The high-resolution spectrum of the Ar–CH 4 complex in the [formula omitted]m region: measurement and ab initio calculation

Abstract

The ν 4 fundamental vibration spectrum of the weakly bound complex Ar–CH 4 in the 7 μm region was discovered, analyzed, and compared with a synthesized spectrum, derived from ab initio calculations. The measurements were made by probing a supersonic gas expansion with a tunable diode laser (TDL) spectrometer. Several bands of Ar–CH 4 associated with different rovibrational transitions of the ν 4 vibration (asymmetric bending vibration) of CH 4 were recorded and analyzed in a spectral region from 1295 to 1330 cm −1. In particular the following transitions were studied: j=1←0 at 1311 cm −1 reported in Z. Naturforsch. 53 (1998) 725, j=0←1 at 1301 cm −1, j=1←1 at 1306 cm −1, j=2←1 at 1316 cm −1, and j=3←2 transitions at 1322 cm −1. Here, j denotes the angular momentum of the methane unit inside the complex. The densest part of the experimental spectrum located in the region between 1300 and 1315 cm −1 is compared in depth with the synthesized ab initio spectra. The close agreement between observed and theoretical ab initio spectra is convincingly demonstrated with respect to the gross spectral features, including also many details of the spectra. The ab initio spectra were computed as described in J. Chem. Phys. 110 (1999) 5639 for the ν 3 excited Ar–CH 4 complex. Briefly, a potential was obtained from symmetry adapted perturbation theory and used as input for a variational solution of the rovibrational problem. After the levels were calculated the line intensities were computed with the ν 4 transition dipole as unit of dipole strength. A Boltzmann distribution was assumed at 7 K.