High resolution spectroscopy of Ar-CH4 and Kr-CH4 in the 7 μ region (j = 1 ← 0 transition)

Abstract

Abstract Diode laser spectra of the rare gas - spherical top van der Waals complexes Ar-CH4 and Kr-CH4 were measured in the wavelength region near 1310 cm-1 and assigned. The most prominent lines of both complexes exhibit three dense but well resolved RP0, QR0, and RQ0 branches, correlated to the R(0) transition of the triply degenerate bending vibration v4 of methane, CH4. A model Hamiltonian based on Coriolis coupled states was applied for the assignment, analysis and fitting of the spectra to within the experimental accuracy of ≈ 15 MHz. The rotational B constants of the upper and lower states determined 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 QP0 and RR0 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 Å and 4.094 Å for Ar-CH4 and Kr-CH4, respectively. The measured small values of the splitting between the K=0 and the K = ±1 levels in the vibrationally excited state (0.39 cm-1 and 0.67 cm-1 for Ar-CH4 and Kr-CH4, respectively), which characterizes the anisotropy of the intermolecular potential, indicated that Kr-CH4 and Ar-CH4 together with Ne-SiH4 represent examples close to the free rotor limit, where the spherical top CH4 is almost free to rotate within the complex. In comparison, the previously analyzed Ar-SiH4 van der Waals molecule is closer to the hindered rotor limit.