Photon-induced desorption of formaldehyde observed with mass fragments m/z = 29 and m/z = 30 (left panel), and carbon monoxide (m/z = 28 in right panel) during UV irradiation of a H₂O:CH₄ ice analog in experiment 5. Results were similar in exp. 3, 4, and 6. Ion currents of different mass fragments are offset for clarity. The black solid line represent an average of the corresponding ion current, shown to better evaluate the shape of the signal. Irradiation intervals are indicated with dashed vertical lines.