Solid State Pathways towards Molecular Complexity in Space

Abstract

Abstract. It has been a long standing problem in astrochemistry to explain how molecules can form in a highly dilute environment such as the interstellar medium. In the last decennium more and more evidence has been found that the observed mix of small and complex, stable and highly transient species in space is the cumulative result of gas phase and solid state reactions as well as gas-grain interactions. Solid state reactions on icy dust grains are specifically found to play an important role in the formation of the more complex "organic" compounds. In order to investigate the underlying physical and chemical processes detailed laboratory based experiments are needed that simulate surface reactions triggered by processes as different as thermal heating, photon (UV) irradiation and particle (atom, cosmic ray, electron) bombardment of interstellar ice analogues. Here, some of the latest research performed in the Sackler Laboratory for Astrophysics in Leiden, the Netherlands is reviewed. The focus is on hydrogenation, i.e., H-atom addition reactions and vacuum ultraviolet irradiation of interstellar ice analogues at astronomically relevant temperatures. It is shown that solid state processes are crucial in the chemical evolution of the interstellar medium, providing pathways towards molecular complexity in space.

Astrochemistry of dust, ice and gas: introduction and overview

“…The Kuiper Airborne Observatory and especially ISO opened up the field of infrared spectroscopy unhindered by the Earth's atmosphere, allowing the first full inventory of interstellar ices. The dominant ice species are simple molecules, H 2 O, CO, CO 2 , CH 4 , NH 3 and CH 3 OH, whose presence has been firmly identified thanks to comparison with laboratory spectroscopy 31,32 . These molecules are precisely the species predicted to be produced by hydrogenation and oxidation of the dominant atoms (O, C and N) and molecules (CO) arriving from the gas on the grains at low temperatures 54 .…”

SURFRESIDE2: An ultrahigh vacuum system for the investigation of surface reaction routes of interstellar interest

“…The effective MWAS O-atom fluxes (i.e., oxygen atoms directly involved in surface reactions) are derived by measuring the final column density of newly formed ozone ice after co-deposition of 16 O atoms and 18 O 2 molecules at 15 K. The reaction O 2 + O → O 3 is barrierless, 39,40 and therefore, we can safely assume that most of the oxygen atoms available for reaction on the surface will recombine to form ozone ice. Codeposition experiments are used in order to avoid limitations in penetration depth of oxygen atoms into molecular oxygen ice.…”

“…6); (ii) measuring the consumption of ozone through hydrogenation reactions and assuming that hydrogen can penetrate only up to 2 ML of ozone ice. In the first case, in order to determine one monolayer of ozone, we first produce 16 O 18 O ice is then exposed to H atoms at 15 K to monitor the destruction of the ozone layer. Assuming that only ∼2 ML of ozone ice is fully hydrogenated, 20 we derive the number of ozone molecules destroyed upon hydrogenation per monolayer, which gives us back the apparent band strength for 16 O 18 O ice is then exposed to D atoms at the same temperature to monitor the destruction of the ozone layer versus the time of exposure.…”

“…6 shows three infrared spectra of ozone ice at different thickness in the range of the ν 3 mode (see Table II). These spectra are acquired during the deuteration of the ∼2 ML of 16 Table I have to be considered as lower limits because (i) fluxes are derived indirectly (effective fluxes), (ii) some of the 16 O can recombine on the surface of the ice, and (iii) 16 O 2 can potentially not be further oxygenated to form ozone because it is trapped into the 18 O 2 matrix. The dissociation rates shown in Table I are obtained by comparing the undissociated molecular component of the beam (i.e., N 2 O, O 2 , N 2 ) measured mass spectrometrically (i.e., with the atom source on and the QMS placed at the center of the main chamber) with the O-and N-atom flux values as derived in Secs.…”

“…Meanwhile it has been confirmed that, except for CO that is efficiently formed in the gas phase, the bulk of interstellar ices (i.e., water, carbon dioxide, methanol, formaldehyde, formic acid, ammonia) is formed in the solid phase through surface reactions. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The focus in this paper is on a new UHV setup able to study atom and radical addition reactions in interstellar a) Current address: Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E. California Blvd., Pasadena, California 91125, USA. Electronic mail: ioppolo@caltech.edu b) Electronic mail: linnartz@strw.leidenuniv.nl ices.…”