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In this work, we focus on the study of radiation induced desorption processes that occurred in acetonitrile ice irradiated by broadband X-rays (6 eV to 2 keV) monitored by FTIR spectroscopy at different radiation fluences. In a previous work, we used the PROCODA code to derive the chemical evolution of the ice. Here, we have obtained that the acetonitrile desorbed column density is at least two orders of magnitude larger than the desorbed column densities of daughter or granddaughter molecular species at chemical equilibrium stage. This indicates that total desorption column density is mainly governed by the father molecule, as also previously hypothesized in experimental studies. This occurs basically because the acetonitrile column density is larger than the other ones. In particular, at chemical equilibrium acetonitrile desorption column density represents almost 98\% of the total, while it is close to 1\% for H, CN and CH$_2$, the species with larger molecular desorption percentages at chemical equilibrium. Another derived quantity is what we called intrinsic desorption rate, which is a number per second for individual species. Some of the larger intrinsic desorption rates were: CH$_3$CN ($6.2\times 10^{-6}$), CN ($6.2\times 10^{-6}$), H ($5.7\times 10^{-6}$), CH$_2$ ($5.7\times 10^{-6}$) and C$_2$N$_2$ ($4.4\times 10^{-6}$). These results help to put constrain in astrochemical models and can be also useful to clarify some astronomical radio observations., Comment: To appear in MNRAS

Methanol is the most complex molecule securely identified in interstellar ices and is a key chemical species for understanding chemical complexity in astrophysical environments. Important aspects of the methanol ice photochemistry are still unclear such as the branching ratios and photo-dissociation cross-sections at different temperatures and irradiation fluxes. This work aims at a quantitative agreement between laboratory experiments and astrochemical modelling of the CH3OH ice UV photolysis. This work also allows us to better understand which processes govern the methanol ice photochemistry present in laboratory experiments. We use ProDiMo to simulate the conditions of laboratory measurements. The simulations start with simple chemistry consisting only of methanol ice and helium to mimic the residual gas in the experimental chamber. A surface chemical network enlarged by photo-dissociation reactions is used to study the chemical reactions within the ice. Additionally, different surface chemistry parameters (surface competition, tunnelling, thermal diffusion and reactive desorption) are adopted to check those that reproduce the experimental results. The chemical models with ProDiMo can reproduce the methanol ice destruction via UV photodissociation at temperatures of 20, 30, 50 and 70 K as observed in the experiments. We note that the results are sensitive to different branching ratios after photolysis and to the mechanisms of reactive desorption. In the simulations of a molecular cloud at 20 K, we observed an increase in the methanol gas abundance of one order of magnitude, with a similar decrease in the solid-phase abundance. Comprehensive astrochemical models provide new insights into laboratory experiments as the quantitative understanding of the processes that govern the reactions within the ice. Ultimately, those insights can help to better interpret astronomical observations., Comment: Accepted for publication in A&A on 08-Feb-2023

In this study, we employed broadband X-rays ($6-2000$ eV) to irradiate the frozen acetone CH$_3$COCH$_3$, at the temperature of 12 K, with different photon fluences up to $2.7\times 10^{18}$ photons cm$^{-2}$. Here, we consider acetone as a representative complex organic molecule (COM) present on interstellar ice grains. The experiments were conduced at the Brazilian synchrotron facility (LNLS/CNPEN) employing infrared spectroscopy (FTIR) to monitor chemical changes induced by radiation in the ice sample. We determined the effective destruction cross-section of the acetone molecule and the effective formation cross-section for daughter species. Chemical equilibrium, obtained for fluence $2\times 10^{18}$ photons cm$^{-2}$, and molecular abundances at this stage were determined, which also includes the estimates for the abundance of unknown molecules, produced but not detected, in the ice. Timescales for ices, at hypothetical snow line distances, to reach chemical equilibrium around several compact and main-sequence X-ray sources are given. We estimate timescales of 18 days, 3.6 and 1.8 months, $1.4\times 10^9-6\times 10^{11}$ years, 600 and $1.2\times 10^7$ years, and $10^7$ years, for the Sun at 5 AU, for O/B stars at 5 AU, for white dwarfs at 1 LY, for the Crab pulsar at 2.25 LY, for Vela pulsar at 2.25 LY, and for Sagittarius A* at 3 LY, respectively. This study improves our current understanding about radiation effects on the chemistry of frozen material, in particular, focusing for the first time, the effects of X-rays produced by compact objects in their eventual surrounding ices., Comment: Accepted in MNRAS. This is a preprint. 14 pages, 7 figures

