Observation and assignment of tunnelling-rotational transitions in the far infrared spectrum of (HF)2

Several ΔK = +1 subbands of the tunnelling-rotational spectrum of (HF)2 were observed for the first time between 20 and 170 cm-1 at a resolution of 0·012 – 0·025 cm-1 in a thermostated long path cell at 245 K and 2 kPa. Assisted by combination differences from the v 1 fundamental, we are able to confirm and extend previous results on the ground state of (HF)2 for K values ranging from 0 to 4. Perturbations are found in some transitions involving high K-levels. Some additional results on the tunnelling splittings in vibrationally excited states (v 1 = 1) are presented and discussed in relation to dynamical and electronic effects upon excitation of the HF stretching motions.