HIGH-RESOLUTION SPECTROSCOPY OF AR-CH4 AND KR-CH4 IN THE 7 MU REGION (J=1[-0 TRANSITION)

Diode laser spectra of the rare gas - spherical top van der Waals comp lexes Ar-CH4 and Kr-CH4 were measured in the wavelength region near 13 10 cm(-1) and assigned. The most prominent lines of both complexes exh ibit three dense but well resolved P-R(0), R-Q(0), and (R)Q(0) branche s, correlated to the R(0) transition of the triply degenerate bending vibration nu(4) of methane, CH4. A model Hamiltonian based on Coriolis coupled states was applied for the assignment, analysis and fitting o f the spectra to within the experimental accuracy of approximate to 15 MHz. The rotational B constants of the upper and lower states determi ned from the three allowed branches appeared to be strongly correlated . The precision in the determination of the rotational B constants of the two complexes was substantially increased by additional recording of several weak transitions in the nearly forbidden P-Q(0) and R-R(0) branches, which were fitted together with the allowed transitions. The separation between the rare gas atom and the methane molecule in the ground vibrational state was determined to be 3.999 Angstrom and 4.094 Angstrom for Ar-CH4 and Kr-CH4, respectively. The measured small valu es of the splitting between the K=0 and the K = +/-1 levels in the vib rationally excited state (0.39 cm(-1) and 0.67 cm(-1) for Ar-CH4 and K r-CH4, respectively), which characterizes the anisotropy of the interm olecular potential, indicated that Kr-CH4 and Ar-CH4 together with Ne- SiH4 represent examples close to the free rotor limit, where the spher ical top CH4 is almost free to rotate within the complex. In compariso n, the previously analyzed Ar-SiH4 van der Waals molecule is closer to the hindered rotor limit.