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

We simulate experimentally the physical conditions present in dense clouds by means of a high vacuum experimental setup at low temperature T approximate to 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, [(CH2)(6)N-4]), ammonium salts of carboxylic acids [(R-COO-)(NH4+)], amides [H2NC(= O) - R], esters [R - C(= O) - O - R'] and species related to polyoxymethylene (POM, [(-CH2O-)(n)]). Furthermore, evidence is presented for the formation of HMT at room temperature, and the important role of H2O 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.