Ultraviolet (UV) ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks,... Show moreUltraviolet (UV) ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks, clouds, and cloud cores. Systematic laboratory studies of the photodesorption rates, between 7 and 14 eV, from CO:N$_{2}$ binary ices, have been performed at the DESIRS vacuum UV beamline of the synchrotron facility SOLEIL. The photodesorption spectral analysis demonstrates that the photodesorption process is indirect, i.e., the desorption is induced by a photon absorption in sub-surface molecular layers, while only surface molecules are actually desorbing. The photodesorption spectra of CO and N$_{2}$ in binary ices therefore depend on the absorption spectra of the dominant species in the sub-surface ice layer, which implies that the photodesorption efficiency and energy dependence are dramatically different for mixed and layered ices compared with pure ices. In particular, a thin (1-2 ML) N$_{2}$ ice layer on top of CO will effectively quench CO photodesorption, while enhancing N$_{2}$ photodesorption by a factor of a few (compared with the pure ices) when the ice is exposed to a typical dark cloud UV field, which may help to explain the different distributions of CO and N$_{2}$H$^{+}$ in molecular cloud cores. This indirect photodesorption mechanism may also explain observations of small amounts of complex organics in cold interstellar environments. Show less
Bertin, M.; Fayolle, E.; Romanzin, C.; Poderoso, H.; Michaut, X.; Philippe, L.; ... ; Fillion, J. 2013
Ultraviolet (UV) ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks,... Show moreUltraviolet (UV) ice photodesorption is an important non-thermal desorption pathway in many interstellar environments that has been invoked to explain observations of cold molecules in disks, clouds, and cloud cores. Systematic laboratory studies of the photodesorption rates, between 7 and 14 eV, from CO:N$_{2}$ binary ices, have been performed at the DESIRS vacuum UV beamline of the synchrotron facility SOLEIL. The photodesorption spectral analysis demonstrates that the photodesorption process is indirect, i.e., the desorption is induced by a photon absorption in sub-surface molecular layers, while only surface molecules are actually desorbing. The photodesorption spectra of CO and N$_{2}$ in binary ices therefore depend on the absorption spectra of the dominant species in the sub-surface ice layer, which implies that the photodesorption efficiency and energy dependence are dramatically different for mixed and layered ices compared with pure ices. In particular, a thin (1-2 ML) N$_{2}$ ice layer on top of CO will effectively quench CO photodesorption, while enhancing N$_{2}$ photodesorption by a factor of a few (compared with the pure ices) when the ice is exposed to a typical dark cloud UV field, which may help to explain the different distributions of CO and N$_{2}$H$^{+}$ in molecular cloud cores. This indirect photodesorption mechanism may also explain observations of small amounts of complex organics in cold interstellar environments. Show less
Fayolle, E.C.; Bertin, M.; Romanzin, C.; Poderoso, H.; Philippe, L.; Michaut, X.; ... ; Fillion, J. 2013
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... Show moreContext. 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
