Photodesorption of ices I: CO, N₂, and CO₂

By K. Oberg, E. Dishoeck, H. Linnartz. 2008

226 citations recorded by the source corpus.

Context. A longstanding problem in astrochemistry is how molecules can be maintained in the gas phase in dense inter- and circumstellar regions at temperatures well below their thermal desorption values. Photodesorption is a non-thermal desorption mechanism, which may explain the small amounts of observed cold gas in cloud cores and disk mid-planes. Aims. This study aims to determine the UV photodesorption yields and to constrain the photodesorption mechanisms of three astrochemically relevant ices: CO, N2 and CO2. In addition, the possibility of co-desorption in mixed and layered CO:N2 ices is explored. Methods. The UV photodesorption of ices is studied experimentally under ultra high vacuum conditions and at astrochemically relevant temperatures (15–60 K) using a hydrogen discharge lamp (7–10.5 eV). The ice desorption is monitored by reflection absorption infrared spectroscopy of the ice and simultaneous mass spectrometry of the desorbed molecules. Results. Both the UV photodesorption yield per incident photon and the photodesorption mechanism are highly molecule specific. The CO photodesorbs without dissociation from the surface layer of the ice, and N2, which lacks a dipole allowed electronic transition in the wavelength range of the lamp, has a photodesorption yield that is more than an order of magnitude lower. This yield increases significantly due to co-desorption when N2 is mixed in with, or layered on top of, CO ice. CO2 photodesorbs through dissociation and subsequent recombination from the top 10 layers of the ice. At low temperatures (15–18 K), the derived photodesorption yields are 2.7(±1.3) × 10 −3 and <2 × 10 −4 molecules photon −1 for pure CO and N2, respectively. The CO2 photodesorption yield is 1.2(±0.7)×10 −3 ×(1−e −(x/2.9(±1.1)) )+1.1(±0.7)×10 −3 ×(1−e −(x/4.6(±2.2) )) molecules photon −1 ,w herex is the ice thickness in monolayers and the two parts of the expression represent a CO2 and a CO photodesorption pathway, respectively. At higher temperatures, the CO ice photodesorption yield decreases, while that of CO2 increases.

Subfields: Astronomy and Astrophysics, Atmospheric Science, Spectroscopy

View Paper: https://www.semanticscholar.org/paper/2fad62d031729390281153da98d89722023007a5

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. R. Garrod
2. E. Herbst
3. P. Caselli
4. V. Wakelam
5. E. Dishoeck
6. Cecilia Ceccarelli
7. K. Oberg
8. E. Bergin

In Atmospheric Science

1. R. Garrod
2. E. Herbst
3. V. Wakelam
4. P. Caselli
5. E. Dishoeck
6. A. Tielens
7. H. Cuppen
8. Cecilia Ceccarelli

In Spectroscopy

1. R. Garrod
2. P. Caselli
3. Cecilia Ceccarelli
4. E. Herbst
5. V. Wakelam
6. E. Dishoeck
7. K. Oberg
8. E. Bergin

Citations in the corpus

• ON THE FORMATION OF CO2 AND OTHER INTERSTELLAR ICES (2011, background): “Öberg et al. (2009c) considered the segregation of ice mixtures in protostellar envelopes, caused by the preferentially higher binding of molecules to each other than to different species.”
• A THREE-PHASE CHEMICAL MODEL OF HOT CORES: THE FORMATION OF GLYCINE (2013, methodology): “Other authors have assumed a swapping mechanism tobe active in laboratory ice experiments (Öberg et al. 2009; Fayolle et al. 2011), which is also implicitly assumed in this model.”
• Gas and grain chemical composition in cold cores as predicted by the Nautilus three-phase model (2016, methodology): “…the last years, quantitative photo-desorption yields have been measured experimentally for pure ices sample using hydrogen discharge lamps: CO (Öberg et al. 2007; Muñoz Caro et al. 2010), H2O (Öberg et al. 2009c), N2, CO2 (Öberg et al. 2009a; Yuan & Yates 2013) and CH3OH (Öberg et al. 2009b).”
• Reaction Networks for Interstellar Chemical Modelling: Improvements and Challenges (2010, background): “As photodesorption proceeds, photodissociation also occurs and new spectral features appear that correspond with reaction products (Öberg et al. 2009a).”
• Microscopic simulation of methanol and formaldehyde ice formation in cold dense cores (2009, methodology): “Higher temperatures allow the radicals to diffuse rapidly enough to form large molecules such as methyl formate and dimethyl ether (Garrod et al. 2008; Öberg et al. 2009d).”
• The Molecular Universe (2011, background): “Recent laboratory studies have focused on putting these ideas on a firm footing identifying the chemical routes involved (Öberg et al. 2009; Hudson et al. 2008).”
• Formation of Complex Molecules in Prestellar Cores: A Multilayer Approach (2017, background): “Although experiments by Öberg et al. (2009) showed high efficiency of photodesorption with a yield of 10−3 molecule per incident photon, newer experiments by Bertin et al. (2016) and Cruz-Diaz et al. (2016) suggest that at least for methanol the yield does not exceed 10−5 molecule per incident…”
• Formation rates of complex organics in UV irradiated CH3OH-rich ices I: Experiments (2009, background): “These yields agree with previous photodesorption studies of other molecules (Westley et al. 1995;Öberg et al. 2007, 2009a,b) and confirms the assumption in several observational and model papers that most ice molecules have similar photodesorption yields, around 10−3 per incident UV photon.”
• The UMIST database for astrochemistry 2012 (2012, methodology): “…(1963), Nahar & Pradhan (1997), Nejad & Millar (1988), Neufeld & Wolfire (2009), Nguyen et al. (2006), Novotný et al. (2010), Öberg et al. (2005), Öberg et al. (2009a), Öjekull et al. (2004), Osamura et al. (1999), Pequignot & Aldrovandi (1986), Petrie et al. (1991), Petrie et al. (2003),…”
• Astrochemistry and compositions of planetary systems (2020, background): “Early experiments demonstrated that UV photodesorption is highly efficient (Westley et al. 1995; Öberg et al. 2007, 2009a,c), while molecular dynamics simulations (Andersson et al. 2006; Andersson and van Dishoeck 2008) revealed that the process can be quite complex and occur through several…”
• Complex organic molecules in protoplanetary disks (2013, methodology): “This is supported by recent estimates of the grain-surface photodestruction of pure methanol ice which also show that photochemistry occurs deep within the bulk ice (Öberg et al. 2009c).”
• The molecular composition of the planet-forming regions of protoplanetary disks across the luminosity regime (2015, methodology): “We use experimentally determined photodesorption yields where available (see, e.g., Öberg et al. 2009a,b,c).”
• Grain Surface Models and Data for Astrochemistry (2017, methodology): “Within this framework, it is also possible to include branching ratios of the expected products, for those species for which photodissociation is the primary process (e.g., H2O and CO2).”
• Our astrochemical heritage (2012, methodology): “…are experimentally found to promote the formation of organic species more complex than CH3OH (Gerakines et al. 1996; Bennett and Kaiser 2007; Öberg et al. 2009a, 2010b) up to amino acids (e.g., Bernstein et al. 2002; Muñoz Caro et al. 2002, 2004) and allow solid species to return into the…”