SIMBAD references

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. N₂ and O₂ 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. Tunable radiation from the DESIRS beamline at the SOLEIL synchrotron in the astrophysically relevant 7 to 13.6eV range is used to irradiate pure N₂ and O₂ 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 N₂ and O₂ isotopolog mixtures are used to investigate the importance of dissociation upon irradiation. N₂ photodesorption mainly occurs through excitation of the b¹Π_u state and subsequent desorption of surface molecules. The observed vibronic structure in the N₂ photodesorption spectrum, together with the absence of N₃ formation, supports that the photodesorption mechanism of N₂ is similar to CO, i.e., an indirect DIET (Desorption Induced by Electronic Transition) process without dissociation of the desorbing molecule. In contrast, O₂ photodesorption in the 7-13.6 eV range occurs through dissociation and presents no vibrational structure. Photodesorption rates of N₂ and O₂ 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.