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

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Abstract
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 R Q 0 and P Q 1; 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 microwaveresonance 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.

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