Laser optogalvanic and photoacoustic studies of some atoms and small molecules

A device designed to perform laser optogalvanic (LOG) measurements on low pressure gases in an electrical discharge has been constructed. The device employs the unique combination of pulsed laser radiation in the visible range, and transverse excitation of a radiofrequency (rf) discharge to obtain high quality spectroscopic data on highly excited atomic and molecular species existing in an electrical discharge. The results of LOG studies on xenon, cesium, and nitrogen are reported. In addition to functioning as a sensitive LOG detector, it has been discovered that the RFLOG device serves admirably as a contactless transducer of the photoacoustic signal. When the laser beam is focused into a region of the discharge cell some distance below the area of the discharge, excitation of ground state species gives rise to an acoustic wave through the photoacoustic effect. The acoustic wave causes a perturbation of the rf discharge which is detected by the RFLOG device. Results of a study on magnetic field induced level crossings in molecular iodine using photoacoustic detection with the RFLOG device are presented. Finally, pulsed excitation has made possible time resolved studies of the response of the rf discharge to laser excitation. In studies on molecular iodine, it has been observed that subsequent to each laser pulse, two distinct signals are obtained. A fast signal, corresponding to absorption by iodine atoms occurs immediately upon pulsed excitation. A slower signal corresponding to absorption by molecular iodine follows some 200μs later. The results are presented, and discussed in terms of the mechanism of the optogalvanic effect in atomic and molecular systems