Statehyphen;specific vibrational predissociation and interconversion tunneling quenching at 3ngr;1and 3ngr;2of (HF)2

摘要

We report the second overtone (Dgr;v=3larr;0) spectra of the freehyphen;HF (ngr;1) and boundhyphen;HF (ngr;2) stretches of (HF)2using laser induced fluorescence. Subbands ofK=0larr;0 andK=1larr;0 are detected near 900 nm with linewidths spanning almost two orders of magnitude. The line broadening (Dgr;ngr;pd) due to vibrational predissociation is not only mode specific but also is state specific. A fit of the spectral lines to a Voigt profile reveals Dgr;ngr;pd=10 GHz for the parallel band of 3ngr;2, and 0.10 and 1.9 GHz for the parallel and the perpendicular bands of 3ngr;1, respectively. The linewidths of these subbands areJand tunneling state independent. TheKhyphen;dependent vibrational predissociation is attributed to nearhyphen;resonant centrifugal interaction of theK=1 state with theK=1 combination mode of the bound HF stretch (3ngr;2) and the antisymmetric bend (ngr;5). The exceedingly statehyphen;specific behavior is at variance with elementary density of states arguments. Spectroscopic constants of these twoKsubbands and two tunneling states (A+andB+) of 3ngr;1are determined from their rotationally resolved manifolds. For the parallel band, we obtain band origins ngr;0=11thinsp;273.501 cmminus;1(A+), 112thinsp;73.499 cmminus;1(B+), rotational constantsBmacr;=0.221thinsp;177 cmminus;1(A+), 0.221thinsp;179 cmminus;1(B+), and centrifugal distortion constantsD=2.02times;10minus;6cmminus;1(A+), 2.05times;10minus;6cmminus;1(B+).For the perpendicular band, ngr;0=11thinsp;299.850 cmminus;1(A+), 11thinsp;299.847 cmminus;1(B+), andBmacr;=0.222thinsp;02 cmminus;1(A+), 0.222thinsp;04 cmminus;1(B+). The interconversion tunneling splitting is found to be 0.0024 cmminus;1, showing that the tunneling motion of the dimer could be quenched entirely. For the 3ngr;2where only theRbranch is resolved, the breadth of the lines prevents accurate determination of its spectroscopic constants. The band is estimated to center at 11thinsp;043.09 cmminus;1with a rotationalBmacr; constant of 0.2240 cmminus;1. All the constants indicate that a stronger hydrogen bond is formed at higher valence vibrational states. The shifts of the freehyphen; and the boundhyphen;HF stretching frequencies from that of the monomer are minus;99.306 and minus;329.72 cmminus;1, respectively. Finally, we present an analysis for the rotational dependence of the tunneling in states ofv1, which suggests that the transition state, under the assumption ofC2hgeometry, has the HF units oriented at 33deg; with respect to the Fndash;F axis.