LASER-INDUCED FLUORESCENCE SPECTROSCOPY OF AR2HF AT NU(HF)=3 - AN EXAMINATION OF 3-BODY FORCES

The vibrational spectrum of Ar2HF in the 11 320-11 430 cm(-1) region i s recorded by intracavity laser-induced fluorescence. The intramolecul ar vibrational state, Sigma(0), in combination with the intermolecular vibrations, assigned as Pi(in-plane), Pi(out-of-plane) and Sigma(1), of the complex have been observed. The Sigma(0) state correlates adiab atically with j = 0 of HF (upsilon = 3); the Pi(in-plane), Pi(out-of-p lane), and Sigma(1) states correlate adiabatically with j = 1 of HF (u psilon = 3), respectively. We have determined the vibrational band ori gins (and rotational constants) of nu(0) = 11 323.784 cm(-1) (A = 0.12 0 15, B = 0.058 30, C = 0.038 94 cm(-1)), nu(0) = 11 387.730 cm(-1) (A = 0.122 68, B = 0.057 05, C = 0.038 42 cm(-1)), nu(0) = 11 426.815 cm (-1) (A = 0.120 27, B = 0.058 15, C = 0.038 71 cm(-1)) and nu(0) = 11 427.400 cm(-1) (A = 0.120 26, B = 0.058 15, C = 0.038 71 cm(-1)) for S igma(0), Pi(in-plane), Pi(out-of-plane), and Sigma(1) states, respecti vely. The vibrational red shift for the pure HF stretch from upsilon(H F) = 0-3 is 49.023 cm(-1). The in-plane and out-of-plant bending frequ encies are 63.947 and 103.031 cm(-1). The Sigma(1) state, which may be viewed as the Ar2FH structure is located 103.616 cm(-1) above the Ar2 HF Sigma(0) state. The spectral line shapes appear to be well fitted b y a Doppler profile with FWHM approximate to 120 MHz, indicating that the predissociation linewidths have a Lorentzian component of less tha n 10 MHz. These results are compared with those of Farrell and Nesbitt [J. Chem. Phys. 105, 9421 (1996)] for upsilon(HF) = 1. The present ex perimental data set is also compared with the quantitative predictions by Ernesti and Hutson [Phys. Rev. A 51, 239 (1995)] and therefore ser ves as a rigorous test for modeling nonadditivity of intermolecular in teractions and their vibrational dependence. These comparisons show th at the vibrational dependence of three-body terms is accurate in the r egion of potential minimum. For configurations far from the energy min imum, appreciable discrepancies appear to exist. The vibrational varia tion of the Pi(in-plane) bending frequency is relatively poorly predic ted, which strongly suggests the inadequacy in the present modeling of the intriguing nonadditive forces for this prototypical system. (C) 1 997 American Institute of Physics.