State-to-state photodissociation dynamics of triatomic molecules: H2O in the B band

Full quantum calculations of the line shape for H2O perturbed by Ar at temperatures from 20 to 300 K

This work theoretically studied the spectral line shape of H2O perturbed by Ar in the temperature range of 20-300 K for the pure rotational lines below 360 cm-1, as well as three lines (31, 2 ← 44, 1, 54, 2 ← 41, 3, and 73, 5 ← 60, 6) in the v2 band. In order to perform precise dynamical calculations at low collision energies, a full-dimensional long-range potential energy surface was constructed for the H2O-Ar system for the first time to correct the long range of our newly developed intermolecular potential energy surface. Subsequently, the six line-shape parameters (pressure-broadening and -shifting parameters, their speed dependencies, and the complex Dicke parameters) were determined from the generalized spectroscopic cross section by the full quantum time-independent close-coupling approach on this new potential energy surface. Our theoretical results are in good agreement with the available experimental observations. Furthermore, the influence of the speed-dependence and Dicke narrowing effects on the line contour was revealed by comparing the differences among the Hartmann-Tran, quadratic-speed-dependent Voigt, and Voigt profiles. The temperature dependence of each line-shape parameter was further parameterized using the triplet-power-law for three pure rotational 61, 6 ← 52, 3, 41, 4 ← 32, 1, and 31, 3 ← 22, 0 lines. These line-shape parameters will provide a comprehensive set of theoretical references for subsequent experimental measurements.

Isotope Effects on State-to-State Photodissociation Dynamics of D2S in Its First Absorption Band

State-to-state photodissociation dynamics of D2S in its first absorption band were explored by utilizing recently developed diabatic potential energy surfaces (PESs). Quantum dynamics calculations, involving the first two strongly coupled 1A″ states, were executed employing a Chebyshev real wavepacket method. The nonadiabatic channel via the conical intersection (CI) is facile, direct, and fast, leading to the production of rotationally and vibrationally cold SD(X̃2Π). The calculated absorption spectrum, product state distributions, and angular distributions are in reasonable agreement with the experimental results, although some discrepancies exist at 193.3 nm. Compared with H2S, there are obvious isotope effects on rotational state distributions for D2S photodissociation in its first absorption band. Moreover, we scrutinize the variation of product state distributions as a function of photon energy and the vibrational mediated photodissociation of the parent molecule. Due to the diverse shapes of the three fundamental vibrational wave functions, photoexcited wavepackets access distinct segments of the upper-state PES, resulting in a disparate absorption spectrum and ro-vibrational distributions via the nonadiabatic transition. This study provides a comprehensive figure of the isotopic effect and wavelength dependence on the photofragmentation behaviors from D2S photodissociation, which should attract more experimental and theoretical attention to this prototypical system.

New accurate diabatic potential energy surfaces for the two lowest 1A'' states of H2S and photodissociation dynamics in its first absorption band

In this work, state-to-state photodissociation dynamics of H2S in its first absorption band has been studied quantum mechanically with a new set of coupled potential energy surfaces (PESs) for the first two 1A'' excited states, which were developed at the explicitly correlated internally contracted multi-reference configuration interaction level with the cc-pVQZ-F12 basis set and a large active space. The calculated absorption spectrum, product state distributions, and angular distributions are in excellent agreement with available experimental data, validating the accuracy of the PESs and the non-adiabatic couplings. Detailed analysis of the dynamics reveals that there are strong non-adiabatic couplings between the bound 11B1 and dissociative 11A2 states around the Franck-Condon region, leading to very fast predissociation to ro-vibrationally cold SH(X̃) fragments, during which marginal angular anisotropy of the PESs is involved. This study provides quantitatively accurate characterization of the electronic structure and detailed fragmentation dynamics of this prototypical photodissociation system, which is desirable for improving astrochemical modelling.

