Search

The topology of multidimensional potential energy surfaces defines the bimolecular collision outcomes of open–shell radicals with molecular partners. Understanding these surfaces is crucial for predicting the inelastic scattering and chemical transformations of increasingly complex radical–molecule collisions. To characterize the inelastic scattering mechanisms of nitric oxide (NO) radicals with large alkanes, we generated the collision complexes comprised of NO with propane or n-butane. The infrared action spectroscopy and infrared-driven dynamics of NO–propane and NO–(n-butane) collision complexes in the CH stretch region were recorded, while also comparing the results to the analogous experiments carried out for NO–CH4 and NO–ethane. The infrared spectroscopy is analyzed using rovibrational simulations to characterize the transition bands and to determine the vibrational predissociation lifetimes of NO–propane and NO–(n-butane). Due to pseudo Jahn–Teller dynamics, the NO–propane and NO–(n-butane) decay mechanisms from IR activation appear similar to those for NO–ethane previously reported from this laboratory (J. P. Davis et al. Faraday Discuss., 2024, 251, 262–278). Furthermore, the NO (X²Π, v′′ = 0, J′′, Fn, Λ) product state distributions from NO–alkane fragmentation reveal a strong electron-spin polarization and a propensity for NO products to rotate in the plane of the π* molecular orbital, yielding mechanistic insights into the inelastic scattering outcomes. We hypothesize that a geometric phase may be present, impacting the relative population distributions, in addition to the accessible pathway timescales. [ABSTRACT FROM AUTHOR]

To empower high school and undergraduate students to become citizen scientists, we describe the design, construction, and operation of a multisensor device based on Arduino microcontroller technology to record atmospheric measurements relevant to secondary organic aerosols (SOAs). The handheld "AIRduino" multisensor device was built and deployed by undergraduate and high school students to simultaneously measure the concentrations of ozone (O[subscript 3]), volatile organic compounds (VOCs), temperature, humidity, and aerosol particulate matter with 2.5 and 10 µm diameters (PM[subscript 2.5] and PM[subscript 10]). Paired with a smartphone via Bluetooth, the data are streamed in real-time to an app interface for on-demand SOA profiling, and the results are recorded with on-board storage using a storage device (SD) card. Furthermore, the AIRduino SOA measurements are calibrated with the regional atmospheric data from the CAPABLE site at the NASA Langley Research Center. Subsequently, the regional chemistry composition was monitored for several days with the AIRduino device to correlate gas-phase molecule concentrations to the formation of aerosols. The mobile AIRduino multisensor device is low-cost and readily accessible for students, thus making it an ideal vehicle to obtain crowd-sourced SOA measurements and to foster excitement in science, technology, engineering, and mathematics.

The article focuses on a general discussion about the scattering of larger molecules, particularly examining the differences in potential energy surfaces (PES) between HDO, H2O, and D2O. Topics include the reason for stronger back scattering in HDO compared to H2O and D2O, the role of symmetry and potential terms in scattering phenomena, and specific insights into the n10 term affecting back-scattering.

In bimolecular collisions between open-shell radicals and increasingly-larger alkanes, the relative impact configurations open the possibility of reactive and nonreactive outcomes that are isomer specific. To model the interaction potential between molecular scattering partners, observables are needed from experiments that can quantify both the initial molecular orientations and internal energies on multidimensional potential energy surfaces. Recent work by our group demonstrated that upon infrared (IR) excitation, the dynamics of the nitric oxide–methane collision complex (NO–CH4) are dependent on the initial monomer geometries, as small changes in configuration substantially affect the energies, electronic couplings, and predissociation pathways due to the Jahn–Teller effect. This study focuses on the isomer-specific scattering mechanisms between NO and ethane (C2H6), encoded in the spectroscopic and dynamical signatures of the NO–C2H6 collision complex. IR action spectroscopy with 1 + 1 resonance-enhanced multiphoton ionization of NO products was employed to characterize the fundamental CH stretch transitions of NO–C2H6, as well as to initiate the nonreactive decay mechanisms of the complex. Furthermore, velocity map imaging (VMI) was utilized to explore the dynamics prior to and following IR excitation of NO–C2H6, imprinted on the NO photoproducts. This work compares the dynamics from NO–C2H6 and NO–CH4 vibrational predissociation, in which substantial differences are observed in the energy exchange mechanisms during the evolution of the collision complexes to products. [ABSTRACT FROM AUTHOR]

