THE VIBRATIONAL 2ND OVERTONES OF HF DIMER - A QUARTET

We complete the study of the HF stretches (v1 and V2) of (HF)2 at N = v1 + v2 = 3. A previous publication [J. Chem. Phys. 98, 9266 (1993)] reported the observations of the free-HF and hydrogen-bonded-HF stretches at (v1,v2) = (3,0) and (0,3). In this paper, second overtone (DELTAN = 3 <-- 0) spectra of the vibrations mixed between the two HF subunits are presented. Spectroscopic constants of the K subbands and tunneling states (A+ and B+) of the two mixed modes (2,1) and (1,2) are determined from their lifetime-broadened but rotationally resolved manifolds. For the (2,1) mode, we observe only a parallel band, K = 0 <-- 0, and obtain band origins nu0 = 11 552.897 cm-1 (A+), 11 552.509 cm-1 (B+), rotational constants BBAR = 0.220 86 cm-1 (A+), 0.22094 cm-1 (B+). For the (1,2) mode, a perpendicular band, K = 1 <-- 0, is observed at nu0 = 11 536.95 cm-1 (A+), 11 536.93 cm-1 (B+) with BBAR = 0.222 cm-1 for both A+ and B+ states. The hydrogen interconversion tunneling splittings are determined to be 0.387 and 0.02 cm-1 for the K = 0 levels of (2, 1) and the K = 1 levels of (1,2), respectively, demonstrating a strong dependence on K rotation and the importance of transition-dipole coupling in the tunneling process. Based on our present and previous results, we provide an overview of all the four components of the quartet by comparing five unique characteristics: vibrational symmetry, band origin, relative transition strength, hydrogen interconversion tunneling, and vibrational predissociation. Systematic comparison is also made against ab initio calculations of Jensen, Bunker, Karpfen, Kofranek, and Lischka [J. Chem. Phys. 93, 6266 (1990)]. A brief analysis suggests that the pure overtone modes can be described sufficiently by a local mode picture, whereas the mixed modes have strong normal mode characters. It is also concluded that the ab initio calculations do not reproduce the observations correctly and more adequate representation of the high vibrationally excited states of the HF dimer is required.