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 Evib≈ 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, JKaKc=J1,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, ΔKa= 0, and ΔKc=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 Evib≈ 11 300 cm–1, where ρvib≈ 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.