Nitric oxide (NO) radicals are ubiquitous chemical intermediates present in the atmosphere and in combustion processes, where laser-induced fluorescence is extensively used on the NO (A2Σ+ ← X2Π) band to report on fuel-burning properties. However, accurate fluorescence quantum yields and NO concentration measurements are impeded by electronic quenching of NO (A2Σ+) to NO (X2Π) with colliding atomic and molecular species. To improve predictive combustion models and develop a molecular-level understanding of NO (A2Σ+) quenching, we report the velocity map ion images and product state distributions of NO (X2Π, v″ = 0, J″, Fn, Λ) following nonreactive collisional quenching of NO (A2Σ+) with molecular oxygen, O2 (X3Σg−). A novel dual-flow pulse valve nozzle is constructed and implemented to carry out the NO (A2Σ+) electronic quenching studies and to limit NO2 formation. The isotropic ion images reveal that the NO–O2 system evolves through a long-lived NO3 collision complex prior to formation of products. Furthermore, the corresponding total kinetic energy release distributions support that O2 collision coproducts are formed primarily in the c1Σu− electronic state with NO (X2Π, v″ = 0, J″, Fn, Λ). The product state distributions also indicate that NO (X2Π) is generated with a propensity to occupy the Π(A″) Λ-doublet state, which is consistent with the NO π* orbital aligned perpendicular to nuclear rotation. The deviations between experimental results and statistical phase space theory simulations illustrate the key role that the conical intersection plays in the quenching dynamics to funnel population to product rovibronic levels. [ABSTRACT FROM AUTHOR]

Laser-induced fluorescence excitation and dispersed fluorescence spectra of a model flexible bichromophore, 1,1-diphenylethane (DPE), have been recorded under jet-cooled conditions in the gas phase in the region near the first pair of near-degenerate excited states (S1 and S2). The S1 and S2 origin transitions have been identified at 37 397 and 37 510 cm−1, a splitting of 113 cm−1. This splitting is four times smaller than the excitonic splitting calculated by ab initio methods at the EOM-CCSD/cc-pVDZ level of theory (410 cm−1), which necessarily relies on the Born-Oppenheimer approximation. Dispersed fluorescence spectra provide a state-to-state picture of the vibronic coupling. These results are compared with the results of a multimode vibronic coupling model capable of treating chromophores in asymmetric environments. This model was used to predict the splitting between S1 and S2 origins close to the experiment, reduced from its pure excitonic value by Franck-Condon quenching. Quantitative accuracy is achieved by the model, lending insight into the state-to-state mixing that occurs between individual S1 and S2 vibronic levels. The S2 origin is determined to be mixed with S1(v) levels by two mechanisms common to internal conversion in almost any setting; namely, (i) mixing involving near-degenerate levels with large vibrational quantum number changes that are not governed by Δv = 1 Herzberg-Teller (HT) selection rules, and (ii) mixing with levels with larger energy gaps that do follow these selection rules. In DPE, the asymmetric ring flapping vibrational mode R ¯ dominates the HT coupling. [ABSTRACT FROM AUTHOR]

The electronic quenching of NO (A2Σ+) with molecular partners exemplifies the rich non-adiabatic dynamics that occurs on multiple, coupled potential energy surfaces (PESs). The mechanistic details of the electronic quenching depend sensitively on the nature and strength of the intermolecular interactions between NO (A2Σ+) and the molecular partner. In this paper, we reveal the electronic quenching mechanisms of NO (A2Σ+) with H2O, a non-adiabatic process with an extremely large cross section of 121 Å2 near room temperature. In doing so, we demonstrate that the NO (A2Σ+) + H2O PES funnels a wide range of initial intermolecular orientations to the same minimum-energy geometry. Furthermore, we reveal low-energy pathways to conical intersections between NO (A2Σ+) + H2O and NO (X2Π) + H2O that primarily involve decreasing the intermolecular distance and elongating a single O–H bond of H2O. Based on these geometric distortions, we predict that nonreactive electronic quenching will be associated with significant vibrational excitation in a local O–H stretch mode in H2O. Reactive quenching will produce a H-atom and HONO, an important intermediate in atmospheric and combustion chemistry and a precursor to the hydroxyl radical. Overall, our work provides the first detailed theoretical study of the mechanism of the electronic quenching of NO (A2Σ+) with a polyatomic molecular partner, as well as makes concrete predictions to inform future velocity map imaging experiments. [ABSTRACT FROM AUTHOR]