Constructing Diabatic Potential Energy Matrices with Neural Networks Based on Adiabatic Energies and Physical Considerations: Toward Quantum Dynamic Accuracy

A permutation invariant polynomial-neural network (PIP-NN) approach for constructing the global diabatic potential energy matrices (PEMs) of the coupled states of molecules is proposed. Specifically, the diabatization scheme is based merely on the adiabatic energy data of the system, which is ideally a most convenient way due to not requiring additional ab initio calculations for the data of the derivative coupling or any other physical properties of the molecule. Considering the permutation and coupling characteristics of the system, particularly in the presence of conical intersections, some vital treatments for the off-diagonal terms in diabatic PEM are essentially needed. Taking the photodissociation of H₂O(X~/B~)/NH₃(X~/A~) and nonadiabatic reaction Na(3p) + H₂ → NaH(Σ⁺) + H for example, this PIP-NN method is shown to build up the global diabatic PEMs effectively and accurately. The root-mean-square errors of the adiabatic potential energies in the fitting for three different systems are all small (<10 meV). Further quantum dynamic calculations show that the absorption spectra and product branching ratios in both H₂O(X~/B~) and NH₃(X~/A~) nonadiabatic photodissociation are well reproduced on the new diabatic PEMs, and the nonadiabatic reaction probability of Na(3p) + H₂ → NaH(Σ⁺) + H obtained on the new diabatic PEMs of the 1²A₁ and 1²B₂ states is in reasonably good agreement with previous theoretical result as well, validating this new PIP-NN method.

ABC+D: A time-independent coupled-channel quantum dynamics program for elastic and ro-vibrational inelastic scattering between atoms and triatomic molecules in full dimensionality

We discuss the details of a time-independent quantum mechanical method and its implementation for full-dimensional non-reactive scattering between a closed-shell triatomic molecule and a closed-shell atom. By solving the time-independent Schrödinger equation within the coupled-channel framework using a log-derivative method, the state-to-state scattering matrix (S-matrix) can be determined for inelastic scattering involving both the rotational and vibrational modes of the molecule. Various approximations are also implemented. The ABC+D code provides an important platform for understanding an array of physical phenomena involving collisions between atoms and molecules.

Full-Dimensional Quantum Dynamics Studies of Ro-vibrationally Inelastic Scattering of H2O with Ar: A Benchmark Test of the Rigid-Rotor Approximation

While the rigid-rotor (RR) approximation is usually considered to be accurate for describing pure rotationally inelastic scattering involving diatoms in their ground or low-lying vibrational states, its validity in scattering involving polyatomic molecules has not been fully examined. The existence of soft/anharmonic vibrational modes in polyatomic molecules could make rotational-vibrational energy transfer rather efficient, thus undermining the premise of the RR approximation. In this work, we conduct a benchmark test of the RR approximation in the rotationally inelastic scattering of the H₂O(v₂ = 0, 1) + Ar system by comparing with full-dimensional quantum scattering calculations. We demonstrate that the error in the RR rate coefficient for v₂ = 0 is less than 5%, while it can reach up to 20% for some initial states within the v₂ = 1 manifold. These results indicate that the RR approximation gradually deteriorates with increasing quantum number v₂. Vibrational relaxation dynamics of this system was also studied, and it is found that transitions from initial states with a large rotational quantum number of projection on the a principal axis are more efficient. These results shed valuable light on ro-vibrationally inelastic scattering involving polyatomic molecules.

Full-dimensional quantum studies of vibrational energy transfer dynamics between H2O and Ar: theory assessing experiment

We report the first full-dimensional quantum mechanical calculations of the ro-vibrational inelastic scattering dynamics between water molecules and argon atoms on an accurate potential energy surface, using a recently developed time-independent quantum method based on the close-coupling approach. The state-to-state integral cross-sections and rate coefficients show strong observance of gap laws. The calculated thermal rate coefficients for the relaxation of the stretching fundamental states of H₂O are in good agreement with experimental values, while those for the bending overtone state are approximately five times smaller than the values extracted through a previous kinetic modeling of fluorescence decay data. Our state-specific quantum scattering results suggest the need to reassess the kinetic modeling of the experimental data. This work advanced our understanding of the quantum dynamics of vibrationally inelastic energy transfer processes involving polyatomic molecules.

