Spectroscopic characterization of NH(A³Π): Linewidths and energy level shifts

The study of linewidths and energy level shifts in the NH (imidogen) molecule provides critical insights into its spectroscopic properties, quantum dynamics, and interactions with external environments. This study investigates the mechanisms responsible for the broadening and shifting observed in the rotational, vibrational, and electronic transitions of the NH(A³Π→ X³Σ^-) band. The analysis places particular emphasis on radiative lifetimes and the influence of spectral perturbations. To this end, high-level multireference configuration interaction (MRCI) calculations were carried out to construct accurate potential energy curves (PECs) for the X³Σ^-, A³Π, and 1⁵Σ^- states, and included newly developed analytical representations for the bound states. For the ground state, we obtained R_e = 1.9558 a₀, ω_e = 3282.6 cm^-1, and D_e = 29084 cm^-1, with the A³Π state lying approximately 29538 cm^-1 above. These values show good agreement with the available experimental data (R_e = 1.9581 a₀, ω_e = 3282.3 cm^-1, and D_e = 29044 cm^-1). Our main results show that these calculations provide a deeper understanding of the molecular energy structure and the dynamical mechanisms underlying linewidth variations and energy level displacements in light diatomic radicals. Using these results, it was possible to calculate the energy shifts as well as the variations in the lifetimes of the vibrational levels of the A³Π state. We calculated the lifetimes of the rotational levels for the first three vibrational energy levels, with results showing good agreement (better than 20%) with previously reported experimental data.