Wavelength-dependent UV photodesorption of pure N2 and O2 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$^{1}${$Pi$}$_u$ 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.