Relaxation in the 1A u state of glyoxal. I. Collision-free lifetimes of single vibrational levels

Abstract

Collision free lifetimes are reported for 26 single vibrational levels of the 1Au state of glyoxal (CHOCOH). A nitrogen laser-pumped tunable dye laser is used to excite primarily single vibronic levels of glyoxal in the pressure range 10−5–10−1 Torr. The resulting fluorescence can either be spectrally resolved to provide single vibronic level observation or the entire 1Au emission can be detected. The fluorescence is time-resolved with 100 nsec resolution, and two or more decades of decay are typically observed. A detailed discussion of the apparatus and techniques is presented. Evidence is presented for very rapid rotation and vibration collisional energy transfer, and implications of these effects on extrapolated zero-pressure lifetimes are discussed. By observing an entire vibronic band, effects of rapid rotational redistribution on observed loss rates can be minimized, and thus by varying the total glyoxal pressure, one can determine single vibronic level loss rate constants. Such data are presented for 26 vibronic levels, with cross sections which range from gas kinetic to seven times faster than gas kinetic. These collisional loss rate constants are dominated by vibrational relaxation; an observable but less important loss channel is collision induced intersystem crossing. Implications of these data are discussed.