Lifetime measurements of the collision-free slow fluorescence from glyoxalS1T1 gateway levels in a beam

Rotationally cold (40 K) glyoxal molecules were prepared by seeding in a pu lsed argon jet and were excited in the 0(0)(0) band of the S-1 <-- S-0 tran sition by a pulsed dye laser. Using an intracavity etalon, the resolution w as improved compared to previous work from this laboratory. A total of 23 l ong-lived S-1/T-1 gateway levels were identified by means of their slow (30 -80 mu s) fluorescence, including 10 found in the earlier study. For eight of them, time-resolved measurements of the fluorescence decay were made. In the range 40-120 mu s, they yielded well-defined, single exponentials, ind icating that each S-1 gateway level couples with only one T-1 level. Lifeti mes between tau = 23 and 83 mu s were measured. In some cases a lengthening of tau by up to 50% with increasing laser intensity was observed. II is as cribed to additional S-1/S-0 coupling within the power-broadened absorption line. Two gateway levels exhibited quantum beats at 76 and 64 kHz, respect ively. The mechanism producing such slow beats within the small-molecule, t wo-state S-1/T-1 coupling is not yet understood.