Average of mass spectra recorded during the ¹²C-methanol and carbon dioxide mixture injection to form the ice film (orange histogram), tracing the QMS mass-fragmentation patterns of pure methanol and carbon dioxide; average of a few spectra that were recorded at the beginning of the ice-film irradiation (blue histogram). Both mass spectra are scaled to m/z = 44 () for comparison. From top to bottom: mixtures with increasing proportions of methanol in relation to carbon dioxide. See text for details.