Power-dependence analysis of the IR-FEL effect on CH₃OH ice. The three different power settings, i.e., 5.76, 57.6, and 115 mJ, are controlled by attenuating a 115 mJ beam with 13, 3, and 0 dB, respectively. Error bars correspond to the three sigma limit of the instrumental error as derived from the integrated noise signal of the off-resonance difference spectrum in the same band width.