Co Ice Photodesorption: A Wavelength-Dependent Study

Abstract

UV-induced photodesorption of ice is a non-thermal evaporation process that can explain the presence of cold molecular gas in a range of interstellar regions. Information on the average UV photodesorption yield of astrophysically important ices exists for broadband UV lamp experiments. UV fields around low-mass pre-main sequence stars, around shocks and in many other astrophysical environments are however often dominated by discrete atomic and molecular emission lines. It is therefore crucial to consider the wavelength dependence of photodesorption yields and mechanisms. In this work, for the first time, the wavelength-dependent photodesorption of pure CO ice is explored between 90 and 170 nm. The experiments are performed under ultra high vacuum conditions using tunable synchrotron radiation. Ice photodesorption is simultaneously probed by infrared absorption spectroscopy in reflection mode of the ice and by quadrupole mass spectrometry of the gas phase. The experimental results for CO reveal a strong wavelength dependence directly linked to the vibronic transition strengths of CO ice, implying that photodesorption is induced by electronic transition (DIET). The observed dependence on the ice absorption spectra implies relatively low photodesorption yields at 121.6 nm (Lyman α), where CO barely absorbs, compared to the high yields found at wavelengths coinciding with transitions into the first electronic state of CO (A 1 Π at 150 nm); the CO photodesorption rates depend strongly on the UV profiles encountered in different star formation environments.

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

“…As expected from the work by Fayolle et al (2011), an ice irradiated with light from the MDHL running on the H 2 :He mixture (suppressed 160 nm emission) photodepletes less rapidly than the ice irradiated with pure H 2 that exhibits a molecular emission that is roughly two times bigger. Obviously, this photodepletion value is different for the different lamp settings, fully in agreement with the main findings of the work presented in this paper.…”

“…(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. Carbon monoxide is an excellent example, since it strongly absorbs 160 nm photons while Lyman-α photons are clearly off resonance, as has been shown in independent experiments (Fayolle et al 2011;Cruz-Diaz et al 2014a).…”

“…These measurements have been performed with broadband MDHLs. In a monochromatic and wavelength dependent study Fayolle et al (2011) showed that CO photodesorption occurs predominantly at longer wavelengths between 135 and 165 nm, following a desorption induced by electronic transition (DIET) mechanism. For wavelengths around Lyman-α the photodesorption is much less efficient.…”

“…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.…”

Astrochemistry of dust, ice and gas: introduction and overview

“…Temperature programmed desorption experiments now routinely probe thermal desorption of both pure and mixed ices, providing binding energies that can be used in models [43][44][45] 54 . The process of photodesorption has been measured quantitatively and found to be much more efficient than previously thought 55,56 , and to depend strongly on the wavelength of the incident radiation field 57,58 . Details of the formation of complex organic molecules by UV irradipc, 1 arcsec corresponds to n×100 AU diameter, or n× 100× the Sun-Earth distance.…”