Pressure dependence of fluorescence quantum yields and collision-induced rotational relaxation of single rotational levels of H2CO(Ã 1A2, 41)

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Abstract
Pressure dependent values of SRL fluorescence quantum yields Φf(J′, K′) in the 1–120 mTorr range are reported for 75 rotational levels (E′rot = 37–1127 cm−1) of S1 H2CO (41). High lying rotational levels tend to show an increase in Φf with increasing pressure of S0 H2CO, whereas low lying rotational levels tend to show a decrease. For some levels, collision‐induced ΔJ′ transitions occur 5–10 times faster than the gas kinetic collision rate but probably with a much slower rate for ΔK′ transitions. A ΔK′‐constrained, collision‐induced rotational relaxation model involving mainly ΔJ′ = ±1 transitions is proposed to explain the observed pressure dependence of Φf(J′, K′).