Rotational state specific dissociation dynamics of D2O via the C̃(010) state: The effect of bending vibrational excitation

The rotational state resolved photodissociation dynamics of D2O via the [Formula: see text](010) state has been investigated by using the D-atom Rydberg tagging time-of-flight technique combined with a tunable vacuum ultraviolet light source. The D-atom action spectrum of the [Formula: see text](010) ← [Formula: see text](000) band and the corresponding time-of-flight (TOF) spectra of D-atom photoproducts formed following the excitation of D2O to individual rotational transition have been measured. By comparison with the action spectrum of the [Formula: see text](000) ← [Formula: see text](000) band, the bending vibrational constant of the [Formula: see text] state for D2O can be determined to be v2 = 1041.37 ± 0.71 cm−1. From the TOF spectra, the product kinetic energy spectra, the vibrational state distributions of OD products, and the state resolved anisotropy parameters have been determined. The experimental results indicate a dramatic variation in the OD product state distributions for different rotational excitations. This illuminates that there are two distinctive coupling channels from the [Formula: see text](010) state to the low-lying electronic states: the homogeneous electronic coupling to the Ã1B1 state, resulting in vibrationally hot OD(X) products, and the Coriolis-type coupling to the [Formula: see text]1A1 state, producing vibrationally cold but rotationally hot OD(X) and OD(A) products. Furthermore, the three-body dissociation channel is confirmed, which is attributed to the [Formula: see text] → 1A2 or [Formula: see text] → Ã pathway. In comparison with the previous results of D2O photolysis via the [Formula: see text](000) state, it is found that the v2 vibration of the parent molecule enhances both the vibrational and rotational excitations of OD products.

Unimolecular Dissociation Dynamics of Electronically Excited HCO( ): Rotational Control of Nonadiabatic Decay

The photoinduced unimolecular decay of the electronically excited HCO( ) is investigated in a combined experimental-theoretical study. The molecule is excited to the (1, n2, 0) combination bands, which decay...

Vibrationally excited molecular hydrogen production from the water photochemistry

Vibrationally excited molecular hydrogen has been commonly observed in the dense photo-dominated regions (PDRs). It plays an important role in understanding the chemical evolution in the interstellar medium. Until recently, it was widely accepted that vibrational excitation of interstellar H₂ was achieved by shock wave or far-ultraviolet fluorescence pumping. Here we show a further pathway to produce vibrationally excited H₂ via the water photochemistry. The results indicate that the H₂ fragments identified in the O(¹S) + H₂(X¹Σ_g⁺) channel following vacuum ultraviolet (VUV) photodissociation of H₂O in the wavelength range of λ = ~100-112 nm are vibrationally excited. In particular, more than 90% of H₂(X) fragments populate in a vibrational state v = 3 at λ~112.81 nm. The abundance of water and VUV photons in the interstellar space suggests that the contributions of these vibrationally excited H₂ from the water photochemistry could be significant and should be recognized in appropriate interstellar chemistry models.

A Time-Independent Quantum Approach to Ro-vibrationally Inelastic Scattering between Atoms and Triatomic Molecules

A full-dimensional time-independent quantum mechanical theory for ro-vibrationally inelastic scattering of triatomic molecules with atoms is formulated. The Jacobi-Radau coordinate system used in the calculation allows not only a near perfect description of the vibrational problem but also the adaptation of the exchange symmetry for A₂B type triatoms. The S-matrix elements are obtained by solving the close-coupling equations with contracted basis using the log-derivative method. This method is applied to the inelastic scattering of the water molecule by a chlorine atom, which sheds light on the energy gap law in energy transfer in atom-triatom collisions.