The unimolecular dissociation dynamics of the dimethyl-substituted Criegee intermediate (CH3)2COO is examined experimentally using velocity map imaging to ascertain the translational and internal energy distributions of the OH and H2CC(CH3)O radical products. The energy profile of key features along the reaction coordinate is also evaluated theoretically. Unimolecular decay of (CH3)2COO is initiated by vibrational activation in the CH stretch overtone region and the resultant OH X²Π3/2 (v = 0) products are state-selectively ionized and imaged. Analysis reveals an isotropic spatial distribution, indicative of a 3 ps lower limit for the timescale of dissociation, and a broad and unstructured total kinetic energy release distribution. The energy released to products is partitioned principally as internal excitation of the H2CC(CH3)O fragments with modest translational excitation of the fragments and a small degree of OH rotational excitation. The total kinetic energy release distribution observed for (CH3)2COO is compared with that predicted for statistical partitioning over product quantum states, and contrasted with recent experimental and quasi-classical trajectory results for syn-CH3CHOO [N. M. Kidwell et al., Nat. Chem. 8, 509 (2016)]. [ABSTRACT FROM AUTHOR]

Vibrationally and rotationally resolved electronic spectra of diphenylmethane-d5 (DPM-d5) are reported in the isolated-molecule environment of a supersonic expansion. While small, the asymmetry induced by deuteration of one of the aromatic rings is sufficient to cause several important effects that change the principle mechanism of vibronic coupling between the close-lying S1 and S2 states, and spectroscopic signatures such coupling produces. The splitting between S1 and S2 origins is 186 cm-1, about 50% greater than its value in DPM-d0 (123 cm-1), and an amount sufficient to bring the S2 zero-point level into near-resonance with the v = 1 level in the S1 state of a low-frequency phenyl flapping mode, vR = 191 cm-1 . Dispersed fluorescence spectra bear clear evidence that △v(R) = 1 Herzberg-Teller coupling dominates the near-resonant internal mixing between the S1 and S2 manifolds. The fluorescence into each pair of Franck-Condon active ring modes shows an asymmetry that suggests incorrectly that the S1 and S2 states may be electronically localized. From rotationally resolved studies, the S0 and S1 states have been well-fit to asymmetric rotor Hamiltonians while the S2 state is perturbed and not fit. The transition dipole moment (TDM) orientation of the S1 state is nearly perpendicular to the C2 symmetry axes with 66(2)%:3(1)%:34(2)% a:b:c hybrid-type character while that of the S2 origin contains 50(10)% a:c-type (S1) and 50(10)% b-type (S2) character. A model is put forward that explains qualitatively the TDM compositions and dispersed emission patterns without the need to invoke electronic localization. The experimental data discussed here serve as a foundation for a multi-mode vibronic coupling model capable of being applied to asymmetric bichromophores, as presented in the work of B. Nebgen and L. V. Slipchenko ["Vibronic coupling in asymmetric bichromophores: Theory and application to diphenylmethane-d5," J. Chem. Phys. (submitted)]. [ABSTRACT FROM AUTHOR]

Vibrationally and rotationally resolved electronic spectra of diphenylmethane-d5 (DPM-d5) are reported in the isolated-molecule environment of a supersonic expansion. While small, the asymmetry induced by deuteration of one of the aromatic rings is sufficient to cause several important effects that change the principle mechanism of vibronic coupling between the close-lying S1 and S2 states, and spectroscopic signatures such coupling produces. The splitting between S1 and S2 origins is 186 cm-1, about 50% greater than its value in DPM-d0 (123 cm-1), and an amount sufficient to bring the S2 zero-point level into near-resonance with the v = 1 level in the S1 state of a low-frequency phenyl flapping mode, vR = 191 cm-1 . Dispersed fluorescence spectra bear clear evidence that △v(R) = 1 Herzberg-Teller coupling dominates the near-resonant internal mixing between the S1 and S2 manifolds. The fluorescence into each pair of Franck-Condon active ring modes shows an asymmetry that suggests incorrectly that the S1 and S2 states may be electronically localized. From rotationally resolved studies, the S0 and S1 states have been well-fit to asymmetric rotor Hamiltonians while the S2 state is perturbed and not fit. The transition dipole moment (TDM) orientation of the S1 state is nearly perpendicular to the C2 symmetry axes with 66(2)%:3(1)%:34(2)% a:b:c hybrid-type character while that of the S2 origin contains 50(10)% a:c-type (S1) and 50(10)% b-type (S2) character. A model is put forward that explains qualitatively the TDM compositions and dispersed emission patterns without the need to invoke electronic localization. The experimental data discussed here serve as a foundation for a multi-mode vibronic coupling model capable of being applied to asymmetric bichromophores, as presented in the work of B. Nebgen and L. V. Slipchenko ["Vibronic coupling in asymmetric bichromophores: Theory and application to diphenylmethane-d5," J. Chem. Phys. (submitted)]. [ABSTRACT FROM AUTHOR]