The physical evolution of Young Stellar Objects (YSOs) is accompanied by an enrichment of the molecular complexity, mainly triggered by the heating and energetic processing of the astrophysical ices. In this paper, a study of how the ice column density varies across the protostellar evolution has been performed. Tabulated data of H$_2$O, CO$_2$, CH$_3$OH, HCOOH observed by ground- and space-based telescopes toward 27 early-stage YSOs were taken from the literature. The observational data shows that ice column density and spectral index ($\alpha$), used to classify the evolutionary stage, are well correlated. A 2D continuum radiative transfer simulation containing bare and grains covered by ices at different levels of cosmic-ray processing were used to calculate the Spectral Energy Distributions (SEDs) in different angle inclinations between face-on and edge-on configuration. The H$_2$O:CO$_2$ ice mixture was used to address the H$_2$O and CO$_2$ column density variation whereas the CH$_3$OH and HCOOH are a byproduct of the virgin ice after the energetic processing. The simulated spectra were used to calculate the ice column densities of YSOs in an evolutionary sequence. As a result, the models show that the ice column density variation of HCOOH with $\alpha$ can be justified by the envelope dissipation and ice energetic processing. On the other hand, the ice column densities are mostly overestimated in the cases of H$_2$O, CO$_2$ and CH$_3$OH, even though the physical and cosmic-ray processing effects are taken into account., Comment: Accepted for publication in The Astrophysical Journal. 29 pages, 16 figures

In dense and cold regions of the interstellar medium (ISM), molecules may be adsorbed onto dust grains to form the ice mantles. Once formed, they can be processed by ionizing radiation coming from stellar or interstellar medium leading to formation of several new molecules in the ice. Among the different kind of ionizing radiation, cosmic rays play an important role in the solid-phase chemistry because of the large amount of energy deposited in the ices. The physicochemical changes induced by the energetic processing of astrophysical ices are recorded in a intrinsic parameter of the matter called complex refractive index (CRI). In this paper, we present for the first time a catalogue containing 39 complex refractive indices (n, k) in the infrared from 2.0 - 16.6 micrometer for 13 different water-containing ices processed in laboratory by cosmic ray analogs. The calculation was done by using the NKABS (acronym of determination of N and K from ABSorbance data) code, which employs the Lambert-Beer and Kramers-Kronig equations to calculate the values of n and k. The results are also available at the website: http://www1.univap.br/gaa/nkabs-database/data.htm. As test case, a H2O:NH3:CO2:CH4 ice was employed in a radiative transfer simulation of a prototoplanetary disk to show that these data are indispensable to reproduce the spectrum of YSOs containing ices.

We investigate the effects produced mainly by broadband soft X-rays up to 2 keV (plus fast (keV) photoelectrons and low-energy (eV) induced secondary electrons) in the ice mixtures containing H2O:CO2:NH3:SO2 (10:1:1:1) at two different temperatures (50 K and 90 K). The experiments are an attempt to simulate the photochemical processes induced by energetic photons in SO2-containing ices present in cold environments in the ices surrounding young stellar objects (YSO) and in molecular clouds in the vicinity of star-forming regions, which are largely illuminated by soft X-rays. The measurements were performed using a high vacuum portable chamber from the Laboratorio de Astroquimica e Astrobiologia (LASA/UNIVAP) coupled to the spherical grating monochromator (SGM) beamline at the Brazilian Synchrotron Light Source (LNLS) in Campinas, Brazil. In-situ analyses were performed by a Fourier transform infrared (FTIR) spectrometer. Sample processing revealed the formation of several organic molecules, including nitriles, acids, and other compounds such as H2O2, H3O+, SO3, CO, and OCN-. The dissociation cross section of parental species was in the order of 2-7E-18 cm2. The ice temperature seems not to affect the stability for SO2 in the presence of X-rays. Formation cross sections of produced new species were also determined. Molecular half-lives at ices towards YSOs due to the presence of incoming soft X-rays were estimated. The low obtained values, employing two different models of radiation field of YSOs (TW Hydra and typical T Tauri star), reinforce that soft X-rays are indeed a very efficient source of molecular dissociation in such environments., Comment: Accepted to be published on The Astrophysical Journal. Paper contains 30 pages, 12 figures, 7 tables

