High-resolution infrared laser spectroscopy of supercooled hexafluorides of heavy elements

A tunable injection laser spectrometer was used to investigate the ν₃ band of sulfur, tungsten, and uranium hexafluorides. In order to eliminate hot bands superimposed on the fundamental ν₃ transitions, the gases were supercooled in a supersonic jet. The gas under study constituted 1% of a helium mixture. It was found that in a supersonic jet the rotational temperature of the molecules was reduced to 40 °K. The isotopic structure of the ν₃ band of supercooled WF₆ was resolved and four Q branches corresponding to the ¹⁸⁶W, ¹⁸⁴W, ¹⁸³W, and ¹⁸²W isotopes were observed. The isotope shift was 0.31 cm^–1/amu. For uranium hexafluoride, Q branches of the ν₃ band of uranium isotopes with atomic weights of 238 and 235 were obtained. The isotope shift was 0.650±0.005 cm^–1. The fine structure of the Q branch was resolved, the position of the maximum for U²³⁸F₆ being 627.680 cm^–1.