Knowledge Bank

The $(HF)2$ hydrogen bonded dimer has been a prototype system for high-resolution spectroscopy since the pioneering studies of its microwave spectra by Dyke, Howard, and Klemperer in $1972.1$ Subsequently the HF stretching fundamentals were studied in $1983,2$ a low frequency fundamental analyzed in the far infrared in $1987,3$ HF stretching overtone spectra investigated by FTIR $spectroscopy4$ and finally full dimensional potential energy hypersurfaces developed of near to spectroscopic $accuracy.5,6$ All these were ``first'' achievements prototypical for any type of hydrogen bonded dimer of this kind. Here we present the first study of the $N=2$ HF stretching overtone polyad by very high resolution cw-diode laser cavity ring-down spectroscopy in pulsed slit jet expansions developed $recently7$ (instrumental bandwidth about 1 MHz corresponding to a resolving power of $2\times 108$). An analysis of all polyad subbands in terms of spectroscopic constants, tunneling splittings, Lorentzian predissociation and Doppler contributions to the linewidths will be $presented.8$ The results agree well with full six-dimensional $calculations9$ but disagree with simple models or approximate calculations that have been presented in the past.