Measurement and analysis of the infrared-active stretching fundamental (ν3) of UF6

High-resolution spectra of the infrared-active stretching fundamental ν3 of 238UF6 have been obtained between 620.6 and 633.5 cm-1 using tunable semiconductor diode lasers. Interference from hot bands was suppressed by cooling the UF6 in a supersonic expansion, and useful monomer concentrations were produced with effective temperatures of <100 K. Portions of the band from P(77) to R(66) are illustrated. All transitions from the vibrational ground state have been assigned, and the Q branch has been fully analyzed. A total of 43 line frequencies and 110 frequency differences extending in J to P(77), Q(91), and R(67) has been used to fit seven spectroscopic constants. The ground- and excited-state values of the rotational constant B could be individually determined, and the U--F bond length in the ground vibrational state is r0 = 1.9962 +- 0.0007 A. The Q branch of 235UF6 has also been analyzed and the 235UF6--238UF6 ν3 isotope shift measured to be 0.603 79 +- 0.000 17 cm-1. The isotope shift and the Coriolis constant zeta3 have been used to refine the general quadratic intramolecular force field of UF6, and the Cartesian displacement coordinates of both infrared-active fundamentals are illustrated and compared with those of SF6