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. Measurements were made with tunable diode laser spectrometers using a pulsed supersonic jet and a long-path lowtemperature absorption cell. Close to the R(0) transition of the methane n4 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 Rbranches are located. As the first step, the Ne-CH4 spectrum was recorded and analyzed in a supersonic 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 compact. 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 higher 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 ?.