UV-photoprocessing of interstellar ice analogs: New infrared spectroscopic results

Abstract

Abstract. We simulate experimentally the physical conditions present in dense clouds by means of a high vacuum experimental setup at low temperature T ≈ 12 K. The accretion and photoprocessing of ices on grain surfaces is simulated in the following way: an ice layer with composition analogous to that of interstellar ices is deposited on a substrate window, while being irradiated by ultraviolet (UV) photons. Subsequently the sample is slowly warmed up to room temperature; a residue remains containing the most refractory products of photo-and thermal processing. In this paper we report on the Fourier transform-infrared (FT-IR) spectroscopy of the refractory organic material formed under a wide variety of initial conditions (ice composition, UV spectrum, UV dose and sample temperature). The refractory products obtained in these experiments are identified and the corresponding efficiencies of formation are given. The first evidence for carboxylic acid salts as part of the refractory products is shown. The features in the IR spectrum of the refractory material are attributed to hexamethylenetetramine (HMT, [(CH 2 ) 6 N 4 ]), ammonium salts of carboxylic acids [(R-COO − )(NH + )], amides [H 2 NC(=O)-R], esters [R-C(=O)-O-R ] and species related to polyoxymethylene (POM, [(-CH 2 O-) n ]). Furthermore, evidence is presented for the formation of HMT at room temperature, and the important role of H 2 O ice as a catalyst for the formation of complex organic molecules. These species might also be present in the interstellar medium (ISM) and form part of comets. Ongoing and future cometary missions, such as Stardust and Rosetta, will allow a comparison with the laboratory results, providing new insight into the physico-chemical conditions present during the formation of our solar system.

Detection of cometary amines in samples returned by Stardust

“…Analyses of the ice residues after irradiation using a variety of analytical techniques demonstrate that these materials contain a much more complex suite of volatile species than originally present in the ice as well as a refractory organic component. In addition, hexamethylenetratramine (HMT, C 6 H 12 N 4 ) was observed to be one of the major products produced by irradiation and warming of various astrophysically relevant ice mixtures (Bernstein et al 1995;Muñoz-Caro and Schutte 2003;Muñoz-Caro et al 2004). HMT will produce a variety of amino acids upon acid hydrolysis (Wolman et al 1971) and analyses of irradiated ice residues after acid hydrolysis demonstrate that they do contain a rich mixture of amino acids not present in the original ice mixture (Bernstein et al 2002;Muñoz-Caro et al 2002;Elsila et al 2007;R.…”

MICMOC/MICMOS: Photochemistry of van der Waals solids and the rise of the organic molecular complexity

“…Among them [27] is one of the most extended and pertinent. The IR spectrum displayed here is very generic and does not change much when similar, but somewhat different relative compositions are considered.…”

“…Before we describe the possible relationship between these organic residues and organic matter found in meteorites, it is important to emphasize that acid hydrolysis treatment of such residues, a classical treatment employed to break up the original macromolecules present in them, has held as many as 16 amino-acids [26,27] as well as carboxylic acids [28]. Very recently, di-petides have been found in such residues [29].…”

Synthesis of analogues of cometary organic matter: thermochemical evolution and preparation of in-situ observations

“…Hexamethylenetetramine, also called HMT (C 6 H 12 N 4 ), has already been identified by numerous authors and several analytical techniques in organic residues remaining at 300 K after VUV photolysis of ices mixtures and subsequent heating at 300K [5][6]. Recently, the precise chemical mechanism of HMT formation has been revealed by the PIIM team [7].…”