We present the study of the effects of high energy cosmic rays (CRs) over the astrophysical ices, observed toward the embedded class I protostar Elias 29, by using computational modeling and laboratory data. Its spectrum was observed with {\it Infrared Space Observatory - ISO}, covering 2.3 - 190 $\mu$m. The modeling employed the three-dimensional Monte Carlo radiative transfer code RADMC-3D (Dullemond et al. 2012) and laboratory data of bombarded ice grains by CRs analogs, and unprocessed ices (not bombarded). We are assuming that Elias 29 has a self-irradiated disk with inclination $i =$ 60$^{\circ}$, surrounded by an envelope with bipolar cavity. The results show that absorption features toward Elias 29, are better reproduced by assuming a combination between unprocessed astrophysical ices at low temperature (H$_2$O, CO, CO$_2$) and bombarded ices (H$_2$O:CO$_2$) by high energy CRs. Evidences of the ice processing around Elias 29 can be observed by the good fitting around 5.5-8.0 $\mu$m, by polar and apolar ice segregation in 15.15-15.25 $\mu$m, and by presence of the CH$_4$ and HCOOH ices. Given that non-nitrogen compounds were employed in this work, we assume that absorption around 5.5-8.0 $\mu$m should not be associated with NH$_4^+$ ion (Shutte & Khanna2003), but more probably with aliphatic ethers (e.g. R1-OCH$_2$-R2), CH$_3$CHO and related species. The results obtained in this paper are important, because they show that the environment around protostars is better modeled considering processed samples and, consequently, demonstrates the chemical evolution of the astrophysical ices., Comment: 51 pages, 16 figures, 4 tables

Formic acid (HCOOH) has been extensively detected in space environments, including interstellar medium (gas and grains), comets and meteorites. Such environments are often subjected to the action of ionizing agents, which may cause changes in the molecular structure, thus leading to formation of new species. Formic acid is a possible precursor of pre-biotic species, such as Glycine (NH2CH2COOH). This work investigates experimentally the physicochemical effects resulting from interaction of heavy and energetic cosmic ray analogues (46MeV 58Ni11+) in H2O:HCOOH (1:1) ice, at 15 K, in ultrahigh vacuum regime, using Fourier transform infrared spectrometry in the mid-infrared region (4000-600 cm-1 or 2.5-12.5 microns). After the bombardment, the sample was slowly heated to room temperature. The results show the dissociation cross-section for the formic acid of 2.4x10^-13 cm2, and half-life due to galactic cosmic rays of 8x10^7 yr. The IR spectra show intense formation of CO and CO2, and small production of more complex species at high fluences.

Enceladus, a small icy moon of Saturn, is one of the most remarkable bodies in the solar system. This moon is a geologically active object, and despite the lower temperatures on most of its surface, the geothermally heated south polar region presents geysers that spouts a plume made of water (approximately ninety percent), carbon dioxide, methane, ammonia, and methanol, among other molecules. Most of the upward-moving particles do not have the velocity to escape from the gravitational influence of the moon and fall back to the surface. The molecules in the ice are continuously exposed to ionizing radiation, such as UV and X-rays photons, cosmic rays, and electrons. Over time, the ionizing radiation promotes molecular bond rupture, destroying and also forming molecules, radicals, and fragments. We analyse the processing of an ice mixture analogue to the Enceladus fallout ice in cold resurfaced areas (north pole) by 1 keV electrons. The main goal is to search for complex species that have not yet been detected in this moon, and to determine relevant physico-chemical parameters, such as destruction and formation cross-sections and the half-life of the studied molecules in the ice. The experiment consisted of the electron irradiation of an Enceladus-like ice mixture (H2O,CO2,CH4,NH3, and CH3OH) in an ultra-high vacuum chamber at 20 K. The analysis was made by infrared spectrometry in the mid-infrared region (4000-800 cm-1 or 2.5-12.5 microns).The absolute dissociation cross-sections of the parent molecules, the formation cross-section of daughter species, and the half-life of the parental species in a simulated Enceladus irradiation scenario were determined. Among the produced species, CO (carbon monoxide), OCN- (cyanate anion), HCONH2 (formamide), and H2CO (formaldehyde) were tentatively detected.

