Cross section of supercooled238UF6 in multiphoton absorption induced by 16 micrometer Raman-laser radiation

Measurements of multiphoton absorption of 16 µm Raman-laser radiation in supercooled238UF6 at 90 K were performed by using a pulsed Laval nozzle with an optical path length of 50 cm. The laser fluence was varied between 50 and 500 mJ/cm2 for four frequencies in the range from 625 to 629 cm−1. The energy absorbed by238UF6 molecules was investigated as a function of laser frequency or fluence, and highly accurate results were obtained with the use of the nozzle whose optical path length is much greater than that of nozzles used before. The results indicated that the absorption cross section at the peak absorption frequency (627.8cm−1) was proportional to the −1/3 power of the fluence.