Frequency measurements in the 10 μm spectrum of 11BCl3

This paper presents a study of the spectroscopy of ¹¹BCl₃ using the Doppler-free saturated absorption technique. Many lines in the degenerate ν₃ fundamental of this molecule have been recorded using the 10P branch of a carbon dioxide laser. The absolute frequencies of the linecentres of these absorptions have been measured to an accuracy of better than 10 kHz, using frequencies calibrated against transitions in SF₆ and OsO₄. Hyperfine structure due to the Cl nuclei was easily resolved, and more detailed structure, which may be attributed to the ¹¹B quadrupole interaction, was also seen. The splittings between individual quadrupole components were measured to an accuracy of better than 2 kHz where SF₆ references were used, and to about 8 kHz in the case of OsO₄. Peak-to-peak linewidths as low as 22 kHz have been recorded, representing the highest resolution data yet obtained for this molecule.