The formation of double and triple C-C bonds from the processing of pure c-C6H12 (cyclohexane) and mixed H2O:NH3:c-C6H12 (1:0.3:0.7) ices by highly-charged, and energetic ions (219 MeV O^{7+} and 632 MeV Ni^{24+}) is studied. The experiments simulate the physical chemistry induced by medium-mass and heavy-ion cosmic rays in interstellar ices analogs. The measurements were performed inside a high vacuum chamber at the heavy-ion accelerator GANIL (Grand Accel\'erat\'eur National d'Ions Lourds) in Caen, France. The gas samples were deposited onto a polished CsI substrate previously cooled to 13 K. In-situ analysis was performed by a Fourier transform infrared (FTIR) spectrometry at different ion fluences. Dissociation cross section of cyclohexane and its half-life in astrophysical environments were determined. A comparison between spectra of bombarded ices and young stellar sources indicates that the initial composition of grains in theses environments should contain a mixture of H2O, NH3, CO (or CO2), simple alkanes, and CH3OH. Several species containing double or triple bounds were identified in the radiochemical products, such as hexene, cyclohexene, benzene, OCN-, CO, CO2, as well as several aliphatic and aromatic alkenes and alkynes. The results suggest an alternative scenario for the production of unsaturated hydrocarbons and possibly aromatic rings (via dehydrogenation processes) in interstellar ices induced by cosmic ray bombardment., Comment: 14 pages, 10 figures, 4 tables. Accepted to be published in MNRAS

An experimental photochemistry study involving gas- and solid-phase amino acids (glycine, DL-valine, DL-proline) and nucleobases (adenine and uracil) under soft X-rays was performed. The aim was to test the molecular stabilities of essential biomolecules against ionizing photon fields inside dense molecular clouds and protostellar disks analogs. In these environments, the main energy sources are the cosmic rays and soft X-rays. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing 150 eV photons. In-situ sample analysis was performed by Time-of-flight mass spectrometer (TOF-MS) and Fourier transform infrared (FTIR) spectrometer, for gas- and solid- phase analysis, respectively. The half-life of solid phase amino acids, assumed to be present at grain mantles, is at least 3E5 years and 3E8 years inside dense molecular clouds and protoplanetary disks, respectively. We estimate that for gas-phase compounds these values increase one order of magnitude since the dissociation cross section of glycine is lower at gas-phase than at solid phase for the same photon energy. The half-life of solid phase nucleobases is about 2-3 orders of magnitude higher than found for amino acids. The results indicate that nucleobases are much more resistant to ionizing radiation than amino acids. We consider these implications for the survival and transfer of biomolecules in space environments., Comment: 10 pages, 5 figures, 2 tables. Accepted to be published in MNRAS

An experimental study on the interaction of heavy, highly charged, and energetic ions (52 MeV Ni^13+) with pure H2O, pure CO2 and mixed H2O:CO2 astrophysical ice analogs is presented. This analysis aims to simulate the chemical and the physicochemical interactions induced by heavy cosmic rays inside dense and cold astrophysical environments such as molecular clouds or protostellar clouds. The measurements were performed at the heavy ion accelerator GANIL (Grand Accelerateur National d'Ions Lourds in Caen, France). The gas samples were deposited onto a CsI substrate at 13 K. In-situ analysis was performed by a Fourier transform infrared (FTIR) spectrometer at different fluences. Radiolysis yields of the produced species were quantified. The dissociation cross sections of pure H2O and CO2 ices are 1.1 and 1.9E-13 cm^2, respectively. In the case of mixed H2O:CO2 (10:1) the dissociation cross sections of both species are about 1E-13 cm^2. The measured sputtering yield of pure CO2 ice is 2.2E4 molec/ion. After a fluence of 2-3E12 ions/cm^2 the CO2/CO ratio becomes roughly constant (~0.1), independent of the of initial CO2/H2O ratio. A similar behavior is observed for the H2O2/H2O ratio which stabilizes at 0.01, independent of the initial H2O column density or relative abundance., Comment: 13 pages, 9 figures, 4 tables. Accepted to be published in Astronomy and Astrophysics

