Photon-induced desorption of HCO (m/z = 30 and m/z = 31 in left panel) and ¹³CO (m/z = 29 in right panel) during UV irradiation of a H₂O:¹³CH₄ ice analog in experiments 10 and 7, respectively. Results were similar for experiments 8 and 9. Ion currents of different mass fragments are offset for clarity. Black solid lines represent an average of the corresponding ion currents, shown to better evaluate the shape of the signals. Irradiation intervals are indicated with dashed vertical lines.