H. C. Chang

THE AR-HF INTERMOLECULAR POTENTIAL : OVERTONE SPECTROSCOPY AND AB INITIO CALCULATIONS

The vibrational dependence of the intermolecular potential of Ar–HF is investigated through the spectra of levels correlating with HF(v=3). We have previously reported measurements of the (vbKn)=(3000), (3100), and (3110) levels of Ar–HF using intracavity laser‐induced fluorescence in a slit supersonic jet [J. Chem. Phys. 98, 2497 (1993)]. These levels are found to be well reproduced (within 0.1 cm−1) by the Ar–HF H6(4,3,2) potential [J. Chem. Phys. 96, 6752 (1992)]. The second overtone experiments are extended to include the (3002) state which is coupled to (3110) through Coriolis interaction, and the (3210) state which is more sensitive to higher‐order anisotropic terms in the potential. The observations establish that the level (3002) lies 0.229 cm−1 below (3110), with upper state rotational constant B=0.085 89 cm−1. This is in good accord with the predictions of the H6(4,3,2) potential. The (3210) state lies at 11 484.745 cm−1 with B=0.099 79 cm−1. The band origin is 1.7 cm−1 higher than predicted, an...

The vibrational second 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 (ΔN=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 ν0=11 552.897 cm−1 (A+), 11 552.509 cm−1 (B+), rotational constants B=0.220 86 cm−1 (A+), 0.220 94 cm−1 (B+). For the (1,2) mode, a perpendicular band, K=1←0, is observed at ν0=11 536.95 cm−1 (A+), 11 536.93 cm−1 (B+) with B=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 th...

Observation of ArHF(3000) and its combination modes by laser‐induced fluorescence

Laser‐induced fluorescence is used to obtain the second overtone spectrum of ArHF. The method exploits intracavity circulating power of a Ti–sapphire ring laser to pump the weakly bound complex generated in a supersonic slit jet from v=0 to v=3. Fundamental (Δv=−1) emission is monitored using an infrared PbS detector. Intense fluorescence allows recording of the rotationally resolved sub‐Doppler spectra of (3000)←(0000), (3100)←(0000), and (3110)←(0000) transitions. We determine vibrational band origins of ν0=11 339.034 cm−1, 11 412.438 cm−1, 11 422.378 cm−1 and rotational constants of B=0.103 30 cm−1, 0.102 76 cm−1, 0.101 18 cm−1 for the (3000), (3100), and (3110) bands, respectively. Both the band origins and the rotational constants indicate that the weak Ar–HF van der Waals bond is strengthened as the HF stretch is vibrationally excited to higher states. All the observations are in near perfect accord with extrapolations of related constants in the HF stretching states of v=0–2.

State-specific vibrational predissociation and interconversion tunneling quenching at 3ν1 and 3ν2 of (HF)2

We report the second overtone (Δv=3←0) spectra of the free‐HF (ν1) and bound‐HF (ν2) stretches of (HF)2 using laser induced fluorescence. Subbands of K=0←0 and K=1←0 are detected near 900 nm with linewidths spanning almost two orders of magnitude. The line broadening (Δνpd) 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 Δνpd=10 GHz for the parallel band of 3ν2, and 0.10 and 1.9 GHz for the parallel and the perpendicular bands of 3ν1, respectively. The linewidths of these subbands are J and tunneling state independent. The K‐dependent vibrational predissociation is attributed to near‐resonant centrifugal interaction of the K=1 state with the K=1 combination mode of the bound HF stretch (3ν2) and the antisymmetric bend (ν5). The exceedingly state‐specific behavior is at variance with elementary density of states arguments. Spectroscopic constants of these two K subbands and two tunneling states (A+ and B+) of 3ν...

High-overtone spectroscopy and photodissociation of hydrogen fluoride complexes

Intracavity laser-induced fluorescence investigations of the HF(v= 3) states of hydrogen fluoride binary complexes have been summarized. Together with the previous measurements made by other authors at lower stretching states, our present investigations of ArHF, N2HF, (HF)2 and HFHCl allow several global features of the weakly bound complexes upon valence vibrational excitation to be obtained. It is found for all four complexes that both the Van der Waals bond strength and the B rotational constant increase smoothly with v. The effects are more pronounced in the complexes where HF acts as a hydrogen donor (bound HF) rather than as an acceptor (free HF). Additionally vibrational predissociation rates show a regular increase in v when the bound-HF stretch is vibrationally excited, contrary to the complex pattern in the excitation of free HF. From v= 1 to v= 3, the increase of the predissociation rate in the bound-HF stretch is much greater than linear but somewhat less than geometric.