Deeply inside dense molecular clouds and protostellar disks, the interstellar ices are protected from stellar energetic UV photons. However, X-rays and energetic cosmic rays can penetrate inside these regions triggering chemical reactions, molecular dissociation and evaporation processes. We present experimental studies on the interaction of heavy, highly charged and energetic ions (46 MeV Ni^13+) with ammonia-containing ices in an attempt to simulate the physical chemistry induced by heavy ion cosmic rays inside dense astrophysical environments. The measurements were performed inside a high vacuum chamber coupled to the heavy ion accelerator GANIL (Grand Accelerateur National d'Ions Lourds) in Caen, France.\textit{In-situ} analysis is performed by a Fourier transform infrared spectrometer (FTIR) at different fluences. The averaged values for the dissociation cross section of water, ammonia and carbon monoxide due to heavy cosmic ray ion analogs are ~2x10^{-13}, 1.4x10^{-13} and 1.9x10^{-13} cm$^2$, respectively. In the presence of a typical heavy cosmic ray field, the estimated half life for the studied species is 2-3x10^6 years. The ice compaction (micropore collapse) due to heavy cosmic rays seems to be at least 3 orders of magnitude higher than the one promoted by (0.8 MeV) protons . In the case of the irradiated H2O:NH3:CO ice, the infrared spectrum at room temperature reveals five bands that were tentatively assigned to vibration modes of the zwitterionic glycine (+NH3CH2COO-)., Comment: Accepted to be published in Astronomy and Astrophysics; Number of pages: 12; Number of Figures: 7; Number of Tables: 5

Titan, the largest satellite of Saturn, has an atmosphere chiefly made up of N2 and CH4 and includes traces of many simple organic compounds. This atmosphere also partly consists of haze and aerosol particles which during the last 4.5 gigayears have been processed by electric discharges, ions, and ionizing photons, being slowly deposited over the Titan surface. In this work, we investigate the possible effects produced by soft X-rays (and secondary electrons) on Titan aerosol analogs in an attempt to simulate some prebiotic photochemistry. The experiments have been performed inside a high vacuum chamber coupled to the soft X-ray spectroscopy beamline at the Brazilian Synchrotron Light Source, Campinas, Brazil. In-situ sample analyses were performed by a Fourier transform infrared spectrometer. The infrared spectra have presented several organic molecules, including nitriles and aromatic CN compounds. After the irradiation, the brownish-orange organic residue (tholin) was analyzed ex-situ by gas chromatographic (GC/MS) and nuclear magnetic resonance (1H NMR) techniques, revealing the presence of adenine (C5H5N5), one of the constituents of the DNA molecule. This confirms previous results which showed that the organic chemistry on the Titan surface can be very complex and extremely rich in prebiotic compounds. Molecules like these on the early Earth have found a place to allow life (as we know) to flourish., Comment: To appear in Journal of Physical Chemistry A.; Number of pages: 6; Number of Figures: 5; Number of Tables: 1; Number of references:49; Full paper at http://pubs.acs.org/doi/abs/10.1021/jp902824v

Benzene molecules, present in the proto-planetary nebula CRL 618, are ionized and dissociated by UV and X-ray photons originated from the hot central star and by its fast wind. Ionic species and free radicals produced by these processes can lead to the formation of new organic molecules. The aim of this work is to study the photoionization and photodissociation processes of the benzene molecule, using synchrotron radiation and time of flight mass spectrometry. Mass spectra were recorded at different energies corresponding to the vacuum ultraviolet (21.21 eV) and soft X-ray (282-310 eV) spectral regions. The production of ions from the benzene dissociative photoionization is here quantified, indicating that C6H6 is more efficiently fragmented by soft X-ray than UV radiation, where 50% of the ionized benzene molecules survive to UV dissociation while only about 4% resist to X-rays. Partial ion yields of H+ and small hydrocarbons such as C2H2+, C3H3+ and C4H2+ are determined as a function of photon energy. Absolute photoionization and dissociative photoionization cross sections have also been determined. From these values, half-life of benzene molecule due to UV and X-ray photon fluxes in CRL 618 were obtained., Comment: The paper contains 8 pages, 9 figures and 4 tables. Accepted to be published on MNRAS on 2008 November 26

