Rotational and fine structure of open-shell molecules in nearly degenerate electronic states

TL;DR

This paper develops a comprehensive Hamiltonian model to analyze the rotational and fine structure of nearly degenerate electronic states in open-shell molecules, incorporating various interactions and providing tools for precise molecular constant determination.

Contribution

It introduces a generalized Hamiltonian without symmetry restrictions and derives intensity formulas and selection rules for complex electronic state interactions.

Findings

The model accurately simulates fine structures using SR, SO, and Coriolis constants.

Simultaneous simulation of interacting levels yields precise molecular constants.

Analysis offers insights into vibronic interactions and molecular dynamics.

Abstract

An effective Hamiltonian without symmetry restriction has been developed to model the rotational and fine structure of two nearly degenerate electronic states of an open-shell molecule. In addition to the rotational Hamiltonian for an asymmetric top, this spectroscopic model includes energy separation between the two states due to difference potential and zero-point energy difference, as well as the spin-orbit (SO), Coriolis, and electron spin-molecular rotation (SR) interactions. Hamiltonian matrices are computed using orbitally and fully symmetrized case (a) and case (b) basis sets. Intensity formulae and selection rules for rotational transitions between a pair of nearly degenerate states and a nondegenerate state have also been derived using all four basis sets. It is demonstrated using real examples of free radicals that the fine structure of a single electronic state can be simulated with either a SR tensor or a combination of SO and Coriolis constants. The related molecular constants can be determined precisely only when all interacting levels are simulated simultaneously. The present study suggests that analysis of rotational and fine structure can provide quantitative insights into vibronic interactions and related effects.