Collisional energy transfer in highly vibrationally excited H2CO(X̃ 1A1)

Abstract

Collisional energy transfer has been investigated in highly vibrationally excited H₂CO (X̃ ¹A₁) at E_vib≅11 400 cm⁻¹ using the method of stimulated emission pumping–transient absorption spectroscopy (SEP‐TAS). Total depopulation and state‐to‐state rate constants were measured for several rotational levels of the 2₄4₄ vibrational state. For H₂CO self‐relaxation the depopulation rate constant of the 6_1,5 level was measured to be k₀=(3.12±0.13)×10⁻⁹ cm³/molecule s. An analysis of the state‐to‐state data with a simplified master equation approach yielded rate constants k₁=(7.7±1.2)×10⁻¹⁰, k₂=(1.2±0.3)×10⁻¹⁰, and k₃=(0.6±0.3)×10⁻¹⁰ cm³/molecule s for collisions with ΔJ=1, 2, and 3, respectively, and ΔK_a=0, ΔK_c=ΔJ, the reverse rate constants being given by microscopic reversibility. Thus, the ΔJ=±1 steps account for ∼50% of all inelastic collisions, which can be rationalized in terms of a simple dipole–dipole interaction. At E_vib≅11 400 cm⁻¹, where ρ_vib≅0.42/cm⁻¹, collisions seem to conserve the vibrational character in spite of the fact that the energy gap between adjacent vibrational states is a fraction of the transferred rotational energy.