H2 formation on Mg-rich amorphous silicates

We present the results of an experimental study on the interaction of D atoms with Mg-rich amorphous silicates. The effects of D irradiation have been analyzed by infrared spectroscopy. The results indicate that HD forms by abstraction of hydrogen atoms chemisorbed in the hydroxyl groups of silicate grains. The formation process occurs for grain and atom temperatures relevant to photodissociation regions

H-atom irradiation of solid state formamide at 12 K

The aim of this work is to understand the stability and investigate the chemical evolution of formamide ice due to thermal hydrogenation at simulated interstellar conditions

Synthesis of CO and CO2 Molecules by UV Irradiation of Water Ice-covered Hydrogenated Carbon Grains

We present the results of UV irradiation with Lyα photons of carbon grains with a water ice cap at 11 K. Formation of CO and CO2 molecules takes place during irradiation. An estimation of the formation cross section of these molecules by Lyα photons has been obtained from the intensity increase of their infrared stretching bands as a function of the photon fluence. The fraction of carbon in the grains converted to CO and CO2 by UV photons is 0.06 and 0.05, respectively. The spectral profile of the CO stretching feature and that of the CO2 bending mode indicate a polar environment for these molecules. On the basis of the present laboratory results and those obtained in previous work on ion irradiation of similar samples, it has been possible to estimate the contribution of polar CO and CO2 produced on carbon grains by energetic processing to the observed column densities of these molecules for dense clouds whose visual extinction is known. A significant amount of polar CO and CO2 is produced through the mechanism we have studied. Furthermore, we have found that the laboratory profile of the bending band of CO2 produced on carbon grains is compatible with that observed toward the field star Elias 16

THE EFFECTS OF ION IRRADIATION ON THE EVOLUTION OF THE CARRIER OF THE 3.4 MICRON INTERSTELLAR ABSORPTION BAND

Carbon grains in the interstellar medium evolve through exposure to UV photons, heat, gas, and cosmic rays. Understanding their formation, evolution, and destruction is an essential component of evaluating the composition of the dust available for newly forming planetary systems. The 3.4 lm absorption band, attributed to the aliphatic C"H stretch vibration, is a useful probe of the degree to which energetic processing affects hydrogenated carbon grains. Here we report on the effects of ion bombardment of two different kinds of nano-size hydrogenated carbon grains with different hydrogen content. Grain samples, both with and without a mantle of H2O ice, were irradiated with 30 keV He + to simulate cosmic-ray processing in both diffuse and dense interstellar medium conditions. The ion fluences ranged between 1:5 � 10 13 and 7:9 � 10 15 ions cm � 2 . Infrared and Raman spectroscopy were used to study the effects of ion irradiation on grains. In both the dense and diffuse interstellar medium simulations, ion bombardment led to a reduction of the 3.4 lm band intensity. To discuss the effects of cosmic-ray irradiation of interstellar hydrogenated carbon materials we adopt the approximation of 1 MeV monoenergetic protons. An estimate of the C"H bond destruction cross section by 1 MeV protons was made based on experiments using 30 keV He + ions and model calculations. In combination with results from our previous studies, which focused on UV irradiation and thermal H atom bombardment, the present results indicate that the C"H bond destruction by fastcolliding charged particles is negligible with respect to that of UV photons in the diffuse ISM. However, in dense cloud regions, cosmic-ray bombardment is the most significant C"H bond destruction mechanism when the optical depth corresponds to values of the visual extinction larger than � 5 mag. The results presented here strengthen the new interpretation of the evolution of the interstellar aliphatic component (i.e., the C"H bonds in the CH2 and CH3 groups) as evidenced by the presence of the 3.4 lm absorption band in the diffuse medium and the absence of such a signature in the dense cloud environment. The evolutionary transformation of carbon grains, induced by H atoms, UV photons, and cosmic rays, indicates that C"H bonds are readily formed, in situ, in the diffuse interstellar medium and are destroyed in the dense cloud environment

On the electronic structure of small carbon grains of astrophysical interest

In a previous paper Mennella et al. (1995a) studied the evolution of the UV spectrum of small carbon grains due to thermal annealing in the range 250-800°C. The spectral variations were interpreted in terms of internal structural rearrangement of the grains caused by hydrogen loss. The electronic transitions (Ï-Ï* and Ï-Ï*) of the sp2 clusters forming the grains were indicated as the major factors responsible for determining their extinction properties. In this paper we present the results of new measurements aimed at probing the heat-induced structural changes. The thermal evolution of the optical gap and of the Raman spectrum, both sensitive to the sp2 clustering degree, confirms that the observed spectral changes do depend on structural variations. In fact, the Ï electron delocalization of the sp2 clusters determines a link between structural and electronic properties in carbons. We find a basic correlation between the UV peak position and the optical gap. It is interpreted in terms of a dependence of the dipole matrix momentum of Ï transitions on the sp2 cluster size. The attribution of the spectral changes to the grain internal structure is corroborated by morphological analyses. Scanning and transmission electron microscope images show that the fluffy structure of the samples as well as the dimension and the shape of the single grains do not change after the annealing process. In the astrophysical context, the present results can be relevant for the attribution of the 217.5 nm feature, as they show that the internal structure of carbon grains, having sizes similar to those expected for the "bump" carriers, controls the interaction with UV photons

