Full-Dimensional Quantum State-to-State Nonadiabatic Dynamics for Photodissociation of Ammonia in its A-Band

Abstract

Full-dimensional state-to-state quantum dynamics of the photodissociation of NH 3 (Ã1A 2 ″) is investigated on newly developed coupled diabatic potential energy surfaces. For the first time, the rovibrational distributions of the nonadiabatically produced NH 2 (X̃2B 1 ) product have been determined quantum mechanically. In agreement with experimental observations, NH 2 (X̃2B 1 ) produced from the 0 0 and 2 1 states of NH 3 (Ã1A 2 ″) was found to be dominated by its ground vibrational state with an N = K a propensity, shedding light on the quantum-stateresolved nonadiabatic dynamics facilitated by conical intersections and setting the stage for the elucidation of vibrationally mediated photodissociation.SECTION: Spectroscopy, Photochemistry, and Excited States T he breakdown of the Born−Oppenheimer approximation in molecular systems is responsible for many important chemical processes, including vision and photosynthesis. Conical intersections (CIs) are the primary cause for the failure of the Born−Oppenheimer approximation. 1−3 Advances in our understanding of nonadiabatic dynamics have relied heavily on prototypical systems where the electron−nuclear interactions can be accurately characterized. Photodissociation of small molecules provides such an ideal proving ground. 4,5 Indeed, we have recently demonstrated that state-to-state multichannel quantum nonadiabatic dynamics of the photodissociation of water in its second absorption band can be quantitatively understood by wave-packet dynamics on accurate coupled potential energy surfaces (PESs). 6−8The photodissociation of ammonia in its A-band, which has been the object of innumerable experimental and theoretical studies over the past 30 years, provides another archetypical example of nonadiabatic dynamics. 4 Its six internal degrees of freedom offer dynamical features richer than those in the triatomic water molecule. In addition to the adiabatic dissociation channel leading to the H + NH 2 (Ã2A 1 ) products, nonadiabatic dissociation of photoexcited NH 3 (Ã1A 2 ″) through a seam of CIs between the ground and first excited electronic states of NH 3 leads to the lower energy H + NH 2 (X̃2B 1 ) products. As a result, it serves as an attractive system to understand, at the state-to-state level, multidimensional nonadiabatic dynamics and an array of other important dynamical properties including tunneling, mode specificity, and intramolecular vibrational energy redistribution (IVR).It is well established that the A←X̃absorption spectrum of ammonia is dominated by a long progression corresponding to umbrella mode (2 n ) excitations, which are attributed to the pyramidal-to-planar transition. 9 The diffuse characteristic of the absorption peaks reflects strong predissociation on the excited electronic state due to a small barrier just outside the quasibound Franck−Condon (FC) region. Lifetimes of these

The ortho-to-para ratio of interstellar NH2: quasi-classical trajectory calculations and new simulations

“…The potential energy surface (PES) used in the QCT calculation is from that devised by Zhu and Yarkony (ZY) and their coworkers which is a 2x2 quasi-diabatic permutation invariant potential matrix designed for studying the photodissociation of ammonia (NH 3 ) in its first absorption band. Quantum dynamical studies on these PES have achieved excellent agreement with experimental data, validating the PES Xie et al 2014). Only the lower adiabat, which corresponds to the ground electronic state of NH 3 , was used in our calculations reported here.…”

Calculation of state-to-state differential and integral cross sections for atom-diatom reactions with transition-state wave packets

“…These characteristics make this approach very appealing for tetra-atomic reactions, where full-dimensional state-to-state calculations are still too expensive. [51][52][53] On the other hand, single arrangement channel quantum dynamical calculations are routinely performed in four atom systems [54][55][56] and the TSWP approach has thus important implications in state-to-state calculations of such reactions. As advocated by Manthe and co-workers, 19, 37 this approach can be coupled with the MCTDH propagator, making it an even more powerful method for state-to-state studies of larger molecular systems.…”

Beyond Kohn–Sham Approximation: Hybrid Multistate Wave Function and Density Functional Theory

“…39,40 We employed 12 CSFs for the ground and excited states (Figure 2). Specifically, each σ NH bond is described by a spin-adapted covalent configuration $false|{\sigma }_{\text{cov}}^{\text{NHa}}>=N_{\text{cov}}^{\mathrm{a}}false(\hat{A}false\{{\phi }_c^6{n}^2\phi {\overline{s}}_{H}false\}+\hat{A}false\{{\phi }_c^6{n}^2\overline{\phi }{s}_{H}false\}false)$ along with two ionic states, and the n σ * state is represented by $false|{\sigma }_{\text{ex}}^{\text{NHa}}>=N_{\text{ex}}^{\mathrm{a}}false(\hat{A}false\{{\phi }_c^8\overline{n}{s}_{H}false\}+\hat{A}false\{{\phi }_c^{8}n{\overline{s}}_{H}false\}false)$.…”