Rotational Modulation of Ã2A″-State Photodissociation of HCO via Renner-Teller Nonadiabatic Transitions

By examining the product-state distribution of a prototypical nonadiabatic predissociation system, HCO(òA″-X̃²A'), we demonstrate that the dissociation dynamics is strongly modulated by parent rotational quantum numbers. The predissociation of the nominal (ν_C-H = 0, ν_bend, ν_C-O = 1) vibronic levels of the òA″ state surprisingly gives rise to both vibrational ground and excited states of the CO product, despite the assumed spectator nature of the CO moiety. This anomaly is attributed to the dependence of the lifetime of the vibronic resonance facilitated by the Renner-Teller interaction on the parent rotational angular momentum quantum numbers coupled with transient intensity borrowing from nearby vibronic resonances with ν_C-O = 0. This unique phenomenon is a purely quantum mechanical behavior that has no classical analogue.

Photodissociation Dynamics of H2O via the Ẽ' (1B2) Electronic State

Photodissociation dynamics of H₂O via the Ẽ'¹B₂ state were studied using the high-resolution H atom photofragment translational spectroscopy method, in combination with the tunable vacuum ultraviolet free electron laser (VUV FEL). The measured translational energy spectra allow us to determine the respective quantum state population distributions for the nascent OH(X²Π) and OH(A²Σ⁺) photofragments. Analyses of the quantum state population distributions show both the ground and electronically excited OH fragments to be formed with moderate vibrational excitation but with highly rotational excitation. Unlike the dissociation via the lower-lying electronic states, where OH(X) is the major fragment, the OH(A) products are predominant via the Ẽ' state. These products are mainly ascribed to a fast dissociation on the B̃¹A₁ state surface after nonadiabatic transitions from the initial excited Ẽ' state to the B̃ state. Meanwhile, another dissociation pathway from the Ẽ' state to the ¹B₂ 3pb₂ state, followed by coupling to the ¹A₂ 3pb₂ state, is also observed, which yields the OH(X) + H and O(³P) + 2H products.

Water Photolysis and Its Contributions to the Hydroxyl Dayglow Emissions in the Atmospheres of Earth and Mars

Airglow is a well-known phenomenon in the Earth's upper atmosphere, which arises from the emissions of energetic atoms and molecules. The Meinel band emission from high vibrationally excited OH(X) radicals is one of the more important contributors to the airglow from the mesosphere/lower thermosphere. The H + O₃ reaction has long been regarded as the dominant source of these OH(X, high v) radicals. Here we demonstrate that vacuum ultraviolet (VUV) photolysis of water vapor at λ ∼ 112.8 nm represents another source of exceptionally highly vibrationally excited OH(X) radicals, with a nascent vibrational state population distribution that maximizes at v = 9 and extends to at least the v = 15 level. Atmospheric chemistry modeling indicates that OH(X, high v) radicals from H₂O photolysis might be detectable in the OH Meinel band dayglow in the upper atmosphere of Earth and should dominate the corresponding emission from the Martian atmosphere. VUV photolysis of H₂O also produces electronically excited OH(A) radicals, and simultaneous detection of emissions from OH(X, high v) and OH(A) is shown to offer a route to identifying high-oxygen exoplanetary atmospheres.

Electronically Excited OH Super-rotors from Water Photodissociation by Using Vacuum Ultraviolet Free-Electron Laser Pulses

The fragmentation dynamics of water in a superexcited state play an important role in the ionosphere of the planets and in the photodissociation region (PDR) of the planetary nebula. In this Letter, we experimentally study the fragmentation dynamics of H₂O with the energy above its ionization potential initiated by vacuum ultraviolet free-electron laser pulses. The experimental results indicate that the binary fragmentation channels H + OH and the triple channels O + 2H both present at 96.4 nm photolysis. Electronically excited OH super-rotors (v = 0, N ≥ 36, or v = 1, N ≥ 34), with the internal energy just above the OH (A) dissociation energy, are observed for the first time, which are only supported by the large centrifugal barriers. An absolute cross section of these super-rotors is estimated to be 0.7(±0.3) × 10^-18 cm². The tunnelling rates of these extremely rotationally excited states are also analyzed. This work shows a spectacular example of energy transfer from a photon to fragment rotation through photodissociation.

