Radiative lifetimes and dipole moments of the A 2Σ+, B 2Σ+, and C 2Σ+ states of OH

Predissociation dynamics of the hydroxyl radical (OH) based on a five-state spectroscopic model

Multi-reference configuration interaction potential energy curves (PECs) and spin-orbit couplings for the X 2Π, A 2Σ+, 1 2Σ-, 1 4Σ-, and 1 4Π states of OH are computed and refined against empirical energy levels and transitions to produce a spectroscopic model. Predissociation lifetimes are determined by discretizing continuum states in the variational method nuclear motion calculation by restricting the calculation to a finite range of internuclear separations. Varying this range gives a series of avoided crossings between quasi-bound states associated with the A 2Σ+ and continuum states, from which predissociation lifetimes are extracted. 424 quasi-bound A 2Σ+ state rovibronic energy levels are analyzed, and 374 predissociation lifetimes are produced, offering good coverage of the predissociation region. Agreement with measured lifetimes is satisfactory, and a majority of computed results were within experimental uncertainty. A previously unreported A 2Σ+ state predissociation channel that goes via X 2Π is identified in the calculations. A Python package, binSLT, produced to calculate predissociation lifetimes, associated line broadening parameters, and lifetime uncertainties is made available. The PECs and other curves from this work will be used to produce a rovibronic ExoMol line list and temperature-dependent photodissociation cross sections for the hydroxyl radical.

Multireference space without first solving the configuration interaction problem

We further develop an idea to generate a compact multireference space without first solving the configuration interaction problem previously proposed for the ground state (GS) (Glushkov, Chem. Phys. Lett. 1995, 244, 1). In the present contribution, our attention is focused on low-lying excited states (ESs) with the same symmetry as the GS which can be adequately described in terms of an high-spin open-shell formalism. Two references Møller-Plesset (MP) like perturbation theory for ESs is developed. It is based on: (1) a main reference configuration constructed from the parent molecular orbitals adjusted to a given ES and (2) secondary double excitation configuration built on the GS like orbitals determined by the Hartree-Fock equations subject to some orthogonality constraints. It is shown how to modify the MP zeroth-order Hamiltonian so that the reference configurations and corresponding excitations are eigenfunctions of it and are compatible with orthogonality conditions for the GS and ES. Intruder states appearance is also discussed. The proposed scheme is applied to the GS, ES, and excitation energies of small molecules to illustrate and calibrate our calculations.

A THEORETICAL ANALYSIS OF THE DISSOCIATION OF OH RADICAL: FINE-STRUCTURE DISTRIBUTIONS OF THE O(3PJ) PRODUCT

A theoretical treatment is presented for the nonadiabatic photodissociation of hydroxyl radical. Five electronic states, X 2Π, A 2Σ+, 14Σ-, 12Σ- and 14Π, are included in this simulation. Based on the accurate ab initio calculations of the potential energy curves, transition dipole moments and nonadiabatic couplings between the relevant states, the dissociation dynamics are investigated using the time-dependent quantum wave packet approach. The direct dissociation, following the excitation from the specific vibrational levels of the ground state X 2Π to the repulsive state 12Σ- is examined, where the total and partial cross sections and the branching fractions of spin–orbit fine-structures of O (3 P J) are calculated over a wide range of the incident photon frequencies. For the predissociation via the bound excited state A 2Σ+, the influence of the nonadiabatic interactions in different internuclear regions and the role of the repulsive states in the dissociation starting at different vibrational levels are also analyzed through the spin–orbit branching fractions of products.