Wavelength-dependent UV photodesorption of pure N₂and O₂ices

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 Π 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.

Vacuum-UV spectroscopy of interstellar ice analogs

“…This indicates that photodesorption of some ice species like CO and N 2 is mainly driven by a desorption induced by electronic transition (DIET) process (Fayolle et al 2011(Fayolle et al , 2013. The lowest photodesorption reported by Fayolle et al (2011) at the Ly-α wavelength (121.6 nm) is <6 × 10 −3 molecules per incident photon, coinciding with the low VUV-absorption at this wavelength in Fig.…”

“…The performance of VUV-spectroscopy inevitably leads to exposure of the sample to irradiation, often causing photodestruction or photodesorption of the ice molecules to some extent during spectral acquisition (e.g., d 'Hendecourt et al 1985'Hendecourt et al , 1986Bernstein et al 1995;Gerakines et al 1995Gerakines et al , 1996Schutte 1996;Sandford 1996;Muñoz Caro et al 2003Dartois 2009;Öberg et al 2009a,b;Fillion et al 2012;Fayolle et al 2011Fayolle et al , 2013, and references therein). Photoproduction of new compounds and the decrease of the starting ice column density was monitored by IR spectroscopy.…”

The HIFI spectral survey of AFGL 2591 (CHESS)

“…We do not allow H 2 isotopologues to stick to grains because for H 2 this requires temperatures of < ∼ 4 K. A UV photodesorption yield of 3 × 10 −3 was adopted (e.g., Öberg et al 2009a,b;Fayolle et al 2011aFayolle et al , 2013. Photodissociation processes of solid species are taken from Garrod & Herbst (2006) and Semenov & Wiebe (2011).…”

Controlling the emission profile of an H2discharge lamp to simulate interstellar radiation fields

“…Photodesorption rates for several molecules using synchrotron radiation (Fayolle et al 2011(Fayolle et al , 2013 as well as direct cross section measurements of the main constituents of interstellar ice show strong wavelength dependences in the VUV domain. (Cruz-Diaz et al 2014a,b) Recently, Chen et al (2014) showed that the differences in CO photodesorption rates reported by various groups over the past decade may be partly explained by differences in MDHL emission spectra.…”