Exclusive Neural Network Representation of the Quasi-Diabatic Hamiltonians Including Conical Intersections

We propose a numerically simple and straightforward, yet accurate and efficient neural networks-based fitting strategy to construct coupled potential energy surfaces (PESs) in a quasi-diabatic representation. The fundamental invariants are incorporated to account for the complete nuclear permutation inversion symmetry. Instead of derivative couplings or interstate couplings, a so-called modified derivative coupling term is fitted by neural networks, resulting in accurate description of near degeneracy points, such as the conical intersections. The adiabatic energies, energy gradients, and derivative couplings are well reproduced, and the vanishing of derivative couplings as well as the isotropic topography of adiabatic and diabatic energies in asymptotic regions are automatically satisfied. All of these features of the coupled global PESs are requisite for accurate dynamics simulations. Our approach is expected to be very useful in developing highly accurate coupled PESs in a quasi-diabatic representation in an efficient machine learning-based way.

Origin of the "odd" behavior in the ultraviolet photochemistry of ozone

The origin of the even-odd rotational state population alternation in the ¹⁶O₂(a ¹Δ_g) fragments resulting from the ultraviolet (UV) photodissociation of ¹⁶O₃, a phenomenon first observed over 30 years ago, has been elucidated using full quantum theory. The calculated ¹⁶O₂(a ¹Δ_g) rotational state distribution following the 266-nm photolysis of 60 K ozone shows a strong even-odd propensity, in excellent agreement with the new experimental rotational state distribution measured under the same conditions. Theory indicates that the even rotational states are significantly more populated than the adjacent odd rotational states because of a preference for the formation of the A' Λ-doublet, which can only occupy even rotational states due to the exchange symmetry of the two bosonic ¹⁶O nuclei, and thus not as a result of parity-selective curve crossing as previously proposed. For nonrotating ozone, its dissociation on the excited B¹A' state dictates that only A' Λ-doublets are populated, due to symmetry conservation. This selection rule is relaxed for rotating parent molecules, but a preference still persists for A' Λ-doublets. The A''/A' ratio increases with increasing ozone rotational quantum number, and thus with increasing temperature, explaining the previously observed temperature dependence of the even-odd population alternation. In light of these results, it is concluded that the previously proposed parity-selective curve-crossing mechanism cannot be a source of heavy isotopic enrichment in the atmosphere.

Non-adiabatic Excited-State Molecular Dynamics: Theory and Applications for Modeling Photophysics in Extended Molecular Materials

Optically active molecular materials, such as organic conjugated polymers and biological systems, are characterized by strong coupling between electronic and vibrational degrees of freedom. Typically, simulations must go beyond the Born-Oppenheimer approximation to account for non-adiabatic coupling between excited states. Indeed, non-adiabatic dynamics is commonly associated with exciton dynamics and photophysics involving charge and energy transfer, as well as exciton dissociation and charge recombination. Understanding the photoinduced dynamics in such materials is vital to providing an accurate description of exciton formation, evolution, and decay. This interdisciplinary field has matured significantly over the past decades. Formulation of new theoretical frameworks, development of more efficient and accurate computational algorithms, and evolution of high-performance computer hardware has extended these simulations to very large molecular systems with hundreds of atoms, including numerous studies of organic semiconductors and biomolecules. In this Review, we will describe recent theoretical advances including treatment of electronic decoherence in surface-hopping methods, the role of solvent effects, trivial unavoided crossings, analysis of data based on transition densities, and efficient computational implementations of these numerical methods. We also emphasize newly developed semiclassical approaches, based on the Gaussian approximation, which retain phase and width information to account for significant decoherence and interference effects while maintaining the high efficiency of surface-hopping approaches. The above developments have been employed to successfully describe photophysics in a variety of molecular materials.

