Collisionless decay, vibrational relaxation, and intermediate case quenching of S/sub 1/ formaldehyde
The decay of fluorescence from the 4/sup 0/ and 4/sup 1/ levels of the S/sub 1/(A /sup 1/A/sub 2/) state of H/sub 2/CO and D/sub 2/CO has been monitored as a function of pressure after selective, pulsed laser excitation. For D/sub 2/CO, single exponential decays modified by 4/sup 0/bold-arrow-left-right4/sup 1/ energy transfer were observed over the entire pressure range 4 x 10/sup -5/--4 Torr. The zero pressure lifetimes tau/sub 0/(4/sup 0/) =7.8 +- 0.7 ..mu..s and tau/sub 0/(4/sup 1/) =6.0 +- 0.4 ..mu..s are probably the radiative lifetimes. The rate of 4/sup 1/..-->..4/sup 0/ energy transfer in D/sub 2/CO was found to be (9.6 +- 0.4) x 10/sup -10/ cm/sup 3/ molecule/sup -1/ s/sup -1/, about three times the gas kinetic rate. For H/sub 2/CO at pressures above 0.1 Torr, fluorescence decays were also single exponentials modified by 4/sup 0/ bold-arrow-left-right4/sup 1/ energy transfer. However, in the range 2 x 10/sup -4/--0.1 Torr, the decays of the individual 4/sup 0/ and 4/sup 1/ vibronic levels were typically biexponential. The zero pressure decay occurs on a timescale at least 20 times faster than the radiative lifetime of approx.5 ..mu..s. The Stern-Volmer plots of tau/sup -1/ vs pressure give quenching rates between 2.2 x 10/sup -9/ and 6.5 x 10/sup -9/ cm/sup 3/ molecule/sup -1/ s/sup -1/ for both fast and slow components below approx.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 x 10/sup -11/ cm/sup 3/ molecule/sup -1/ s/sup -1/ above 1 Torr. The low pressure quenching rates and zero pressure lifetimes for H/sub 2/CO depend significantly on the K' rotational quantum number within 4/sup 0/. The biexponential decays for H/sub 2/CO 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.
- Research Organization:
- Department of Chemistry, University of California, and Materials and Molecular Research Division of the Lawrence Berkeley Laboratory, Berkeley, California 94720
- OSTI ID:
- 6537290
- Journal Information:
- J. Chem. Phys.; (United States), Vol. 69:11
- Country of Publication:
- United States
- Language:
- English
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