39 research outputs found
Single and Double Photoionization and Photodissociation of Toluene by Soft X-rays in Circumstellar Environment
The formation of polycyclic aromatic hydrocarbons (PAHs) and their methyl
derivatives occurs mainly in the dust shells of asymptotic giant branch (AGB)
stars. The bands at 3.3 and 3.4 m, observed in infrared emission spectra
of several objects, are attributed C-H vibrational modes in aromatic and
aliphatic structures, respectively. In general, the feature at 3.3 m is
more intense than the 3.4 m. Photoionization and photodissociation
processes of toluene, the precursor of methylated PAHs, were studied using
synchrotron radiation at soft X-ray energies around the carbon K edge with
time-of-flight mass spectrometry. Partial ion yields of a large number of ionic
fragments were extracted from single and 2D-spectra, where electron-ion
coincidences have revealed the doubly charged parent-molecule and several
doubly charged fragments containing seven carbon atoms with considerable
abundance. \textit{Ab initio} calculations based on density functional theory
were performed to elucidate the chemical structure of these stable dicationic
species. The survival of the dications subjected to hard inner shell ionization
suggests that they could be observed in the interstellar medium, especially in
regions where PAHs are detected. The ionization and destruction of toluene
induced by X-rays were examined in the T Dra conditions, a carbon-rich AGB
star. In this context, a minimum photodissociation radius and the half-life of
toluene subjected to the incidence of the soft X-ray flux emitted from a
companion white dwarf star were determined.Comment: 11 pages, 4 figures, accept for publication in Ap
Molecules with a peptide link in protostellar shocks: a comprehensive study of L1157
Interstellar molecules with a peptide link -NH-C(=O)-, like formamide
(NHCHO), acetamide (NHCOCH) and isocyanic acid (HNCO) are
particularly interesting for their potential role in pre-biotic chemistry. We
have studied their emission in the protostellar shock regions L1157-B1 and
L1157-B2, with the IRAM 30m telescope, as part of the ASAI Large Program.
Analysis of the line profiles shows that the emission arises from the outflow
cavities associated with B1 and B2. Molecular abundance of
and are derived for
formamide and isocyanic acid, respectively, from a simple rotational diagram
analysis. Conversely, NHCOCH was not detected down to a relative
abundance of a few . B1 and B2 appear to be among the richest
Galactic sources of HNCO and NHCHO molecules. A tight linear correlation
between their abundances is observed, suggesting that the two species are
chemically related. Comparison with astrochemical models favours molecule
formation on ice grain mantles, with NHCHO generated from hydrogenation of
HNCO.Comment: 11 pages, 9 figures. Accepted for publication in MNRAS Main Journal.
Accepted 2014 August 19, in original form 2014 July
Dissociation of the benzene molecule by UV and soft X-rays in circumstellar environment
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 2
Ionisation 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 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 MNRA
Photostability of gas- and solid-phase biomolecules within dense molecular clouds due to soft X-rays
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 MNRA
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 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&
Photodissociation of organic molecules in star-forming regions
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
(CH3COOH), a glycine (NH2CH2COOH) 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 CH3COOH survive the strong
ionization field. CH3CO+, COOH+ and CH 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 H2O abundance since the net result of
this process converts H2O into OH + H+. The COOH+ ion plays
an important role in ion-molecule reactions to form large
biomolecules like glycine.