New results on thermal and photodesorption of CO ice using the novel InterStellar Astrochemistry Chamber (ISAC)

Abstract

Aims. We present the novel InterStellar Astrochemistry Chamber (ISAC), designed for studying solids (ice mantles, organics, and silicates) in interstellar and circumstellar environments: characterizing their physico-chemical properties and monitoring their evolution as caused by (i) vacuum-UV irradiation; (ii) cosmic ray irradiation; and (iii) thermal processing. Experimental study of thermal and photodesorption of the CO ice reported here simulates the freeze-out and desorption of CO on grains, providing new information on these processes. Methods. ISAC is an UHV set-up, with base pressure down to P = 2.5 × 10 −11 mbar, where an ice layer is deposited at 7 K and can be UV-irradiated. The evolution of the solid sample was monitored by in situ transmittance FTIR spectroscopy, while the volatile species were monitored by QMS. Results. The UHV conditions of ISAC allow experiments under extremely clean conditions. Transmittance FTIR spectroscopy coupled to QMS proved to be ideal for in situ monitoring of ice processes that include radiation and thermal annealing. Thermal desorption of CO starting at 15 K, induced by the release of H 2 from the CO ice, was observed. We measured the photodesorption yield of CO ice per incident photon at 7, 8, and 15 K, respectively yielding 6.4 ± 0.5 × 10 −2 , 5.4 ± 0.5 × 10 −2 , and 3.5 ± 0.5 × 10 −2 CO molecules photon (7.3-10.5 eV) −1 . Our value of the photodesorption yield of CO ice at 15 K is about one order of magnitude higher than the previous estimate. We confirmed that the photodesorption yield is constant during irradiation and independent of the ice thickness. Only below ∼5 monolayers ice thickness the photodesorption rate decreases, which suggests that only the UV photons absorbed in the top 5 monolayers led to photodesorption. The measured CO photodesorption quantum yield at 7 K per absorbed photon in the top 5 monolayers is 3.4 molecules photon −1 . Conclusions. Experimental values were used as input for a simple model of a quiescent cloud interior. Photodesorption seems to explain the observations of CO in the gas phase for densities below 3-7 ×10 4 cm −3 . For the same density of a cloud, 3 × 10 4 cm −3 , thermal desorption of CO is not triggered until T = 14.5 K. This has important implications for CO ice mantle build up in dark clouds.

Grain Surface Models and Data for Astrochemistry

“…On the other hand, microwave discharge lamps have a significantly higher flux; however, independent measurements of CO photodesorption have shown that the derived yield is dependent on having a well-calibrated lamp spectrum. Muñoz Caro et al (2010) determined a desorption yield for pure CO ice of (3.5 ± 0.5 × 10 −2 ) photon −1 which is one order of magnitude higher than that from Öberg et al (2009a). Recent investigations showed that the FUV spectrum, in particular the ratio of Lyman-α (121.6 nm) to H 2 molecular emission bands (110-180 nm), is sensitive to the gas pressure, the type of lamp, the gas composition (pure H 2 versus a mixture of H 2 and He) and the window properties (Chen et al 2014;Ligterink et al 2015).…”

New experimental sublimation energy measurements for some relevant astrophysical ices

“…To help understand how it can be possible to observe those molecules in the gas phase in many lines of sight, the effect of irradiation on ices has been studied. Muñoz-Caro et al (2010) demonstrate that if the effect of UV photons (induced by cosmic rays) on CO ices is taken into account, an equilibrium between solid and gas phase is reached, because of ice desorption from dust produced by secondary photon irradiation. But this is not the only interaction producing desorption of icy components.…”

“…Seperuelo perform a series of experiments where the desorption rate induced by H, Ni and Fe ions are estimated, concluding that the effect of heavy ions is even more important than the effect of protons in the ISM. The works performed by Muñoz-Caro et al (2010) andSeperuelo Duarte et al (2010), are relevant for the ISM because, even though CO can be mixed with water ice, from the absorption profile studied by Ehrenfreund et al (1996), it is possible to deduce that CO could be embedded in a nearly separate ice phase.…”

“…If one knows the sublimation energy for a particular ice, its desorption rate under astrophysical conditions can be inferred. This is relevant to estimating either the residence time in a specific surface of astrophysical interest at a certain temperature (Sandford & Allamandola 1993) or the gas composition for a specific temperature in a typical frozen surface present in planets, satellites, comets, or on the refractory surface of interstellar grains (Muñoz-Caro et al 2010).…”

“…In temperature programmed desorption (TPD) method, the experiments (Acharyya et al 2007;Bolina et al 2005;Bischop et al 2006;and Muñoz-Caro et al 2010) are performed using ultra high vacuum (UHV) conditions (pressure ≤10 −10 mbar). The necessary experimental apparatus is similar to the previous method, but for this one, mass spectroscopy is used instead.…”