Nonstationary processes in matrix-isolated methane probed by optical and current emission spectroscopy

Abstract

Nonstationary processes in the Ar matrix doped with CH4 were studied with a focus on the behavior of radiolysis products – H atoms and CH radicals. Samples of 25 μm thickness were irradiated with 1.5 keV electron beam. It has been shown that because of the small penetration depth of electrons the bulk of the matrix is excited preferentially by photons with an energy of 9.8 eV, the most intense emission band of the matrix (excitons self-trapped into the configuration of Ar2∗), In other words “internal photolysis” occurs. The spectrally resolved emission of H atoms and CH radicals, along with the total yield of desorbing particles, were monitored in a correlated manner under the concurrent action of photons and external heating. These dependences, called nonstationary luminescence (NsL) and nonstationary desorption (NsD), were compared with the measured yield of thermally stimulated exoelectron emission (TSEE), that enabled us to reveal the appearance of the CH radical in the neutralization reaction CH3+ + e− → CH3∗ followed by dissociation of CH3∗. Thus, the CH radical was assumed to be a signature of the CH3∗ species. The antibate behavior of the CH NsL and the NsD in the temperature range 37–43 K, where diffusion in the Ar matrix occurs, was interpreted as a result of the recombination of CH3 radicals with the formation of C2H6 and the energy release spent for desorption. Self-oscillations (with τ of about 10 s) of increasing amplitude were detected, indicating self-oscillating processes in the bulk of the Ar matrix doped with 5% CH4.