Laboratory experiments on cosmic dust analogues: the structure of small carbon grains

In this paper we present new results of our experiments aimed to study the internal structure of cosmic analogue carbon grains. The samples, produced by arc discharge between two carbon electrodes in an argon atmosphere, were annealed in the temperature range 250-780°C in order to produce modification of the internal grain structure. These changes were monitored by analysing the variations of the extinction profile between 190 and 2600 nm and of the optical gap as a function of the annealing temperature. The shift of the UV peak position towards longer wavelengths. the overall increase of the extinction coefficient and the closing of the gap as the temperature increases are all consistent with the evolution of carbon grains outlined by Mennella et al. (Astrophys. J., 444, 288, 1995 ; Astrophys. J. Suppl. Ser., 100, 149, 1995). It provides a growth in number and size of the sp2 clusters forming the grains during the annealing. The relevance of the electronic structure of the aromatic clusters in the extinction processes and the dependence of the energy Ï transitions on their size are confirmed by the present results. These results may be relevant in the context of interstellar bump attribution, as they show that the internal structure of small carbon grains is dominant in extinction processes. © 1995

Ultraviolet Spectral Changes in Amorphous Carbon Grains Induced by Ion Irradiation

Small carbon grains, processed by UV radiation and cosmic rays, have been proposed as carriers of the 217.5 nm bump present in the interstellar extinction curves (Hecht 1986; Sorrell 1990). In this paper, we present the results of an experiment aimed at simulating, in a -rst approximation, the cosmic-ray irradiation active in space. We have studied the e†ects induced by 3 keV Heions on the UV spectrum of small cosmic analog carbon grains. Two di†erent kinds of grains have been analyzed. They were produced by vapor conden- sation in hydrogen and argon quenching atmospheres. Spectrophotometric measurements have been carried out on grains as they were produced and after ion irradiation in the spectral range 0.19E2 km. Relevant UV spectral changes are observed after ion irradiation: while the UV absorption band shifts from 203 to 215 nm in hydrogenated amorphous carbon grains, an opposite trend is observed for the samples produced in the argon atmosphere. In this case the UV band moves from 240 to 218 nm. These spectral changes are well correlated with the optical gap variations and are therefore interpreted in terms of grain microstructure changes induced by the interactions with ions. At the highest ion Nuence considered, the two carbons tend to have a similar microstructure, as testi-ed by the UV peak position and optical gap values because of a saturation e†ect of the two competitive processes, amorphization and graphitization, which occur in carbon samples during ion irradiation (Compagnini & Calcagno 1996). The results of the present experiment suggest that hydrogenated amorphous carbon grains cannot be transformed into graphite grains by cosmic-ray irradiation. Moreover, the efficiency of ion irradiation in destroying well-ordered aromatic structures poses the problem of the survival itself of polycrystalline or pure graphite particles in the interstellar medium. Subject headings: cosmic rays E dust, extinction E methods: laboratory E ultraviolet: IS

Thermal treatment under high-vacuum of tars relevant in combustion and material science

The composition of tars, typically derived from coal and heavy fuel processing or formed in fuel-rich combustion, determines their transformation into carbons relevant in combustion and environmental fields as well as for material production. The speciation of the huge number of aromatic components of tars, usually found in form of viscous black liquid or solid, is not straightforward because of the tar complexity and high molecular weight, spanning from few hundreds up to thousands of Da. To this regard, the pre-separation of tar in lighter and heavier fractions simplifies the further characterization of its composition. The present work reports a fractionation method of a typical sample of combustion-formed tar based on moderate heating in high-vacuum conditions (10-6 mbar). It was preliminarily tested on a single polycyclic aromatic hydrocarbon, coronene, and on synthetic mixtures of polycyclic aromatic hydrocarbons, presumed to be the basic aromatic moieties of tar components. Lighter components obtained by condensation/deposition as thin films and/or crystals, as well as the heavier residue, were analysed by optical microscopy and spectroscopy. The separation procedure allowed to get more information on the components distribution also inferring the self-organization in cluster assembly and/or crystal forms