Investigation of the gas-phase B̃–X̃ electronic spectra of CH–Ar by laser-induced fluorescence

Abstract

Gas-phase methyidyne–argon (CH–Ar) van der Waals complexes have been detected spectroscopically by laser-induced fluorescence (LIF) in the region of the CH B 2Σ−–X 2Πr (0,0) and (1,0) bands near 363.5 and 388.9 nm, respectively. They are formed by a supersonic free-jet expansion of argon gas seeded with CH radicals generated from the 248 nm photolysis of CHBr2Cl. The excitation spectra reveal a number of rovibronic bands which are assigned to various stretching and/or bending motions of the CH–Ar complex. From the excitation spectra, lower limits for the ground and exited state binding energies are estimated. Rotational analysis based on combination differences and computer simulations of eight of the rovibronic bands yields an average ground state value of B″av = 0.174 ± 0.004 cm−1 and excited state constants ranging from B′=0.086–0.116 cm−1. This indicates that the CH–Ar van der Waals bond is lengthened considerably upon electronic excitation. A splitting of the ground state rotational energy levels, related to the anisotropy of the intermolecular potential and Coriolis coupling, is also observed. Based on an analysis of the rovibronic structure of the CH–Ar bands in terms of a hindered internal rotational model describing the interaction of Ar(1S0) atom with a CH monomer, a linear equilibrium geometry is inferred for the excited state and a ‘‘T’’ shaped geometry for the ground state. These results are compared to those obtained from gas-phase LIF studies of OH–Ar complexes recorded near the OH A 2Σ+–X 2Πi (0,0) and (1,0) bands.