Photodissociation dynamics of H2O and D2O via the D[combining tilde](1A1) electronic state

Photodissociation dynamics of H₂O and D₂O via the D[combining tilde] state by one-photon excitation have been investigated using the H/D atom Rydberg tagging time-of-flight technique. The TOF spectra of the H/D-atom product in both parallel and perpendicular polarizations have been measured. Product translational energy distributions and angular distributions have been derived from TOF spectra. By simulating these distributions, quantum state distributions of the OH/OD product as well as the state-resolved angular anisotropy parameters were determined. The most important pathway of H₂O/D₂O dissociation via the D[combining tilde] state leads to highly rotationally excited OH/OD(X, v = 0) products, while vibrationally excited OH/OD products with v≥ 1 comprise only one third of the total OH/OD(X) population. The branching ratios of OH(A)/OH(X) and OD(A)/OD(X) have also been determined, 1.0/3.0 for H₂O at 122.12 nm and 1.0/2.2 for D₂O at 121.95 nm, which are reasonably consistent with the values predicted by the previous theory.

Striking Isotopologue-Dependent Photodissociation Dynamics of Water Molecules: The Signature of an Accidental Resonance

Investigations of the photofragmentation patterns of both light and heavy water at the state-to-state level are a prerequisite for any thorough understanding of chemical processing and isotope heterogeneity in the interstellar medium. Here we reveal dynamical features of the dissociation of water molecules following excitation to the C̃(010) state using a tunable vacuum ultraviolet source in combination with the high-resolution H(D)-atom Rydberg tagging time-of-flight technique. The action spectra for forming H(D) atoms and the OH(OD) product state distributions resulting from excitation to the C̃(010) states of H₂O and D₂O both show striking differences, which are attributable to the effects of an isotopologue-specific accidental resonance. Such accidental-resonance-induced state mixing may contribute to the D/H isotope heterogeneity in the solar system. The present study provides an excellent example of competitive state-to-state nonadiabatic decay pathways involving at least five electronic states.

Hydroxyl super rotors from vacuum ultraviolet photodissociation of water

Hydroxyl radicals (OH) play a central role in the interstellar medium. Here, we observe highly rotationally excited OH radicals with energies above the bond dissociation energy, termed OH "super rotors", from the vacuum ultraviolet photodissociation of water. The most highly excited OH(X) super rotors identified at 115.2 nm photolysis have an internal energy of 4.86 eV. A striking enhancement in the yield of vibrationally-excited OH super rotors is detected when exciting the bending vibration of the water molecule. Theoretical analysis shows that bending excitation enhances the probability of non-adiabatic coupling between the [Formula: see text] and [Formula: see text] states of water at collinear O-H-H geometries following fast internal conversion from the initially excited [Formula: see text] state. The present study illustrates a route to produce extremely rotationally excited OH(X) radicals from vacuum ultraviolet water photolysis, which may be related to the production of the highly rotationally excited OH(X) radicals observed in the interstellar medium.

Two-state diabatic potential energy surfaces of ClH2 based on nonadiabatic couplings with neural networks

A neural network-fitting procedure based on nonadiabatic couplings is proposed to generate two-state diabatic PESs with conical intersections.

Dynamical interference in the vibronic bond breaking reaction of HCO

First-principles treatments of quantum molecular reaction dynamics have reached the level of quantitative accuracy even in cases with strong non-Born-Oppenheimer effects. This achievement permits the interpretation of puzzling experimental phenomena related to dynamics governed by multiple coupled potential energy surfaces. We present a combined experimental and theoretical study of the photodissociation of formyl radical (HCO). Oscillations observed in the distribution of product states are found to arise from the interference of matter waves-a manifestation analogous to Young's double-slit experiment.