The alkyl and aromatic CH stretch infrared spectra of inden-2-ylmethyl (I2M, C10H9) and trihydronaphthyl (THN, C10H11) radicals have been recorded under jet-cooled conditions in the ground (D0) and first electronically excited (D1) states using resonant ion-dip infrared (RIDIR) spectroscopy. Previously, the vibronic spectroscopy of a series of C10H9 and C10H11 hydronaphthyl radicals were investigated and their thermochemical properties were evaluated with isomer specificity [J. A. Sebree et al., J. Phys. Chem. A 11, 6255-6262 (2010)]. We show here that one of the m/z 129 spectral carriers characterized in that work was misidentified as 2-hydronaphthyl (2-HN) radical, appearing in a discharge of 1,2-dihydronaphthalene in close proximity to 1-hydronaphthyl radical. The D0-RIDIR spectrum in the alkyl CH stretch region positively identifies the m/z 129 isomer as I2M, whose two-color resonant two-photon ionization (2C-R2PI) spectrum was recently reported by Schmidt and co-workers [T. P. Troy et al., Chem. Sci. 2, 1755-1765 (2011)]. Here, we further characterize the I2M and THN radicals by recording their gas phase IR spectra in the alkyl and aromatic CH stretch regions, and explore the spectroscopic consequences of electronic excitation on the CH stretch absorptions. A local-mode CH stretch Hamiltonian incorporating cubic stretch-bend coupling between anharmonic CH stretches and CH2 scissor modes is utilized to describe their Fermi resonance interactions. Excellent agreement between the experimental and theoretical results facilitates the interpretation of the D0- and D1-state RIDIR spectra of I2M, revealing that upon excitation the alkyl CH stretches decrease in frequency by 70 cm-1, while the allyl-like CH stretches experience a modest blueshift. In comparison, the photophysics of THN are strikingly different in that the IR transitions that possess vibrational motion along the CβH and CδH bonds are absent in the D1-RIDIR spectrum yet are predicted to be present from the theoretical model. Several hypotheses are considered to account for the perturbations to these vibrations. [ABSTRACT FROM AUTHOR]

Theinfrared (IR) spectroscopy of the alkyl CH stretch region (2750–3000cm–1) of a series of bicyclic hydrocarbons and freeradicals has been studied under supersonic expansion cooling in thegas phase, and compared with a theoretical model that describes thelocal mode stretch–bend Fermi resonance interactions. The doubleresonance method of fluorescence-dip infrared (FDIR) spectroscopywas used on the stable molecules 1,2-dihydronaphthalene, 1,4-dihydronaphthalene,tetralin, indene, and indane using the S0–S1origin transition as a monitor of transitions. Resonant ion-dipinfrared (RIDIR) spectra were recorded for the trihydronaphthyl (THN)and inden-2-yl methyl (I2M) radicals. The previously developed modelHamiltonian (J. Chem. Phys.2013, 138, 064308) incorporates cubic stretch–bend couplingwith parameters obtained from density functional theory methods. Fulldimensional calculations are compared to reduced dimensional Hamiltonianresults in which anharmonic CH stretches and CH2scissormodes are Fermi coupled. Excellent agreement between theoretical resultsis found. Scale factors of select terms in the reduced dimensionalHamiltonian, obtained by fitting the theoretical Hamiltonian predictionsto the experimental spectra, are found to be similar to previous work.The resulting Hamiltonian predicts successfully all the major spectralfeatures considered in this study. A simplified model is introducedin which the CH2groups are decoupled. This model enablesthe assignment of many of the spectral features. The model resultsare extended to describe the CH stretch spectrum of the chair andtwist-boat conformers of cyclohexane. The chair conformer is usedto illustrate the shortcomings of the CH2decoupling model. [ABSTRACT FROM AUTHOR]

