Wavelength-dependent UV photodesorption of pure N2 and O2 ices
  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 13.6 eV 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 b^1Pi_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 10E-3 and 10E-2 photodesorbed
molecules per incoming photon.
