Diode laser spectroscopy of the weakly bound complex Ne-CH4

The infrared absorption spectrum of the weakly bound rare-gas-spherical-top complex Ne-CH4 was discovered and analyzed for the first time. Measurement s were made with tunable diode laser spectrometers using a pulsed supersoni c jet and a long-path low-temperature absorption cell. Close to the R(0) tr ansition of the methane v(4) fundamental band at 1311.430 cm(-1),the Ne-CH4 spectrum was recorded as a very compact absorption pattern. Within a total wave-number range of about 0.1 cm(-1), P-, Q-, and R-branches are located. As the first step, the Ne-CH4 spectrum was recorded and analyzed in a supe rsonic jet at low rotational temperature of about 5 K. Three branches were identified, of which the P- and R-branches were partially resolved and the Q-branch remained unresolved. Compared with the previously measured spectra of Ar-CH4 and Kr-CH4 [Z. Naturforsch. A, 53, 725 (1998).], the absorption pattern in the spectrum of Ne-CH4 is much denser and considerably more comp act. However, by analogy with the spectra of Ar-CH4 and Kr-CH4, assignment and analysis were carried out using a Hamiltonian model that incorporates a Coriolis interaction between the total angular momentum of the complex and the angular momentum of the methane monomer. This analysis then allowed us to assign the same spectrum as recorded in a long-path (160 m) cell at a h igher temperature of 62 K. The observed rotational constant for Ne-CH4, B" = 0.129(9) cm(-1), corresponds to an effective intermolecular separation of 3.8 Angstrom.