Wavelength-Dependent UV Photodesorption of Pure \(N_2\) and \(O_2\) Ices

Abstract

Context: Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high. Aims: \(N_2\) and \(O_2\) are expected to play key roles in astrochemical reaction networks, both in the solid state and in the gas phase. Measurements of the wavelength-dependent photodesorption rates of these two infrared-inactive molecules provide astronomical and physical-chemical insights into the conditions required for their photodesorption. Methods: Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6 eV range is used to irradiate pure \(N_2\) and \(O_2\) thin ice films. Photodesorption of molecules is monitored through quadrupole mass spectrometry. Absolute rates are calculated by using the well-calibrated CO photodesorption rates. Strategic \(N_2\) and \(O_2\) isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. Results: \(N_2\) photodesorption mainly occurs through excitation of the \(b^1\sqcap_u\) state and subsequent desorption of surface molecules. The observed vibronic structure in the \(N_2\) photodesorption spectrum, together with the absence of \(N_3\) formation, supports that the photodesorption mechanism of \(N_2\) is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, \(O_2\) photodesorption in the 7−13.6 eV range occurs through dissociation and presents no vibrational structure. Conclusions: Photodesorption rates of \(N_2\) and \(O_2\) integrated over the far-UV field from various star-forming environments are lower than for CO. Rates vary between \(10^{-3}\) and \(10^{-2}\) photodesorbed molecules per incoming photon.