Multiple-photon absorption of a CO 2-pumped CF 4 laser radiation in UF 6

Abstract

Pressure, fluence and intensity dependent measurements of the multiple-photon absorption (MPA) of 16-μm CF 4 laser pulses in UF 6 are reported. Pumping of the CF 4 gas by single longitudinal mode pulses from an injection controlled TEA-CO 2 laser yielded up to 40 mJ pulses of 16-μm laser radiation which were used in the present study. The pressure dependent results at constant fluence (〈φ〉), show that at UF 6 pressures ( P) lower than 0.5 Torr the average number of photons absorbed per molecule (〈 n〉) increases with P and that at higher pressures 〈 n〉 levels off. This indicates that, similar to what is found in SF 6, rotational hole filling takes place in UF 6 at low pressures. The fluence dependent results show that at a given P and for 〈φ〉 smaller than about 5 mJ/cm 2, 〈 n〉 increases linearly with 〈φ〉, indicating single-photon absorption, and that at higher values of fluence 〈 n〉 is proportional to 〈φ〉 1.6, which is typical of MPA processes. The use of laser pulses of two different durations (300 ns and 600 ns at FWHM) for the excitation shows that the results of 〈 n〉 versus P and 〈φ〉 are independent of the length of these pulses.