Fivefold differential cross sections for ground-state ionization of aligned H(2) by electron impact

TDCS Calculation for the Ionization of Nitrogen Molecule by Electron Impact

Triple differential cross section (TDCS) results are reported for the electron impact ionization of nitrogen molecules. The TDCSs have been calculated in distorted wave Born formalism using orientation averaged molecular orbital (OAMO) approximation. The TDCS results are presented as average and weighted sum for the outer molecular orbital 3σg, 1πu, 2σu and the inner 2σg molecular orbital. The obtained theoretical TDCSs are compared with the available measurements. The results are analysed in terms of the positions and relative intensities of binary and recoil peaks. Within a first order model and for a complex molecule, a reasonable agreement is obtained with the experimental data in the binary peak region with certain discrepancies in position and magnitude in the recoil peak region.

Development of an electron-ion coincidence apparatus for molecular-frame electron energy loss spectroscopy studies

We report details of an electron-ion coincidence apparatus, which has been developed for molecular-frame electron energy loss spectroscopy studies. The apparatus is mainly composed of a pulsed electron gun, an energy-dispersive electron spectrometer, and an ion momentum imaging spectrometer. Molecular-orientation dependence of the high-energy electron scattering cross section can be examined by conducting measurements of vector correlation between the momenta of the scattered electron and fragment ion. Background due to false coincidences is significantly reduced by introducing a pulsed electron beam and pulsing scheme of ion extraction. The experimental setup has been tested by measuring the inner-shell excitation of N₂ at an incident electron energy of 1.5 keV and a scattering angle of 10.2°.

Stereodynamics of electron-induced dissociative ionization of N2 studied by (e, e+ion) spectroscopy

We report an (e, e+ion) spectroscopy study on electron-induced dissociative ionization of N₂. Vector correlation between the scattered electron and N⁺ ion has been measured for inner-valence ionization of N₂ at an incident electron energy of 1.4 keV and scattering angles of 2.2°, 4.2°, and 8.2°. By analyzing the experimental data, partial ion yield spectra have been obtained for transitions to the C ²Σ, F ²Σ, and 2σ ion states, showing that the individual transitions depend on the momentum transferred to the target, K, in different ways. The molecular-orientation dependence of the ionization cross section has subsequently been examined for the F ²Σ ionization. To account for the angular anisotropy of the scattering cross section, a compact analytical form has been developed. It is elucidated that for small K the F ²Σ ionization preferentially takes place when the molecule has its axis aligned parallel to the momentum transfer vector due to σ_u shape resonance, while the angular distribution drastically changes with K, indicating strong influences of non-dipole interaction on the ionization dynamics. It has been shown that the present method provides a powerful means to explore K-dependent stereodynamics in electron-induced dissociative ionization of molecules.

A recoil ion momentum spectrometer for molecular and atomic fragmentation studies

We report the development and performance studies of a newly built recoil ion momentum spectrometer for the study of atomic and molecular fragmentation dynamics in gas phase upon the impact of charged particles and photons. The present design is a two-stage Wiley-McLaren type spectrometer which satisfies both time and velocity focusing conditions and is capable of measuring singly charged ionic fragments up-to 13 eV in all directions. An electrostatic lens has been introduced in order to achieve velocity imaging. Effects of the lens on time-of-flight as well as on the position have been investigated in detail, both, by simulation and in experiment. We have used 120 keV proton beam on molecular nitrogen gas target. Complete momentum distributions and kinetic energy release distributions have been derived from the measured position and time-of-flight spectra. Along with this, the kinetic energy release spectra of fragmentation of doubly ionized nitrogen molecule upon various projectile impacts are presented.

Momentum imaging spectrometer for molecular fragmentation dynamics induced by pulsed electron beam

A momentum imaging spectrometer has been built for studying the electron impact molecular fragmentation dynamics. The setup consists of a pulsed electron gun and a time of flight system as well as a two-dimensional time and position sensitive multi-hit detector. The charged fragments with kinetic energy up to 10 eV can be detected in 4π solid angles and their three-dimensional momentum vectors can be reconstructed. The apparatus is tested by electron impact ionization of Ar and dissociative ionization of CO2. By analyzing the ion-ion coincidence spectra, the complete and incomplete Coulomb fragmentation channels for CO2(2+) and CO2(3+) are identified. The kinetic energy release (KER) and angular correlation for the two-body breakup channel CO2(2+*) → O(+) + CO(+) are reported. The peak value of total KER is found to be 6.8 eV which is consistent with the previous photoion-photoion coincidence studies, and the correlation angle of O(+) and CO(+) is also explicitly determined to be 172.5°.

Strong molecular alignment dependence of H2 electron impact ionization dynamics

Low-energy (E(0) = 54 eV) electron impact single ionization of molecular hydrogen (H(2)) has been investigated as a function of molecular alignment in order to benchmark recent theoretical predictions [Colgan et al., Phys. Rev. Lett. 101, 233201 (2008) and Al-Hagan et al., Nature Phys. 5, 59 (2009)]. In contrast to any previous work, we observe distinct alignment dependence of the (e,2e) cross sections in the perpendicular plane in good overall agreement with results from time-dependent close-coupling calculations. The cross section behavior can be consistently explained by a rescattering of the ejected electron in the molecular potential resulting in an effective focusing along the molecular axis.

Differential cross sections for ionization of laser-aligned atoms by electron impact

The first experimental data are given for (e,2e) ionization from laser-aligned atoms. A linearly polarized laser excited Mg atoms to the 3¹P₁ state prior to ionization by low energy electrons. The scattered and ejected electrons were detected in coincidence and the differential cross section determined for a range of alignment angles. An asymmetric coplanar geometry was used, with one electron fixed and the other detected at different angles. The data are compared to that from the spherically symmetric 3¹S₀ state. Significant differences are found, in both magnitude and angular distribution.