Thomas Pflüger

Fivefold differential cross sections for ground-state ionization of aligned H2 by electron impact

We discuss the ionization of aligned hydrogen molecules into their ionic ground state by 200 eV electrons. Using a reaction microscope, the complete electron scattering kinematics is imaged over a large solid angle. Simultaneously, the molecular alignment is derived from postcollision dissociation of the residual ion. It is found that the ionization cross section is maximized for small angles between the internuclear axis and the momentum transfer. Fivefold differential cross sections (5DCSs) reveal subtle differences in the scattering process for the distinct alignments. We compare our observations with theoretical 5DCSs obtained with an adapted molecular three-body distorted wave model that reproduces most of the results, although discrepancies remain.

An (e, 2e + ion) study of low-energy electron-impact ionization and fragmentation of tetrahydrofuran with high mass and energy resolutions.

We study the low-energy (E0 = 26 eV) electron-impact induced ionization and fragmentation of tetrahydrofuran using a reaction microscope. All three final-state charged particles, i.e., two outgoing electrons and one fragment ion, are detected in triple coincidence such that the momentum vectors and, consequently, the kinetic energies for charged reaction products are determined. The ionic fragments are clearly identified in the experiment with a mass resolution of 1 amu. The fragmentation pathways of tetrahydrofuran are investigated by measuring the ion kinetic energy spectra and the binding energy spectra where an energy resolution of 1.5 eV has been achieved using the recently developed photoemission electron source. Here, we will discuss the fragmentation reactions for the cations C4H8O(+), C4H7O(+), C2H3O(+), C3H6(+), C3H5(+), C3H3(+), CH3O(+), CHO(+), and C2H3(+).

Wavelength Dependence of the Suppressed Ionization of Molecules in Strong Laser Fields

We study ionization of molecules by an intense laser field over a broad wavelength regime, ranging from 0.8 to 1.5 μm experimentally and from 0.6 to 10 μm theoretically. A reaction microscope is combined with an optical parametric amplifier to achieve ionization yields in the near-infrared wavelength regime. Calculations are done using the strong-field S-matrix theory and agreement is found between experiment and theory, showing that ionization of many molecules is suppressed compared to the ionization of atoms with identical ionization potentials at near-infrared wavelengths at around 0.8 μm, but not at longest wavelengths (10 μm). This is due to interference effects in the electron emission that are effective at low photoelectron energies but tend to average out at higher energies. We observe the transition between suppression and nonsuppression of molecular ionization in the near-infrared wavelength regime (1-5 μm).

Low energy (e, 2e) study from the 1t(2) orbital of CH4.

Single ionization of the methane (CH(4)) 1t(2) orbital by 54 eV electron impact has been studied experimentally and theoretically. The measured triple differential cross sections cover nearly a 4π solid angle for the emission of low energy electrons and a range of projectile scattering angles. Experimental data are compared with theoretical calculations from the distorted wave Born approximation and the molecular three-body distorted wave models. It is found that theory can give a proper description of the main features of experimental cross section only at smaller scattering angles. For larger scattering angles, significant discrepancies between experiment and theory are observed. The importance of the strength of nuclear scattering from the H-nuclei was theoretically tested by reducing the distance between the carbon nuclei and the hydrogen nuclei and improved agreement with experiment was found for both the scattering plane and the perpendicular plane.

Tuning the internuclear distance in ionization of H2

The dependence of triply differential cross sections on the internuclear distance R is explored for ionization of molecular hydrogen by 200 eV electrons for two distinct values R = 1.1 and 1.4 au in comparison with helium as an isoelectronic system with a single nucleus (R = 0). A clear and systematic trend is observed with an increasing recoil-peak contribution with decreasing R. BBK-type model calculations indicate that the inclusion of the passive electron in the target wavefunction is the key ingredient for explaining the data rather than the two-centre character of the system.

Signatures of projectile?nucleus scattering in three-dimensional (e,2e) cross sections for argon

Electron impact ionization (E0 = 195 eV) of the 3p-orbital in argon is investigated experimentally and theoretically. The triple-differential cross sections (TDCS) obtained using a multi-particle momentum spectrometer (reaction microscope) cover more than 80% of the full solid angle for the slow emitted electron up to an energy of 25 eV and a range of projectile scattering angles from −5° to −15°. Inside the projectile scattering plane the TDCS shape is in rather good agreement with a hybrid distorted-wave plus R-matrix (DWBA-RM) calculation. Outside the scattering plane relatively strong electron emission is observed which is reproduced by theory in magnitude but not in shape. A systematic study of the TDCS behaviour and structure in this region indicates that its origin lies in high-order projectile−target interaction.

High-resolution (e, 2e + ion) study of electron-impact ionization and fragmentation of methane

The ionization and fragmentation of methane induced by low-energy (E0 = 66 eV) electron-impact is investigated using a reaction microscope. The momentum vectors of all three charged final state particles, two outgoing electrons, and one fragment ion, are detected in coincidence. Compared to the earlier study [Xu et al., J. Chem. Phys. 138, 134307 (2013)], considerable improvements to the instrumental mass and energy resolutions have been achieved. The fragment products CH4 (+), CH3 (+), CH2 (+), CH(+), and C(+) are clearly resolved. The binding energy resolution of ΔE = 2.0 eV is a factor of three better than in the earlier measurements. The fragmentation channels are investigated by measuring the ion kinetic energy distributions and the binding energy spectra. While being mostly in consistence with existing photoionization studies the results show differences including missing fragmentation channels and previously unseen channels.

Kinematically complete experiments for positron-impact ionization of helium atoms at the NEPOMUC facility

Positron impact ionization of helium is studied with fully resolved momentum vectors of all continuum particles. An imaging multi-particle momentum spectrometer (reaction microscope) detecting all final state particles over the full solid angle was applied. This apparatus was connected to the NEPOMUC facility delivering intense positron beams tuneable over a large energy range with good beam quality. At 80 eV impact energy about 5000 triple coincidence events were collected. Cross sections as function of the longitudinal particle momentum show strong differences compared to respective electron impact ionization data most likely originating from the reversed post collision interaction in both cases. Calculations using the 3 Coulomb wavefunction method show clear discrepancies from the experimental results.

An (e, 2e + ion) investigation of dissociative ionization of methane.

We present in this paper an (e, 2e + ion) investigation of the dissociative ionization of methane by 54 eV electron impact employing the advanced reaction microscope. By measuring two electrons and the ion in the final state in triple coincidence, the species of the ions are identified and the energies deposited into the target are determined. The species and the kinetic energies of the fragmented ion show strong dependence on the intermediate states of the parent ion. Possible decay pathways for the production of different species of ions are analyzed.

Momentum imaging of dissociative electron attachment in biologically relevant molecules

We measured dissociative electron attachment in biologically relevant molecules using momentum imaging for negative ions in an apparatus that combines high resolutions of impact energy, fragment mass and fragment momentum. First investigations of the production of NH2−-ions around the A1 resonance at 5.7 eV impact energy show a clear dependence of the distribution of fragment dissociation angles on the projectile energy.