Tunable VUV photochemistry using vacuum ultraviolet free electron laser combined with H-atom Rydberg tagging time-of-flight spectroscopy

In this article, we describe an experimental setup for studying tunable vacuum ultraviolet photochemistry using the H-atom Rydberg tagging time-of-flight technique. In this apparatus, two vacuum ultraviolet laser beams were used: one is generated by using a nonlinear four-wave mixing scheme in a Kr gas cell and fixed at 121.6 nm wavelength to probe the H-atom product through the Lyman α transition and the other beam, produced by a seeded free electron laser facility, can be continuously tunable for photodissociating molecules in the wavelength range of 50-150 nm with extremely high brightness. Preliminary results on the H₂O photodissociation in the 4d (000) Rydberg state are reported here. These results suggest that the experimental setup is a powerful tool for investigating photodissociation dynamics in the vacuum ultraviolet region for molecules involving H-atom elimination processes.

First-principles C band absorption spectra of SO2 and its isotopologues

The low-energy wing of the C∼B21←X∼¹A₁ absorption spectra for SO₂ in the ultraviolet region is computed for the 32S,³³S,³⁴S and ³⁶S isotopes, using the recently developed ab initio potential energy surfaces (PESs) of the two electronic states and the corresponding transition dipole surface. The state-resolved absorption spectra from various ro-vibrational states of SO₂(X∼¹A₁) are computed. When contributions of these excited ro-vibrational states are included, the thermally averaged spectra are broadened but maintain their key characters. Excellent agreement with experimental absorption spectra is found, validating the accuracy of the PESs. The isotope shifts of the absorption peaks are found to increase linearly with energy, in good agreement with experiment.

Laser-induced dissociation dynamics of triatomic molecule in electronic excited states: A full-dimensional quantum mechanics study

We present a detailed theoretical approach to investigate the laser-induced dissociation dynamics of a triatomic molecule on its electronic excited state in full dimensional case. In this method, the time evolution of the time-dependent system is propagated via combined the split operator method and the expansion of Chebyshev polynomials (or short-time Chebyshev propagation) and the system wave functions are expanded in terms of molecular rotational bases. As an example of the application of this formalism, the dissociation dynamics of H3(+)→H2(+)+H induced by ultrashort UV laser pulses are investigated on new Born-Oppenheimer potential energy surfaces. Our numerical results show that the signals of dissociation products will be easier to observe as the increasing of field strength. Driving by a 266 nm laser beam, the calculated central value of kinetic-energy-release is 2.04 eV which shows excellent agreement with the experimental estimation of 2.1 eV. When the H3(+) ion is rotationally excited, the spatial distribution of product fragments will become well converged.

Photodissociation dynamics of HOD via the B̃ ((1)A1) electronic state

Photodissociation dynamics of HOD from the B̃ state has been studied using H/D atom Rydberg "tagging" time-of-flight technique. Both the OD + H and OH + D channels have been investigated. Product kinetic energy distributions, internal state distributions of the OD/OH product, as well as the OD/OH quantum state specific angular anisotropy parameters have been determined. Overall, the photodissociation dynamics of HOD via the B̃ state is qualitatively similar to that of the H2O and D2O, with quantitative differences arising probably from the change in masses. At different photolysis energies, similar rovibrational distributions and state-resolved angular distributions have been observed for the OH/OD(X) product, while remarkable differences have been observed in the rovibrational distributions and state-resolved angular distributions of the OH/OD(A) product.

Accurate nonadiabatic dynamics

This Perspective addresses the use of coupled diabatic potential energy surfaces (PESs) together with rigorous quantum dynamics in full or reduced dimensional coordinate spaces to obtain accurate solutions to problems in nonadiabatic dynamics.

