Perturbation analysis of the ν = 6 level in the d ³Δ state of CS Based on Its Near-Infrared Absorption Spectrum

The spectrum of CS was recorded in the region of 12 086-12 630 cm ^-1 by employing optical heterodyne concentration modulation laser absorption spectroscopy. Nearly 350 transitions were assigned to the (6, 0) band in the d³Δ-a³π system of CS. The overtone transitions of the (12, 0) band in the a³π₂-a ³π₀ transition were first observed due to the perturbation interaction between d³Δ₁ and a ³π₂. The Λ doubling in the a³π ₁ state was also resolved at high rotational levels. The molecular constants of the a³π (ν = 0) and d³Δ (ν = 6) levels and the perturbation parameters of the d³Δ (ν = 6) level were determined through nonlinear least-squares fitting using effective Hamiltonians. The calculations of mixing fractions of the perturbed states were performed in order to obtain precise information on the perturbations of the d³Δ (ν = 6) levels. The mechanisms for perturbations of d³Δ (ν = 6) with the a³π (ν = 12) and A¹π (ν = 1) levels, especially for the second-order perturbation, were discussed and explained according to first-order nondegenerate perturbation theory.