We present experimental studies on the photoionization and photodissociation processes (photodestruction) of gaseous amino acids and nucleobases in interstellar and interplanetary radiation conditions analogs. The measurements have been undertaken at the Brazilian Synchrotron Light Laboratory (LNLS), employing vacuum ultraviolet (VUV) and soft X-ray photons. The experimental set up basically consists of a time-of-flight mass spectrometer kept under high vacuum conditions. Mass spectra were obtained using photoelectron photoion coincidence technique. We have shown that the amino acids are effectively more destroyed (up to 70-80%) by the stellar radiation than the nucleobases, mainly in the VUV. Since polycyclic aromatic hydrocarbons have the same survival capability and seem to be ubiquitous in the ISM, it is not unreasonable to predict that nucleobases could survive in the interstellar medium and/or in comets, even as a stable cation., Comment: 4 pages, 2 figures. To be published in the Proceedings of the IAU-251 symposium - Organic Mater in Space, Hong Kong, China (2008)

We present experimental results obtained from photoionization and photodissociation processes of abundant interstellar methanol (CH$_3$OH) as an alternative route for the production of H$_3^+$ in dense clouds. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS) employing soft X-ray and time-of-flight mass spectrometry. Mass spectra were obtained using the photoelectron-photoion coincidence techniques. Absolute averaged cross sections for the production of H$_3^+$ due to molecular dissociation of methanol by soft X-rays (C1s edge) were determined. The H$_3^+$'s photoproduction rate and column density were been estimated adopting a typical soft X-ray luminosity inside dense molecular and the observed column density of methanol. Assuming a steady state scenario, the highest column density value for the photoproduced H$_3^+$ was about 10$^{11}$ cm$^2$, which gives the ratio photoproduced/observed of about 0.05%, as in the case of dense molecular cloud AFGL 2591. Despite the small value, this represent a new and alternative source of H$_3^+$ into dense molecular clouds and it is not been considered as yet in interstellar chemistry models., Comment: To be published in the Proceedings of IAU-251 Symposium - Organic Matter in Space, Hong Kong, China

The presence of methyl alcohol or methanol (CH$_3$OH) in several astrophysical environments has been characterized by its high abundance that depends on both the production rate and the destruction rate. In the present work, the photoionization and photodissociation processes of methanol have been experimentally studied, employing soft X-ray photons (100-310 eV) from a toroidal grating monochromator (TGM) beamline of the Brazilian Synchrotron Light Laboratory (LNLS). Mass spectra were obtained using the photoelectron photoion coincidence (PEPICO) method. Kinetic energy distribution and abundances for each ionic fragment have been obtained from the analysis of the corresponding peak shapes in the mass spectra. Absolute photoionization and photodissociation cross sections were also determined. We have found, among the channels leading to ionization, about 11-16% of CH$_3$OH survive the soft X-rays photons. This behavior, together with an efficient formation pathways, may be associated with the high column density observed in star-forming regions. The three main photodissociation pathways are represented by COH$^+$ (or HCO$^+$) ion release (with ejection of H$_2$ + H), the dissociation via C-O bond rupture (with strong charge retention preferentially on the methyl fragment) and the ejection of a single energetic (2-4 eV) proton. Since methanol is very abundant in star forming regions, the produced protons could be an alternative route to molecular hydrogenation or a trigger for secondary dissociation processes or even to promote extra heating of the environment., Comment: The paper contains 7 pages, 4 figures and 3 tables. Accepted to be published on A&A on 2006 December 11

We present experimental results obtained from photoionization and photodissociation processes of abundant interstellar CH$_3$-X type organic molecules like methanol (CH$_3$OH), methylamine (CH$_3$NH$_2$) and acetonitrile (CH$_3$CN) as alternative route for the production of H$_3^+$ in interstellar and star forming environments. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons with energies between 200 and 310 eV and time of flight mass spectrometry. Mass spectra were obtained using the photoelectron-photoion coincidence techniques. Absolute averaged cross sections for H$_3^+$ production by soft X-rays were determined. We have found that, among the channels leading to molecular dissociation, the H$_3^+$ yield could reach values up to 0.7% for single photoionization process and up to 4% for process involving double photoionization. The H$_3^+$ photoproduction cross section due to the dissociation of the studied organic molecules by photons over the C1s edge (200-310 eV) were about 0.2-1.4 $\times$ 10$^{-18}$ cm$^2$. Adopting the typical X-ray luminosity $L_X \gtrsim 10^{31}$ erg s$^{-1}$ which best fit the observational data for AFGL 2591 (Stauber et al. 2005) we derive an estimative for the H$_3^+$ photoproduction rate due to methyl-compounds dissociation process. The highest value for the H$_3^+$ column density from methanol dissociation by soft X-rays, assuming a steady state scenario, was about $10^{11}$ cm$^2$, which gives the fraction of the photoproduced H$_3^+$ of about 0.05%, as in the case of dense molecular cloud AFGL 2591. Despite the extreme small value, this represent a new and alternative source of H$_3^+$ into dense molecular clouds and it is not been considered as yet in interstellar chemistry models., Comment: The paper contains 7 pages, 3 figures and 3 tables. Accepted to be published on MNRAS on 2006 December 14. Received 2006 December 14; in original form 2006 September 19

