Difference spectra obtained before and after 5 min of IR-FEL irradiation on methanol ice at 20 K. The difference between spectra acquired at 80 K and 20 K (without IR irradiation) during a TPD control experiment is also shown in cyan and included in the 2.96 µm plot. To allow for a direct comparison, all spectra are plotted for the same scale; the differences in the noise level between 3 µm and 9 µm are due to the intrinsic wavelength-dependent S/N of the FTIR. The irradiation spectra are offset for clarity.