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Wavelength-dependent UV photodesorption of pure N2 and O2 ices
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$...Show more Context. Ultraviolet photodesorption of molecules from icy interstellar grains can explain observations of cold gas in regions where thermal desorption is negligible. This non-thermal desorption mechanism should be especially important where UV fluxes are high.
Aims: N$_{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.Show less
- All authors
- Fayolle, E.C.; Bertin, M.; Romanzin, C.; Poderoso, H.; Philippe, L.; Michaut, X.; Jeseck, P.; Linnartz, H.V.J.; Öberg, K.; Fillion, J.
- Date
- 2013
- Journal
- Astronomy & Astrophysics
- Volume
- 556
- Pages
- A122