Wavelength-dependent UV photodesorption of pure N₂ and O₂ ices

¹ Sackler Laboratory for Astrophysics, Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden The Netherlands
e-mail: fayolle@strw.leidenuniv.nl
² Laboratoire de Physique Moléculaire pour l’Atmosphère et l’Astrophysique, UPMC Univ. Paris 6, CNRS-UMR7092, 4 place jussieu, 75252 Paris, France
³ Laboratoire de Chimie Physique, UMR 8000 CNRS-Université Paris-Sud, 91405 Orsay, France
⁴ Departments of Chemistry and Astronomy, University of Virginia, Charlottesville, VA 22904, USA

Received: 21 March 2013
Accepted: 13 June 2013

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. 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.

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 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.

Results. 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.

Conclusions. 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.