Collisionless decay, vibrational relaxation, and intermediate case quenching of S1 formaldehyde

Publisher Website
Google Scholar
Abstract
The decay of fluorescence from the 40 and 41 levels of the S1(Ã 1A2) state of H2CO and D2CO has been monitored as a function of pressure after selective, pulsed laser excitation. For D2CO, single exponential decays modified by 40↔41 energy transfer were observed over the entire pressure range 4×10−5–4 Torr. The zero pressure lifetimes τ0(40) =7.8±0.7 μs and τ0(41) =6.0±0.4 μs are probably the radiative lifetimes. The rate of 41→40 energy transfer in D2CO was found to be (9.6±0.4) ×10−10 cm3 molecule−1 s−1, about three times the gas kinetic rate. For H2CO at pressures above 0.1 Torr, fluorescence decays were also single exponentials modified by 40 ↔41 energy transfer. However, in the range 2×10−4–0.1 Torr, the decays of the individual 40 and 41 vibronic levels were typically biexponential. The zero pressure decay occurs on a timescale at least 20 times faster than the radiative lifetime of ∼5 μs. The Stern‐Volmer plots of τ−1 vs pressure give quenching rates between 2.2×10−9 and 6.5×10−9 cm3 molecule−1 s−1 for both fast and slow components below ∼20 mTorr. The relative amplitude of the fast component decreases rapidly with pressure and approaches zero at 0.1 Torr. The slow component plots are dramatically curved and give quenching rates of only about 2.2×10−11 cm3 molecule−1 s−1 above 1 Torr. The low pressure quenching rates and zero pressure lifetimes for H2CO depend significantly on the K′ rotational quantum number within 40. The biexponential decays for H2CO may result from variations in lifetime among the J′ states excited by the laser. The large quenching rate constants and the curvature of the Stern–Volmer plots can be qualitatively understood in terms of recent mixed‐state models of collision‐induced radiationless decay.