Rotational structure and vibrational predissociation in the HF stretching bands of the HF dimer

The rotational structure in the HF stretching bands of the HF dimer has been recorded with nearly Doppler-limited resolution using a tunable difference-frequency laser spectrometer and a long-path cell held at low temperatures and pressures. Two bands are observed; the higher frequency band, corresponding principally to the ‘‘free’’ hydrogen stretch, has the appearance of a B-type or perpendicular band with two prominent Q subbranches RQ0 and PQ1; the lower band is an A-type or parallel band arising primarily from the ‘‘bonded’’ hydrogen vibration. The K=0 subbands of both vibrations have been fully assigned and fit with polynomial expansions in J(J+1) to yield ground state constants in excellent agreement with a previous microwave resonance molecular beam study of the HF dimer. The subbands exhibit doubling and 10:6 intensity ratios for alternate J indicative of the internal rotation tunneling motion proposed in the microwave study. A pressure-independent broadening of the lines to about twice the Doppler width, attributed to vibrational predissociation, is observed for the bonded hydrogen stretching band.