W. Wolff

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.

X-ray photodesorption and proton destruction in protoplanetary discs: pyrimidine

The organic compounds HCN and C2H2, present in protoplanetary disks, may react to form precursor molecules of the nucleobases, such as the pyrimidine molecule, C4H4N2. Depending on the temperature in a given region of the disk, molecules are in the gas phase or condensed onto grain surfaces. The action of X-ray photons and MeV protons, emitted by the young central star, may lead to several physical and chemical processes in such prestellar environments. In this work we have experimentally investigated the ionization, dissociation and desorption processes of pyrimidine in the condensed and the gas phase stimulated by soft X-rays and protons, respectively. Pyrimidine was frozen at temperatures below 130 K and irradiated with X-rays at energies from 394 to 427 eV. In the gas phase experiment, a pyrimidine effusive jet at room temperature was bombarded with protons of 2.5 MeV. In both experiments, the time-of-flight mass-spectrometry technique was employed. Partial photodesorption ion yields as a function of the X-ray photon energy for ions such as C3H2+, HC3NH+ and C4H+ were determined. The experimental results were applied to conditions of the protoplanetary disk of TW Hydra star. Assuming three density profiles of molecular hydrogen, 1 x 10^6, 1 x 10^7 and 1 x 10^8 cm^-3, we determined HC3NH+ ion-production rates of the order of 10^-31 up to 10^-8 ions cm^-3 s^-1. Integrating over 1 x 10^6 yr, HC3NH^+ column density values, ranging from 3.47 x 10^9 to 1.29 x 10^13 cm^-2, were obtained as a function of the distance from central star. The optical depth is the main variable that affects ions production. In addition, computational simulations were used to determine the kinetic energies of ions desorbed from pyrimidine ice distributed between ~ 7 and 15 eV.

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.

Electron-detachment cross sections of halogen negative-ion projectiles for inertial confinement fusion

Negative-ion beams have recently been suggested as sources of high-energy heavy atoms to be used as drivers for inertial confinement fusion (ICF). Owing to their electron affinities limited to a few eV, anions can be efficiently photo-detached in the vicinity of the fusion chamber, with the resulting high-velocity neutral projectiles following ballistic trajectories towards the hydrogen pellet target. Electron-detachment cross sections are needed as parameters to estimate the beam attenuation in the path from the ion source to the hydrogen pellet. Halogen anions are possible projectile choices. In this paper we present experimental data for total electron-detachment cross sections for F-, Cl-, Br - and I- ions incident on N 2 , in the 0.94-74 ke V u -1 energy range. Our measurements can benchmark theory on anion electron detachment at intermediate to high velocities. Comparison between different projectiles shows very similar collision velocity dependencies. A simple geometrical scaling is presented, providing an estimate for electron-detachment cross sections at the Me V u -1 energy range. The presented scaling indicates that the vacuum requirements due to the use of halogen anions for ICF are less critical than previously suggested.

Strong Electronic Selectivity in the Shallow Core Excitation of the CH2Cl2 Molecule.

The photoexcitation and multiphotoionization of the dichloromethane molecule have been studied for photons with energies from 100 eV to the Cl 2p edge, using the time-of-flight multicoincidence technique and synchrotron radiation. The electronic de-excitation gives rise to one to three electrons and an ionic molecule that decays onto smaller moieties through several fragmentation channels. To discern the channels, sets of fragments have been dispersed in time, measured in coincidence, and recorded as a function of incident photon energy. The chlorine ion, Cl(+), has the highest intensity around and above the Cl 2p edge, while the CHnCl(+) ion, corresponding to the loss of one neutral chlorine atom, dominates the mass spectra in the valence region. In addition, strong electronic selectivity has been observed for the core-excited molecule.

Cross-section measurements for the fragmentation of CHClF2 by electron impact

CFC compounds present in the upper atmosphere have a significant effect on the environment, strongly contributing to the increase of the hole in the ozone layer. Recent studies show that low-energy electron impact is an important process in the dissociation of these molecules, creating atomic chlorine, which breaks down ozone molecules. In this work, the CHClF2 fragmentation by electron impact in the 40–400 eV energy range is measured. Total and partial cross sections have been obtained, showing the predominance of the release of neutral chlorine, which amounts to around 60% of the total yield. There is a strong indication that this chlorine is being released as a result of the ionization of electrons from both chlorine and fluorine orbitals.

Multiple ionization of neon induced by Li3+ and C3+ projectiles: influence of projectile screening in the ionization and electron capture channels

Neq + (q = 1,2,3,4) ionization and charge exchange cross sections (total electron capture, single electron capture and transfer ionization) in the collisions with Li3+, with energies between 100 and 900 keV amu−1, and C3+, with energies between 250 and 500 keV amu−1 are reported. Bare Li3+ projectiles give a key benchmark to study the role of projectile screening in collisions involving dressed projectile ions, and the measurements have shown a strong screening effect for all n-fold recoil ion charge states in the ionization channel which, unexpectedly, does not appear for transfer ionization.

Ionization and dissociation of the formic acid molecule by protons and electrons

Ionization and dissociation of the formic acid molecule (HCOOH) by energetic charged particles have been studied for 1 keV electrons and nearly equivelocity 2 MeV protons. Whereas the fragmentation pattern induced by impact of electron and proton show close similarities, some distinctions in the fragmentation yields were observed for these equivelocity projectiles.

Kinetic Energy Release of the Singly and Doubly Charged Methylene Chloride Molecule: The Role of Fast Dissociation

The center of mass kinetic energy release distribution (KERD) spectra of selected ionic fragments, formed through dissociative single and double photoionization of CH2Cl2 at photon energies around the Cl 2p edge, were extracted from the shape and width of the experimentally obtained time-of-flight (TOF) distributions. The KERD spectra exhibit either smooth profiles or structures, depending on the moiety and photon energy. In general, the heavier the ionic fragments, the lower their average KERDs are. In contrast, the light H(+) fragments are observed with kinetic energies centered around 4.5-5.5 eV, depending on the photon energy. It was observed that the change in the photon energy involves a change in the KERDs, indicating different processes or transitions taking place in the breakup process. In the particular case of double ionization with the ejection of two charged fragments, the KERDs present have characteristics compatible with the Coulombic fragmentation model. Intending to interpret the experimental data, singlet and triplet states at Cl 2p edge of the CH2Cl2 molecule, corresponding to the Cl (2p → 10a1*) and Cl (2p → 4b1*) transitions, were calculated at multiconfigurational self-consistent field (MCSCF) level and multireference configuration interaction (MRCI). These states were selected to form the spin-orbit coupling matrix elements, which after diagonalization result in a spin-orbit manifold. Minimum energy pathways for dissociation of the molecule were additionally calculated aiming to give support to the presence of the ultrafast dissociation mechanism in the molecular breakup.

Matrix isolation sublimation: An apparatus for producing cryogenic beams of atoms and molecules

We describe the apparatus to generate cryogenic beams of atoms and molecules based on matrix isolation sublimation. Isolation matrices of Ne and H2 are hosts for atomic and molecular species which are sublimated into vacuum at cryogenic temperatures. The resulting cryogenic beams are used for high-resolution laser spectroscopy. The technique also aims at loading atomic and molecular traps.