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The collision-induced deactivation of H2CO in a specific rotational level in its first electronically excited singlet state, A 1A2, v4=1, JKa,Kc=132,12, has been investigated with a series of colliders, M=He, Ar, Xe, and N2. The target level was populated via the transition A←X, 410, 132,12←133,11 using a pulsed dye laser. The subsequent relaxation was monitored using the technique of transient gain spectroscopy (TGS) via the transition A→X, 201301414, 132,12→131,13, which coincides with the line of an Ar+ laser at λ=488.0 nm. The experiments yielded the overall collisional depopulation rate constants (velocity averaged cross sections) for He, Ar, Xe, and N2, respectively: (5.25±0.20)×10-10 cm3/molecule s (39 Å2), (5.22±0.18)×10-10 cm3/molecule s (86 Å2), (5.37±0.29)×10-10 cm3/molecule s (106 Å2), (8.25±0.29)×10-10 cm3/molecule s (125 Å2), where the uncertainties correspond to the 2σ standard deviations. [ABSTRACT FROM AUTHOR]

Collisional energy transfer has been investigated in highly vibrationally excited H2CO (X 1A1) at Evib≅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 61,5 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 ΔKa=0, ΔKc=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 Evib≅11 400 cm-1, where ρvib≅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. [ABSTRACT FROM AUTHOR]