Knowledge Bank

We have fully resolved the rotational fine structure of the HF dimer in the R(0) to P(1) null gap of the HF monomer. Nearly Doppler-limited linewidths were achieved in the spectrum recorded with a tunable difference-frequency laser by using a long pathlength (64m) at relatively low sample pressure (3.5 Torr) and temperature (234 k). The principal structure has the appearance of a K""=0 subband of a perpendicular (B-type) vibration with a large positive AB. A doubling of the subband is observed, indicative of torsional tunnelling, with a large splitting of the subband origins $(0.302cm-1)$. Each series exhibits a 10:6 intensity alternation with J, due to spin statistics of indistinguishable pairs of H and F atoms, implying a $C2$ symmetry axis for the time-averaged molecular structure. The K=0 ground state constants obtained from combination differences or a least-squares fit to the subband transitions are in essential agreement with these found previously in a microwave resonance study of a supersonic HF molecular been by $Dyke,HowardandKlemperer1$, whose dynamic molecular Structure model is consistent with our findings.