Full-Dimensional Quantum Dynamics of Vibrational Mediated Photodissociation of HOD in Its B Band

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.

The "Hole" Story in Ionized Water from the Perspective of Ehrenfest Dynamics

The radiolysis of liquid water and the radiation-matter interactions that happen in aqueous environments are important to the fields of chemistry, materials, and environmental sciences, as well as the biological and physiological response to extreme conditions and medical treatments. The initial stage of radiolysis is the ultrafast response, or hole dynamics, that triggers chemical processes within complex energetic landscapes that may include reactivity. A fundamental understanding necessitates the use of theoretical methods that are capable of simulating both ultrafast coherence and non-adiabatic energy transfer pathways. In this work, we carry out an ab initio Ehrenfest dynamics study to provide a more complete description of the ultrafast dynamics and reactive events initiated by photoionization of water. After sudden ionization, a range of processes, including hole trapping and transfer, large OH oscillations, proton transfer and subsequent relay, formation of the metastable Zundel complex, and long-lived coherence, are identified and new insight into their driving forces is elucidated.

The Role of Excited-State Proton Relays in the Photochemical Dynamics of Water Nanodroplets

In this work, we applied nonadiabatic excited-state molecular dynamics in tandem with ab initio electronic structure theory to illustrate a complete mechanistic landscape underpinning the ultraviolet absorption-initiated photochemical dynamics in water nanodroplets. The goal is to understand the nonequilibrium excited-state molecular dynamics initiated by the relaxation of a solvated photoelectron and consequential photochemical processes. The lowest-lying excited state shows the proton dissociation for a single water molecule forming intermediate hydronium complexes through a proton relay. At approximately 100 fs, the proton relay process gives rise to the relaxation of the excited state accompanied by a rapid increase in the nonadiabatic coupling strength with the ground state, and the nanodroplet nonradiatively decays. The nonadiabatic transition to the ground state produces excited vibrational states that facilitate the recombination of the dissociated proton and hydroxyl group, eventually leading to the desorption of water molecules from the nanodroplet. Additionally, lifetimes of transient photochemical events are also resolved for the relaxation of a solvated electron, excited-state proton relay, and nonradiative transition.

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.