Thestate-dependent spectroscopy of α-methylbenzyl radical(α-MeBz) has been studied under jet-cooled conditions. Two-colorresonant two-photon ionization (2C-R2PI), laser-induced fluorescence,and dispersed fluorescence spectra were obtained for the D0–D1electronic transition of this prototypicalresonance-stabilized radical in which the methyl group is immediatelyadjacent to the primary radical site. Extensive Franck–Condonactivity in hindered rotor levels was observed in the excitation spectrum,reflecting a reorientation of the methyl group upon electronic excitation.Dispersed fluorescence spectra from the set of internal rotor levelsare combined with the excitation spectrum to obtain a global fit ofthe barrier heights and angular change of the methyl group in bothD0and D1states. The best-fit methyl rotorpotential in the ground electronic state (D0) is a flat-topped3-fold potential (V3″ = 151 cm–1, V6″ = 34 cm–1) while the D1state has a lower barrier (V3′ = 72 cm–1, V6′ = 15 cm–1) with Δφ = ± π/3, π, consistent witha reorientation of the methyl group upon electronic excitation. Theground state results are compared with calculations carried out atthe DFT B3LYP level of theory using the 6-311+G(d,p) basis set, anda variety of excited state calculations are carried out to compareagainst experiment. The preferred geometry of the methyl rotor inthe ground state is anti, which switches to synin the D1state and in the cation. The calculationsuncover a subtle combination of effects that contribute to the shiftin orientation and change in barrier in the excited state relativeto ground state. Steric interaction favors the anticonformation, while hyperconjugation is greater in the synorientation. The presence of a second excited state close by D1is postulated to influence the methyl rotor properties. Aresonant ion-dip infrared (RIDIR) spectrum in the alkyl and aromaticCH stretch regions was also recorded, probing in a complementary waythe state-dependent conformation of α-MeBz. Using a scheme inwhich infrared depletion occurs between excitation and ionizationsteps of the 2C-R2PI process, analogous infrared spectra in D1were also obtained, probing the response of the CH stretchfundamentals to electronic excitation. A reduced-dimension WilsonG-matrix model was implemented to simulate and interpret the observedinfrared results. Finally, photoionization efficiency scans were carriedout to determine the adiabatic ionization threshold of α-MeBz(IP = 6.835 ± 0.002 eV) and provide thresholds for ionizationout of specific internal rotor levels, which report on the methylrotor barrier in the cation state. [ABSTRACT FROM AUTHOR]

Nitric oxide (NO) radicals are ubiquitous chemical intermediates present in the atmosphere and in combustion processes, where laser-induced fluorescence is extensively used on the NO (A2Σ+ ← X2Π) band to report on fuel-burning properties. However, accurate fluorescence quantum yields and NO concentration measurements are impeded by electronic quenching of NO (A2Σ+) to NO (X2Π) with colliding atomic and molecular species. To improve predictive combustion models and develop a molecular-level understanding of NO (A2Σ+) quenching, we report the velocity map ion images and product state distributions of NO (X2Π, v″ = 0, J″, Fn, Λ) following nonreactive collisional quenching of NO (A2Σ+) with molecular oxygen, O2 (X3Σg−). A novel dual-flow pulse valve nozzle is constructed and implemented to carry out the NO (A2Σ+) electronic quenching studies and to limit NO2 formation. The isotropic ion images reveal that the NO–O2 system evolves through a long-lived NO3 collision complex prior to formation of products. Furthermore, the corresponding total kinetic energy release distributions support that O2 collision coproducts are formed primarily in the c1Σu− electronic state with NO (X2Π, v″ = 0, J″, Fn, Λ). The product state distributions also indicate that NO (X2Π) is generated with a propensity to occupy the Π(A″) Λ-doublet state, which is consistent with the NO π* orbital aligned perpendicular to nuclear rotation. The deviations between experimental results and statistical phase space theory simulations illustrate the key role that the conical intersection plays in the quenching dynamics to funnel population to product rovibronic levels. [ABSTRACT FROM AUTHOR]

