INTRAMOLECULAR DYNAMICS OF THE $N = 2$ HF STRETCHING OVERTONE POLYAD OF $(HF)_{2}$ STUDIED BY HIGH-RESOLUTION cw-DIODE LASER CAVITY RING-DOWN SPECTROSCOPY IN A PULSED SLIT JET

$^{1}$ T. R. Dyke, B. J. Howard, and W. Klemperer, J. Chem. Phys. 56 (1972), 2442. $^{2}$ A. S. Pine, and W. J. Lafferty, J. Chem. Phys. 78 (1983), 2154. $^{3}$ K. von Puttkamer, and M. Quack, Mol. Phys. 62 (1987), 1047. $^{4}$ K. von Puttkamer, and M. Quack, Chem. Phys. 139 (1989), 31. $^{5}$ M. Quack, and M. A. Suhm, J. Chem. Phys. 95 (1991), 28. $^{6}$ W. Klopper, M. Quack, and M. A. Suhm, J. Chem. Phys. 108 (1998), 10096. $^{7}$ M. Hippler, and M. Quack, Chem. Phys. Lett. 314 (1999) 273; J. Chem. Phys. (2002) in press. $^{8}$ M. Hippler, L. Oeltjen, and M. Quack, in preparation. $^{9}$ J. Blumberger, L. Oeltjen, M. Quack, Z. Ba\u{c}i\'{c}, and Y. Qiu, in preparation.Author Institution: Physical Chemistry, ETH ZurichThe $(HF)_{2}$ 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 $spectroscopy^{4}$ 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 $recently^{7}$ (instrumental bandwidth about 1 MHz corresponding to a resolving power of $2 \times 10^{8}$). 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 $calculations^{9}$ but disagree with simple models or approximate calculations that have been presented in the past