Ion irradiation effects on frozen methanol (CH3OH)

We present some experimental results on the chemical-physical effects induced by ion irradiation on frozen methanol (CH3OH), pure and mixed with water (H2O). We have studied, by “in situ” infrared (IR) spectroscopy, samples of solid CH3OH and H2O:CH3OH mixtures before and after irradiation, with 3-30 keV ions, at low pressures and low temperature (10 K). We have derived the integrated absorbance values of the main methanol bands. Furthermore we have analyzed the effects produced upon warm-up comparing the spectra of the irradiated H2O:CH3OH mixtures with the unirradiated ones at different temperatures. We found that after ion irradiation of methanol new species (such as H2O, CO2, CO, H2CO and/or (CH3)2OH, CH4) are formed, the profile (shape, width and peak position) of methanol bands changes and estimation of methanol abundance from different bands, in the same spectrum, gives different values. In particular, the CH3OH/H2O ratio estimated in an irradiated sample, from the 1460 cm−1 and 2830 cm−1 bands differs up to a factor 3-4. This study can have relevant astrophysical implications in fact, methanol has been detected as a component of icy grain mantles, from IR spectra of several protostellar sources. However the results concerning solid methanol abundance in the interstellar medium are quite controversial. In this work we focus our attention on the profile, in laboratory spectra, of the three methanol bands (at 1034, 1460, 2830 cm−1) used to compute its abundance in icy grain mantles.