Bimolecular collision outcomes sensitively depend on the chemical functionality and relative orientations of the colliding partners that define the accessible reactive and nonreactive pathways. Accurate predictions from multidimensional potential energy surfaces demand a full characterization of the available mechanisms. Therefore, there is a need for experimental benchmarks to control and characterize the collision conditions with spectroscopic accuracy to accelerate the predictive modeling of chemical reactivity. To this end, the bimolecular collision outcomes can be investigated systematically by preparing reactants in the entrance channel prior to reaction. Herein, we investigate the vibrational spectroscopy and infrared-driven dynamics of the bimolecular collision complex between nitric oxide and methane (NO-CH 4 ). We recorded the vibrational spectroscopy of NO-CH 4 in the CH 4 asymmetric stretching region using resonant ion-depletion infrared spectroscopy and infrared action spectroscopy, thus revealing a significantly broad spectrum centered at 3030 cm -1 that extends over 50 cm -1 . The asymmetric CH stretch feature of NO-CH 4 is explained by CH 4 internal rotation and attributed to transitions involving three different nuclear spin isomers of CH 4 . The vibrational spectra also show extensive homogeneous broadening due to the ultrafast vibrational predissociation of NO-CH 4 . Additionally, we combine infrared activation of NO-CH 4 with velocity map imaging of NO ( X 2 Π, ν″ = 0, J ″, F n , Λ) products to develop a molecular-level understanding of the nonreactive collisions of NO with CH 4 . The anisotropy of the ion image features is largely determined by the probed rotational quantum number of NO ( J ″) products. For a subset of NO fragments, the ion images and total kinetic energy release (TKER) distributions show an anisotropic component at low relative translation (∼225 cm -1 ) indicating a prompt dissociation mechanism. However, for other detected NO products, the ion images and TKER distributions are bimodal, in which the anisotropic component is accompanied by an isotropic feature at high relative translation (∼1400 cm -1 ) signifying a slow dissociation pathway. In addition to the predissociation dynamics following vibrational excitation, the Jahn-Teller dynamics prior to infrared activation need to be considered to fully describe the product spin-orbit distributions. Therefore, we correlate the Jahn-Teller mechanisms of NO-CH 4 to the symmetry-restricted NO ( X 2 Π, ν″ = 0, J ″, F n , Λ) + CH 4 (ν″) product outcomes.

The electronic quenching of NO (A 2 Σ + ) with molecular partners exemplifies the rich non-adiabatic dynamics that occurs on multiple, coupled potential energy surfaces (PESs). The mechanistic details of the electronic quenching depend sensitively on the nature and strength of the intermolecular interactions between NO (A 2 Σ + ) and the molecular partner. In this paper, we reveal the electronic quenching mechanisms of NO (A 2 Σ + ) with H 2 O, a non-adiabatic process with an extremely large cross section of 121 Å 2 near room temperature. In doing so, we demonstrate that the NO (A 2 Σ + ) + H 2 O PES funnels a wide range of initial intermolecular orientations to the same minimum-energy geometry. Furthermore, we reveal low-energy pathways to conical intersections between NO (A 2 Σ + ) + H 2 O and NO (X 2 Π) + H 2 O that primarily involve decreasing the intermolecular distance and elongating a single O-H bond of H 2 O. Based on these geometric distortions, we predict that nonreactive electronic quenching will be associated with significant vibrational excitation in a local O-H stretch mode in H 2 O. Reactive quenching will produce a H-atom and HONO, an important intermediate in atmospheric and combustion chemistry and a precursor to the hydroxyl radical. Overall, our work provides the first detailed theoretical study of the mechanism of the electronic quenching of NO (A 2 Σ + ) with a polyatomic molecular partner, as well as makes concrete predictions to inform future velocity map imaging experiments.

