No abstract

Infrared emission features assigned to gas phase polycyclic aromatic hydrocarbons (PAHs) are observed in space along many lines of sight. In regions where interstellar ices are present, these emissions are largely quenched. It is here that PAHs form agglomerates covered by ice or freeze out on to dust grains, together with volatile species such as H2O, CO, CO2 and NH3. Upon exposure to the Lyα‐dominated interstellar radiation field, PAHs are expected to participate in photo‐induced chemical reactions, explicitly also involving the surrounding ice matrix. In this paper, a systematic laboratory‐based study is presented for the solid‐state behaviour of the PAHs pyrene and benzo[ghi]perylene upon Lyα irradiation in ammonia and mixed NH3:H2O astronomical ice analogues. The results are compared to recently published work focusing on a pure water ice environment. It is found that the ice matrix acts as an ‘electronic solid‐state switch’ in which the relative amount of water and ammonia determines whether positively or negatively charged PAHs form. In pure water ice, cations are generated through direct ionization, whereas in pure ammonia ice, anions form through electron donation from ammonia‐related photoproducts. The solid‐state process controlling this latter channel involves electron transfer, rather than acid–base type proton transfer. In the mixed ice, the resulting products depend on the mixing ratio. The astronomical consequences of these laboratory findings are discussed.

Context. Observations and models show that polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the interstellar medium. Like other molecules in dense clouds, PAHs accrete onto interstellar dust grains, where they are embedded in an ice matrix dominated by water. In the laboratory, mixed molecular ices (not containing PAHs) have been extensively studied using Fourier transform infrared absorption spectroscopy. Experiments including PAHs in ices have started, however, the concentrations used are typically much higher than the concentrations expected for interstellar ices. Optical spectroscopy offers a sensitive alternative. Aims. We report an experimental study of the effect PAH concentration has on the electronic spectra and the vacuum UV (VUV) driven processes of PAHs in water-rich ices. The goal is to apply the outcome to cosmic ices. Methods. Optical spectroscopic studies allow us to obtain in-situ and quasi real-time electronic solid state spectra of two prototypical PAHs (pyrene and coronene) embedded in water ice under VUV photoprocessing. The study is carried out on PAH:H 2 O concentrations in the range of 1:30 000 to pure PAH, covering the temperature range from 12 to 125 K. Results. PAH concentration strongly influences the efficiency of PAH cation formation. At low concentrations, ionization efficiencies are over 60% dropping to about 15% at 1:1000. Increasing the PAH concentration reveals spectral broadening in neutral and cation PAH spectra attributed to PAH clustering inside the ice. At the PAH concentrations expected for interstellar ices, some 10 to 20% may be present as cations. The presence of PAHs in neutral and ion form will add distinctive absorption bands to cosmic ice optical spectra and this may serve as a tool to determine PAH concentrations.

Aims. Gas phase acetylene (C 2 H 2 ) and polyynes (H(-C≡C-) m H) are ubiquitous in the interstellar medium. However, astrochemical models systematically underestimate the observed abundances, supporting the idea that enrichment from the solid state takes place. In this laboratory-based study, we investigate the role C 2 H 2 plays in interstellar ice chemistry and we discuss the way its photoproducts may affect gas phase compositions. Methods. C 2 H 2 ice is investigated under vacuum ultraviolet (VUV) irradiation in its pure form as present in the atmosphere of Titan and in a water-dominated ice as present on grain mantles in molecular clouds and on comets. To disentangle the photochemical network, a unique, complementary combination of infrared and ultraviolet-visible (UV-VIS) spectroscopy is used. Results. From the experimental results, it can be concluded that the VUV-induced solid state C 2 H 2 reaction network is dominated by polymerization resulting in the formation of polyynes at least up to C 20 H 2 and larger polyyne-like molecules. At low temperatures, this process takes place very efficiently and suggests low barriers. When extending this reaction scheme to a water-rich environment, the dominant reaction products are CO and CO 2 but the simultaneous detection of polyyne like molecules is evidence that the reactions as observed in pure C 2 H 2 ice persist. Conclusions. From the spectroscopic evidence as presented in this laboratory study, it is concluded that the formation of polyynes upon VUV irradiation of interstellar ices is a process that may contribute to at least part of the observed gas phase enrichment in space.

Vacuum-Ultraviolet (VUV) radiation is responsible for the photo-processing of simple and complex molecules in several terrestrial and extraterrestrial environments. In the laboratory such radiation is commonly simulated by inexpensive and easy-to-use microwave-powered hydrogen discharge lamps. However, VUV flux measurements are not trivial and the methods/devices typically used for this purpose, mainly actinometry and calibrated VUV silicon photodiodes, are not very accurate or expensive and lack of general suitability to experimental setups. Here, we present a straightforward method for measuring the VUV photon flux based on the photoelectric effect and using a gold photodetector. This method is easily applicable to most experimental setups, bypasses the major problems of the other methods, and provides reliable flux measurements. As a case study, the method is applied to a microwave-powered hydrogen discharge lamp. In addition, the comparison of these flux measurements to those obtained by O2 actinometry experiments allow us to estimate the quantum yield (QY) values QY122 = 0.44 ± 0.16 and QY160 = 0.87 ± 0.30 for solid-phase O2 actinometry.