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.
Cross sections are reported for collisional quenching of low-lying levels of the 1Au state of glyoxal by 24 collision partners at 297 °K. The quenching cross sections varied from 1 to 22 Å2 with polar collision partners exhibiting the larger cross sections. The quenching rate constants were determined by measuring the time-resolved decay of laser-excited glyoxal, as a function of quench gas pressure. Studies of 3Au state production for five of these gases indicate that the principal collisional loss channel from the 1Au state is to the 3Au state. The derived quenching cross sections are well parameterized by several differing quenching models, not all of which should be valid in this case; such correlations have predictive value, but cannot be taken to verify the quenching model.
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