Intermolecular interactions, stereodynamics, and coupled potential energy surfaces (PESs) all play a significant role in determining the outcomes of molecular collisions. A detailed knowledge of such processes is often essential for a proper interpretation of spectroscopic observations. For example, nitric oxide (NO), an important radical in combustion and atmospheric chemistry, is commonly quantified using laser-induced fluorescence on the A 2 Σ + ← X 2 Π transition band. However, the electronic quenching of NO ( A 2 Σ + ) with other molecular species provides alternative nonradiative pathways that compete with fluorescence. While the cross sections and rate constants of NO ( A 2 Σ + ) electronic quenching have been experimentally measured for a number of important molecular collision partners, the underlying photochemical mechanisms responsible for the electronic quenching are not well understood. In this paper, we describe the development of high-quality PESs that provide new physical insights into the intermolecular interactions and conical intersections that facilitate the branching between the electronic quenching and scattering of NO ( A 2 Σ + ) with H 2 , N 2 , and CO. The PESs are calculated at the EOM-EA-CCSD/d-aug-cc-pVTZ//EOM-EA-CCSD/aug-cc-pVDZ level of theory, an approach that ensures a balanced treatment of the valence and Rydberg electronic states and an accurate description of the open-shell character of NO. Our PESs show that H 2 is incapable of electronically quenching NO ( A 2 Σ + ) at low collision energies; instead, the two molecules will likely undergo scattering. The PESs of NO ( A 2 Σ + ) with N 2 and CO are highly anisotropic and demonstrate evidence of electron transfer from NO ( A 2 Σ + ) into the lowest unoccupied molecular orbital of the collision partner, that is, the harpoon mechanism. In the case of ON + CO, the PES becomes strongly attractive at longer intermolecular distances and funnels population to a conical intersection between NO ( A 2 Σ + ) + CO and NO ( X 2 Π) + CO. In contrast, for ON + N 2 , the conical intersection is preceded by an ∼0.40 eV barrier. Overall, our work shines new light into the impact of coupled PESs on the nonadiabatic dynamics of open-shell systems.

Nitric oxide (NO) radicals are ubiquitous chemical intermediates present in the atmosphere and in combustion processes, where laser-induced fluorescence is extensively used on the NO (A 2 Σ + ← X 2 Π) band to report on fuel-burning properties. However, accurate fluorescence quantum yields and NO concentration measurements are impeded by electronic quenching of NO (A 2 Σ + ) to NO (X 2 Π) with colliding atomic and molecular species. To improve predictive combustion models and develop a molecular-level understanding of NO (A 2 Σ + ) quenching, we report the velocity map ion images and product state distributions of NO (X 2 Π, v″ = 0, J″, F n , Λ) following nonreactive collisional quenching of NO (A 2 Σ + ) with molecular oxygen, O 2 (X 3 Σ g - ). A novel dual-flow pulse valve nozzle is constructed and implemented to carry out the NO (A 2 Σ + ) electronic quenching studies and to limit NO 2 formation. The isotropic ion images reveal that the NO-O 2 system evolves through a long-lived NO 3 collision complex prior to formation of products. Furthermore, the corresponding total kinetic energy release distributions support that O 2 collision coproducts are formed primarily in the c 1 Σ u - electronic state with NO (X 2 Π, v″ = 0, J″, F n , Λ). The product state distributions also indicate that NO (X 2 Π) is generated with a propensity to occupy the Π(A″) Λ-doublet state, which is consistent with the NO π * orbital aligned perpendicular to nuclear rotation. The deviations between experimental results and statistical phase space theory simulations illustrate the key role that the conical intersection plays in the quenching dynamics to funnel population to product rovibronic levels.

The photodissociation dynamics of jet-cooled CH 2 BrI were investigated in the near-ultraviolet (UV) region from 280-310 nm using velocity map imaging. We report the translational and internal energy distributions of the CH 2 Br radical and ground state I ( 2 P 3/2 ) or spin-orbit excited I ( 2 P 1/2 ) fragments determined by velocity map imaging of the ionized iodine fragments following 2 + 1 resonance-enhanced multiphoton ionization of the nascent neutral iodine products. The velocity distributions indicate that most of the available energy is partitioned into the internal energy of the CH 2 Br radical with only modest translational excitation imparted to the cofragments, which is consistent with a simple impulsive model. Furthermore, from extrapolation of the velocity distribution results, the first determination of the C-I bond dissociation energy of CH 2 BrI is presented in this work to be D 0 = 16 790 ± 590 cm -1 . The ion images appear anisotropic, indicative of a prompt dissociation, and the derived anisotropy parameters are consistently positive. Additionally, the angular distributions report on the electronic excited state dynamics, which validate recent works characterizing the electronic states responsible for the first absorption band of CH 2 BrI. In the current work, photolysis of CH 2 BrI on the red edge of the absorption spectrum reveals an additional channel producing I ( 2 P 3/2 ) fragments.