A. C. F. Santos

Destruction of formic acid by soft X-rays in star-forming regions

Formic acid is much more abundant in the solid state, both in interstellar ices and cometary ices, than in the in- terstellar 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 pho- todissociation 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 f ormation from formic acid may be through the ice phase reaction.

Mass spectrometry study of the fragmentation of valence and core-shell (Cl 2p) excited CHCl3 and CDCl3 molecules

The dissociative photoionization of the chloroform and chloroform-d molecules has been studied in the valence region and around the chlorine 2p edge. Time-of-flight mass spectrometry in the coincidence mode-namely, photoelectron-photoion coincidence (PEPICO)-was employed. He I lamp and tunable synchrotron radiation were used as light sources. Total and partial ion yields have been recorded as a function of the photon energy. Singly, doubly, and triply ionized species have been observed below (195 eV), on (201 eV), and above (230 eV) the Cl 2p resonances. A definite degree of site-selective fragmentation was observed at the Cl 2p resonance as the relative contributions of several ionic species were seen to go through a maximum at 201 eV. At the same time all stable doubly charged ions were also observed at 198 eV (below the 2p resonances), resulting from direct ionization processes. Isotopic substitution is shown to provide a very efficient means of improving the mass resolution and assignment of unresolved peaks in spectra of CHCl(3), particularly for those fragments differing by a hydrogen atom. It is suggested that ultrafast fragmentation of the system following 2p excitation to a strongly antibonding state contributes to the large amount of Cl(+) observed in the PEPICO spectrum measured at 201 eV. Kinetic energy distributions were determined for the H(+), D(+), and Cl(+) fragments.

Production of H+3 via photodissociation of organic molecules in interstellar clouds

We present experimental results obtained from photoionization and photodissociation processes of abundant interstellar CH3-X type organic molecules like methanol (CH3OH), methylamine (CH3NH2) and acetonitrile (CH3CN) as alternative route for the production of H + 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 spect ra were obtained using the photoelectron-photoion coincidence techniques. Absolute averaged cross sections for H + production by soft X-rays were determined. We have found that, among the channels leading to molecular dissociation, the H + yield could reach values up to 0.7% for single photoionization process and up to 4% for process involving double photoionization. The H + 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 × 10 18 cm 2 . Adopting the typical X-ray luminosity

Photodissociation of organic molecules in star-forming regions III. Methanol

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 late 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.

Photodissociation of organic molecules in star-forming regions. II. Acetic acid

The presence of methyl alcohol or methanol (CH3OH) 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 ion- ization, about 11-16% of CH3OH survive the soft X-rays photons. This behavior, together with an effi cient 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 H2 + 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.

Dissociative photoionization of SiF4 around the Si 2p edge: a new TOFMS study with improved mass resolution

Abstract Mass spectra, with improved mass resolution, have been obtained for the SiF 4 molecule, using time-of-flight mass spectrometry and synchrotron radiation, below and above the Si 2p ionization edge (L III , 111.7 eV). New branching ratios for the ionic dissociation have been determined and compared to previous results. The excellent signal-to-noise ratio has also allowed for the first observation of a rearrangement fragment (F 2 + ) in photoionization studies of the SiF 4 molecule.

Ionization and dissociation of cometary gaseous organic molecules by solar wind particles – I. Formic acid

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 ionization 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 four to five 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 per cent larger than the minimum momentum transfer observed with the equivelocity protons. The ultraviolet (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.

Cellular Phones Helping to Get a Clearer Picture of Kinematics.

The main purpose of this paper is to add to the list of examples of how cell phones may be used as teaching tools in the classroom.1 One very interesting example of this comes from the study of projectile motion, the classical “cannon ball” problem. This problem is central to the study of kinematics, the very first topic a student meets in physics. Our approach exploits the fact that these days almost all students carry cellular phones and the vast majority of the phones have built-in cameras.

A multi-coincidence study of the double, triple photoionization and fragmentation of the SiF4 molecule around the Si 2p edge

Abstract Multiple ionization of the SiF 4 molecule has been studied using synchrotron radiation and time-of-flight mass spectrometry in a multi-coincidence mode, in the photon energy range of 70–109.4 eV, which encompasses the Si 2p edge. Photoionization branching ratios have been measured under efficient ion extraction and the results are compared to previously published data. Two electron–ion coincidence techniques (PE2PICO, and PE3PICO) have been used in the elucidation of the fragmentation mechanisms of highly excited molecular ions, formed following the absorption of a high-energy photon. It is shown that the unstable doubly charged parent molecule, SiF 4 2+ , fragments preferentially (90%) via the asymetric mechanism m 2+ → m 1 2+ + m 2 .

Fragmentation of the CH2Cl2 molecule by proton impact and VUV photons

A comparative study for the fragmentation of a CH2Cl2 molecule has been performed for collisions with 0.2–2.0 MeV H+ beam and 12.0–90.0 eV photons using the time-of-flight coincidence technique. Branching ratios for fragmentation products have been determined as a function of the energy of the projectiles. The present results show that the more the proton energy increases, the more the fragmentation pattern resembles the corresponding photon impact spectra at lower energies. For instance, at 2.0 MeV proton impact, the fragmentation pattern closely resembles the corresponding photon impact pattern at hν = 60 eV. From the analysis of the peak shapes, the fragmentation products were found to be formed with low kinetic energies (<1 eV). The main observed fragments, in the proton impact case and for photons above 30 eV, were associated with the release of a chlorine atom. Combining the information from the molecular orbital energies, one estimates the relative contributions of the molecular orbitals to the total ionization of CH2Cl2 by proton impact. It is also shown for the first time that the fragmentation pattern for charged products in the proton impact spectra can be directly compared to the corresponding fragmentation pattern for photon impact, through the transferred momentum, which depends on the projectile velocity as v−1. This is a clear indication that the main dynamical variable behind the branching ratios is the momentum transfer at high velocities.