Semiclassical Wigner theory of photodissociation in three dimensions: Shedding light on its basis

The semiclassical Wigner theory (SCWT) of photodissociation dynamics, initially proposed by Brown and Heller [J. Chem. Phys. 75, 186 (1981)] in order to describe state distributions in the products of direct collinear photodissociations, was recently extended to realistic three-dimensional triatomic processes of the same type [Arbelo-González et al., Phys. Chem. Chem. Phys. 15, 9994 (2013)]. The resulting approach, which takes into account rotational motions in addition to vibrational and translational ones, was applied to a triatomic-like model of methyl iodide photodissociation and its predictions were found to be in nearly quantitative agreement with rigorous quantum results, but at a much lower computational cost, making thereby SCWT a potential tool for the study of polyatomic reaction dynamics. Here, we analyse the main reasons for this agreement by means of an elementary model of fragmentation explicitly dealing with the rotational motion only. We show that our formulation of SCWT makes it a semiclassical approximation to an approximate planar quantum treatment of the dynamics, both of sufficient quality for the whole treatment to be satisfying.

Laser-driven isomerization of HCN → HNC: the importance of rotational excitation

We report a time-dependent quantum wave packet theory, which is employed to interpret the isomerization dynamics of HCN molecules induced by an intense picosecond infrared laser field. Considering the molecular rotational degrees of the freedom, the wave functions are expanded in terms of molecular rotational bases. Our full-dimensional quantum model includes the full Coriolis coupling in the molecular kinetic energy Hamiltonian and dipole approximation in interaction terms. The numerical results show that the field-induced molecule rotational excitation plays an important role in the isomerization dynamical process. Some phenomena appear such as two-step two-photon absorption and highly oscillatory structure in rotational state distributions. The centrifugal sudden (CS) approximation calculation is also carried out and compared in this work; it is shown that the Coriolis couplings may lead to a significant decrease in the isomerization rate but highly enhanced molecular rotational excitation.

Product fine-structure resolved photodissociation dynamics: the A band of H2O

The photodissociation dynamics of H2O in its first absorption band is investigated on an accurate potential energy surface based on a large number of high-level ab initio points. Several ro-vibrational states of the parent molecule are considered. Different from most previous theoretical studies, the spin-orbit and Λ-doublet populations of the open-shell OH fragment are reported from full-dimensional wave packet calculations. The populations of the two spin-orbit manifolds are in most cases close to the statistical limit, but the Λ-doublet is dominated by the A(") component, thanks largely to the fast in-plane dissociation of H2O(Ã(1)A('')). Comparisons with experimental data and a Franck-Condon model are generally very good, although some discrepancies exist.

New insights into the semiclassical Wigner treatment of photodissociation dynamics

The semiclassical Wigner treatment of Brown and Heller [J. Chem. Phys. 1981, 75, 186] is applied to direct triatomic (or triatomic-like polyatomic) photodissociations with the aim of accurately predicting final state distributions at relatively low computational cost, and having available a powerful interpretative tool. For the first time, the treatment takes rotational motions into account. The proposed formulation closely parallels the quantum description as far as possible. An approximate version is proposed, which is still accurate while numerically much more efficient. In addition to being weighted by usual vibrational Wigner distributions, final phase space states appear to be weighted by new rotational Wigner distributions. These densities have remarkable structures clearly showing that classical trajectories most contributing to rotational state j are those reaching the products with a rotational angular momentum close to [j(j + 1)](1/2) (in ℏ units). The previous methods involve running trajectories from the reagent molecule onto the products. The alternative backward approach [L. Bonnet, J. Chem. Phys., 2010, 133, 174108], in which trajectories are run in the reverse direction, is shown to strongly improve the numerical efficiency of the most rigorous method in addition to being state-selective, and thus, ideally suited to the description of state-correlated distributions measured in velocity imaging experiments. The results obtained by means of the previous methods are compared with rigorous quantum results in the case of Guo's triatomic-like model of methyl iodide photodissociation [J. Chem. Phys., 1992, 96, 6629] and close agreement is found. In comparison, the standard method of Goursaud et al. [J. Chem. Phys., 1976, 65, 5453] is only semi-quantitative.