Study of the frequency factor in the thermal desorption of astrophysical ice analogs: CH4, C2H4, C2H6, CH3OH, CO, CO2, H2O and N2

Abstract

In this work the frequency factor and the influence of the temperature on this parameter, for zeroth order desorption processes, has been experimentally determined for eight molecules of astrophysical interest. In the literature, this parameter has been estimated indirectly, obtaining values that differ by as much as three orders of magnitude from different authors. As a consequence, there are very different desorption rates reported for the same molecule and additionally its temperature dependence has been systematically neglected.The frequency factor is widely used to model the dynamics of these species under low temperature conditions present in some astrophysical environments. The method reported in this work is based on the analysis of the signal of a quartz crystal microbalance acting as a sample-holder, which is able to directly detect molecules desorbing from it.Two different types of desorption experiments were necessary for this study. In a first set of experiments, carried out at a constant rate of warming up, the desorption energy is obtained. The second set of experiments were performed at several constant temperatures to calculate the frequency factor and its relationship with temperature. The reasons for some anomalous behaviour have been analyzed. The dependence of the frequency factor on temperature should be taken into account when the Polanyi-Wigner equation is used for desorption processes. Every molecule has to be independently studied as no global tendency is found for the variation of the frequency factor with temperature.