In order to simulate the effects of energetic charged particles present in the solar wind colliding with the cometary gaseous formic acid molecule (HCOOH), laboratory experiments have been performed. The absolute ionisation and dissociation cross sections for this molecule interacting with solar wind particles were measured employing fast electrons in the energy range of 0.5 to 2 keV and energetic protons with energies varying from 0.128 to 2 MeV. Despite the fact that both projectiles lead to a very similar fragmentation pattern, differences in the relative intensities of the fragments were observed. Formic acid survives about 4-5 times more to the proton beam than to the energetic electron collision.The minimum momentum transfer in the electron impact case was estimated to be 3-38% larger than the minimum momentum transfer observed with the equivelocity protons. The UV photodissociation rates and half-lives for HCOOH are roughly closer to the values obtained with energetic electrons. It is consequently important to take electron impact data into account when developing chemical models to simulate the interplanetary conditions., Comment: 11 pages, 7 figures, 5 tables, Accepted to be published in MNRAS

Fragments from organic molecule dissociation (such as reactive ions and radicals) can form interstellar complex molecules like amino acids. The goal of this work is to experimentally study photoionization and photodissociation processes of acetic acid (CH$_3$COOH), a glycine (NH$_2$CH$_2$COOH) precursor molecule, by soft X-ray photons. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons from a toroidal grating monochromator (TGM) beamline (100 - 310 eV). Mass spectra were obtained using the photoelectron photoion coincidence (PEPICO) method. Kinetic energy distribution and abundances for each ionic fragment have been obtained from the analysis of the corresponding peak shapes in the mass spectra. Absolute photoionization and photodissociation cross sections were also determined. We have found, among the channels leading to ionization, that only 4-6% of CH$_3$COOH survive the strong ionization field. CH$_3$CO$^+$, COOH$^+$ and CH$_3^+$ ions are the main fragments, and the presence of the former may indicate that the production-destruction process of acetic acid in hot molecular cores (HMCs) could decrease the H$_2$O abundance since the net result of this process converts H$_2$O into OH + H$^+$. The COOH$^+$ ion plays an important role in ion-molecule reactions to form large biomolecules like glycine., Comment: Comments: 8 pages, 5 figures, 3 tables. Accepted to be printed in A&A

Formic acid is much more abundant in the solid state, both in interstellar ices and cometary ices, than in the interstellar gas (ice/gas ~ 10^{4}) and this point remains a puzzle. The goal of this work is to experimentally study ionization and photodissociation processes of HCOOH (formic acid), a glycine precursor molecule. The measurements were taken at the Brazilian Synchrotron Light Laboratory (LNLS), employing soft X-ray photons from toroidal grating monochromator TGM) beamline (200 - 310 eV). Mass spectra were obtained using photoelectron photoion coincidence (PEPICO) method. Kinetic energy distributions and abundances for each ionic fragment have been obtained from the analysis of the corresponding peak shapes in the mass spectra. Photoionization and photodissociation cross sections were also determined. Due to the large photodissociation cross section of HCOOH it is possible that in PDRs regions, just after molecules evaporation from the grain surface, formic acid molecules are almost totally destroyed by soft X-rays, justifying the observed low abundance of HCOOH in the gaseous phase. The preferential path for the glycine formation from formic acid may be through the ice phase reaction. Keywords: HCOOH; Photoionization; X-rays; Astrochemistry., Comment: 9 pages, 7 figures, 2 tables. Accepted to be printed in A&A