Communication: State-to-state differential cross sections for H2O(B̃) photodissociation

Photodissociation dynamics of H2S+ via the A2A1 (0, 8, 0) state

The photodissociation dynamics of the hydrogen sulfide cation (H2S+) (X2B1) were investigated using the time-sliced velocity map ion imaging technique. S+ (4Su) product images were measured at four photolysis wavelengths around 393.70 nm, corresponding to the excitation of the H2S+ (X2B1) cation to the A2A1 (0, 8, 0) state. The raw images and the derived total kinetic energy releases (TKERs) spectra exhibited partial rotational resolution for the H2 products. A sensitive dependence on the photolysis wavelength was observed in the TKER spectra and anisotropy parameters. Within a narrow excitation energy range of ∼12 cm-1, the H2 products showed two distinct rotational excitations. Furthermore, clear differences in anisotropy parameters were observed. These phenomena indicate that the rotational excitation of the H2S+ ions plays a role in the non-adiabatic photodissociation dynamics.

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.

Accurate Full-Dimensional Global Diabatic Potential Energy Matrix for the Two Lowest-Lying Electronic States of the H + O2 ↔ HO + O Reaction

A new and more accurate diabatic potential energy matrix (DPEM) is developed for the two lowest-lying electronic states of HO₂, covering both the strong interaction region and reaction asymptotes. The ab initio calculations were performed at the Davidson corrected multireference configuration interaction level with the augmented correlation-consistent polarized valence quintuple-zeta basis set (MRCI+Q/AV5Z). The accuracy of the electronic structure calculations is validated by excellent agreement with the experimental HO₂ equilibrium geometry, fundamental vibrational frequencies, and H + O₂ ↔ OH + O reaction energy. Through the combination of an electronic angular momentum-method and a configuration interaction vector-based method, the mixing angle between the first two ²A″ states of HO₂ was successfully determined. Elements of the 2×2 DPEM were fit to neural networks with a proper account of the complete nuclear permutation inversion symmetry of HO₂. The DPEM correctly predicted the properties of conical intersection seams at linear and T-shape geometries, thus providing a reliable platform for studying both the spectroscopy of HO₂ and the nonadiabatic dynamics for the H + O₂ ↔ OH + O reaction.

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.

High-fidelity first principles nonadiabaticity: diabatization, analytic representation of global diabatic potential energy matrices, and quantum dynamics

Nonadiabatic dynamics, which goes beyond the Born-Oppenheimer approximation, has increasingly been shown to play an important role in chemical processes, particularly those involving electronically excited states. Understanding multistate dynamics requires rigorous quantum characterization of both electronic and nuclear motion. However, such first principles treatments of multi-dimensional systems have so far been rather limited due to the lack of accurate coupled potential energy surfaces and difficulties associated with quantum dynamics. In this Perspective, we review recent advances in developing high-fidelity analytical diabatic potential energy matrices for quantum dynamical investigations of polyatomic uni- and bi-molecular nonadiabatic processes, by machine learning of high-level ab initio data. Special attention is paid to methods of diabatization, high fidelity construction of multi-state coupled potential energy surfaces and property surfaces, as well as quantum mechanical characterization of nonadiabatic nuclear dynamics. To illustrate the tremendous progress made by these new developments, several examples are discussed, in which direct comparison with quantum state resolved measurements led to either confirmation of the observation or sometimes reinterpretation of the experimental data. The insights gained in these prototypical systems greatly advance our understanding of nonadiabatic dynamics in chemical systems.

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.

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.

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.

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

State-to-state photodissociation dynamics of H(2)O in its B band has been investigated quantum mechanically on a new set of non-adiabatically coupled potential energy surfaces for the lowest two (1)A' states of H(2)O, which are developed at the internally contracted multi-reference configuration interaction level with the aug-cc-pVQZ basis set. Quantum dynamical calculations carried out using the Chebyshev propagator yield absorption spectra, product state distributions, branching ratios, and differential cross sections, which are in reasonably good agreement with the latest experimental results. Particular focus is placed here on the dependence of various dynamical observables on the photon energy. Detailed analyses of the dynamics have assigned the diffuse structure in absorption spectrum to short-time recurring dynamics near the HOH conical intersection. The non-adiabatic dissociation to the ground state OH product via the HOH conical intersection is facile, direct, fast, and produces rotationally hot OH(X̃) products. On the other hand, the adiabatic channel on the excited state leading to the OH(Ã) product is dominated by long-lived resonances, which depend sensitively on the potential energy surfaces.