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.