Infrared optical depth spectra of the a-C:H sample, irradiated with ¹³²Xe^{21 +} at 300 K (upper panel), 100 K (center panel), and 40 K (lower panel), as a function of fluence. The lower spectrum in each panel is the difference between the spectrum recorded at the highest fluence and the pristine material scaled to the CH stretching mode intensity to show the newly formed bonds in the material with a higher contrast. The upper right insets show the integrated optical depth for the CH bonds (squares) as a function of fluence. The lines are exponential fits that allow us to determine the destruction cross section (see text for details). At 40 K the inset shows the appearance of a band around 4120 cm^-1 associated with H₂ formed and trapped in the bulk.