High-resolution spectroscopy of the $ν3$ band of $WF6$ and $ReF6$ in a supersonic jet

We have recorded the Fourier-transform infrared (FTIR) spectrum of the $ν3$ fundamental band of $WF6$ in a continuous supersonic jet expansion with an instrumental bandwidth of 0.0024 cm⁻¹ (FWHM, full width at half maximum, unapodized), using a Bomem DA.002 spectrometer. Some parts of this band have also been recorded with 0.0007 cm⁻¹ bandwidth using a diode laser spectrometer combined with a pulsed slit jet expansion. A multiple-pass arrangement has been used for the slit jet to observe low-intensity lines. In each case, we have used a $WF6:He$ mixture with a seeding ratio 1:3 leading to a rotational temperature of ca. 50 K. This work extends the previous investigation of Takami and Kuze [J. Chem. Phys. 80, 5994 (1984)] to much higher J transitions. In both P and R branches, rotational lines have been recorded for J up to 46–48. We have used a tensorial Hamiltonian adapted to the group chain $O(3)⊃Oh$ and developed to the third order for the analysis of the spectra. A least-squares fit for each of the four main isotopic species: ¹⁸²$WF6,$ ¹⁸³$WF6,$ ¹⁸⁴$WF6,$ and ¹⁸⁶$WF6$ results in band centers (in this order) 714.538 19, 714.214 06, 713.895 44, and 713.266 21 cm⁻¹. We report furthermore first results on the high-resolution spectra of $ν3$ of $ReF6,$ which exhibits a fourfold degenerate electronic ground state of $Gg′$ species in the $OhS$ group. Supersonic jet-FTIR spectra show a moderately structured relatively broad band, whereas the diode laser spectroscopy of the seeded jet in the range 708–733 cm⁻¹ results in line resolved spectra of high complexity. A preliminary analysis is discussed, while a complete analysis still represents an appreciable challenge.