Vibrationally excited formaldehyde. The relationship between vibrational structure and collisional properties

Abstract

Collisional energy transfer has been investigated in highly vibrationally excited H₂CO(X¹A₁) at E_vib≈ 11 300 cm^–1 using the method of stimulated emission pumping-transient absorption spectroscopy (SEP–TAS). The experiments yielded total depopulation rate constants as well as state-to-state rate constants of several specific channels for selected rotational levels belonging to the 2₄4₄ vibrational state. Total depopulation rate constants at room temperature for M = H₂CO and He were found to be near k(H₂CO)≈ 3× 10^–9 cm³ molecule^–1 s^–1 and k(He)≈ 3.7 × 10^–10 cm³ molecule^–1s^–1, for levels of the type 2₄4₄, J_KaK_c=J_1,J–1. The relaxation was found to proceed primarily via purely rotational transitions. An analysis of the state-to-state data with a master equation approach revealed the dominance of ΔJ=±1, ΔK_a= 0, and ΔK_c=ΔJ steps, which account for ≈50% of the total decay. The observed propensity rule can be rationalized in terms of a simple dipole–dipole interaction. The results indicate that on the timescale of the present study, at E_vib≈ 11 300 cm^–1, where ρ_vib≈ 0.42 per cm^–1, collisions conserve the vibrational character despite the fact that the energy spacing of two adjacent vibrational states is only a fraction of the transferred rotational energy. The possible role of Coriolis coupling in highly vibrationally excited molecules for enhancing vibrational energy transfer rates is discussed.