultrashort photodissociation dynamics of a diatomic molecule on different excitation channels is investigated by Stark shift with a strong and approximately constant electric pulse, that 'straddles' the pump pulse excitation. The scheme implies first the creation of the light-induced potentials (LIP) in the presence of the control field, then the selective crossing to a single excited LIP by the ultrashort pump pulse, and finally the turn-off of the control field. We observe: (1) blue-shifting of the photospectra controlled by the field amplitude; (2) non-selective photodissociation of a single exit channel in all the interval of energies despite selective adiabatic passage of the wave packet and some control of the branching ratio; (3) high yields of population transfer to weakly bonded states in the excited potentials with ultrashort pulses and no frequency tuning. [ABSTRACT FROM AUTHOR]

Abstract: Photodissociation dynamics of OCS and CS2 adsorbed on water ice films at 90K was investigated at 193nm by measuring the time-of-flight spectra of photodissociated S(1D) and S(3P) atoms. The translational energy distributions are well represented by a combination of Gaussian and Maxwell–Boltzmann (M–B) distributions corresponding to the fast and slow time-of-flight components, respectively. Formation of the sulfur dimer on the ice surface is proposed as a secondary photoproduct. The fine-structure branching ratios of S(3P J ) from OCS and CS2 adsorbed on H2O or D2O water ice films were also determined. [Copyright &y& Elsevier]

Abstract: In recent years, the photodissociation dynamics of aryl halides has been a subject of intensive studies, which is closely related to the atmospheric chemistry. Here we present a review on the photochemistry of aryl halides, with emphasis on the recent progress in photodissociation dynamics at 266nm by using photofragment translational spectroscopy. The ab initio calculations have also been employed to investigate those photodissociation processes. It has been found that the photodissociation of aryl halides at 266nm is attributed to the nonadiabatic process via intersystem crossings from bound singlet excited state to triplet excited state and/or via internal conversion from bound singlet excited state to ground state. Also, the substitution effects in the photodissociation dynamics of aryl halides are discussed. [Copyright &y& Elsevier]

Abstract: The photodissociation dynamics of CH2Br2 was investigated near 234 and 267nm. A two-dimensional photofragment ion velocity imaging technique coupled with a [2+1] resonance-enhanced multiphoton (REMPI) ionization scheme was utilized to obtain the angular and translational energy distributions of the nascent Br (2 P 3/2) and Br* (2 P 1/2) atoms. The obtained translational energy distributions of Br and Br* are found consist of two components which should be come from the radical channel and secondary dissociation process, respectively. It is suggested that the symmetry reduction from to C s during photodissociation invokes a non-adiabatic coupling between the 2B1 and A1 states. Consequently, the higher internal energy distribution of Br channel than Br* formation channel and the broader translational energy distribution of the former are presumed correlate with a variety of vibrational excitation disposal at the crossing point resulting from the larger non-adiabatic crossing from 2B1 to A1 state than the reverse crossing. Moreover, the measured anisotropy parameter β indicate that fragments recoil along the Br–Br direction mostly in the photodissociation. [Copyright &y& Elsevier]

Product state distributions of the CO produced in the 308-nm photolysis of acetaldehyde show clear evidence of two dissociation mechanisms. One is attributed to the conventional transition state mechanism predicted by theory, with high rotational and translational energy of the CO and a pronounced v⊥J vector correlation. However, as much as 15% of the reaction flux proceeds via another pathway that produces low CO rotational and translational energy, very high CH4 internal energy, and no correlation between the CO velocity and angular momentum vectors. The attributes of this channel are dynamically similar to the recently reported ‘roaming atom’ mechanism in formaldehyde. We therefore speculate that the second pathway in acetaldehyde also occurs via a roaming mechanism in the CH3 + HCO exit channel that decays into the CH4 + CO channel. [ABSTRACT FROM AUTHOR]

In this review article, recent studies on the photodissociation dynamics of carboxylic acids carried out in our laboratory are presented. The dynamics are investigated by mapping the energy partitioning in the nascent photoproduct OH using laser-induced fluorescence spectroscopy. To understand the effect of the nature of the C&z.sbnd;C bond on the dissociation dynamics, both saturated (acetic) as well as unsaturated (acrylic and propiolic) carboxylic acids are investigated. In all of the carboxylic acids studied, a high percentage of the available energy is partitioned into the product translational state, indicating the presence of an exit barrier in the dissociative potential energy surface. Based on the energy partitioning, the quantum yield and the OH formation rate, the photoexcitation dynamics of carboxylic acids are revealed. [Copyright &y& Elsevier]

The dynamics of formation of oxygen atoms after UV photoexcitation of SO in the gas-phase was studied by pulsed laser photolysis-laser-inducedfluorescence 'pump-and-probe' technique in a flow reactor. SO at room-temperature was excited at the KrCl excimer laser wavelength (222.4 nm) and O(P) photofragments were detected under collision-free conditions by vacuum ultraviolet laser-induced fluorescence. The use of narrow-band probe laser radiation, generated viaresonant third-order sum-difference frequency conversion of dye laser radiation in Krypton, allowed the measurement of the nascent O(P=2,1,0) fine-structure state distribution: n =2/n=1/n=0 = (0.88 ± 0.02)/(0.10 ± 0.01)/(0.02 ± 0.01). Employing NOphotolysis as a reference, a value of Φ(P) = 0.13 ± 0.05 for the absolute O(P) atom quantum yield was determined. The measured O(P) quantum yield is compared with the results of earlier fluorescence quantum yield measurements. A suitable mechanism is suggested in which the dissociation proceeds via internal conversion from high rotational states of the initially excited SO(~CB (1, 2, 2) vibronic level to nearby continuum states of the electronic ground state. [ABSTRACT FROM AUTHOR]