Search

We have measured the double differential cross sections (DDCS) ({ital d}{sup 2}{sigma}/{ital d}{var_epsilon}{sub {ital ed}}{Omega}{sub {ital e}}) of low-energy electron emission in the ionization of H{sub 2} bombarded by bare carbon ions of energy 30 MeV. The energy and angular distributions of the electron DDCS have been obtained for 12 different emission angles and for electron energies varying between 0.1 and 300 eV. We have also deduced the single differential and total ionization cross section from the measured DDCS. The data have been compared with the predictions of first Born approximations and the CDW-EIS (continuum distorted wave{endash}eikonal initial state) model. The CDW-EIS model provides an excellent agreement with the data. {copyright} {ital 1996 The American Physical Society.}

Absolute ionization cross sections for 28{endash}114-keV helium ion impact on atomic hydrogen, differential in energy and angle of the ejected electrons, have been obtained from crossed-beam measurements and previously measured cross sections for molecular hydrogen targets. A radio frequency discharge source produced a mixed atomic and molecular target with a typical dissociation fraction of 74{percent}. Energy spectra were measured from 1.5 to 130 eV by an electrostatic analyzer with a resolution of 5{percent}. The angular range was 15{degree}{endash}160{degree}. Results are compared with calculations based on the first Born, continuum-distorted-wave{endash}eikonal-initial-state, and classical trajectory Monte Carlo methods. Total electron yields are obtained by combining calculations that are separately performed for liberating the target and projectile electrons. Model potentials are used to represent the interparticle-separation-dependent screening of the nuclear charges experienced by the electrons. Though projectile electron emission is a negligible component of the total ionization cross section for the present collision energies, its contribution is significant for particular regions of the spectrum of ejected electrons. Through comparison with our proton-impact data [Kerby {ital et} {ital al}., Phys. Rev. A {bold 51}, 2256 (1995)], differences and similarities are demonstrated owing to the common asymptotic charge of these two projectiles but differing nuclear charges more » and the electron carried by the He{sup +} ion. Comparisons are also made illustrating the differences between atomic and molecular hydrogen targets. {copyright} {ital 1996 The American Physical Society.} « less

Absolute cross sections, differential in energy and angle of ejection of the secondary electrons, were determined for electron emission from He{sup +}+H{sub 2} collisions at 28{endash}114 keV by measuring electron energy spectra from 1.5 to 300 eV at several angles from 15{degree} to 160{degree}. A rotatable electrostatic analyzer was used with an energy resolution of 5{percent} and an angular acceptance of 4.6{degree} full width at half maximum. The double-differential cross sections were integrated over angle, energy, or both to obtain single-differential and total ionization cross sections. The latter are in excellent agreement with previous experimental results. Agreement of the experimental double-differential cross sections with plane-wave Born approximation calculations is generally poor. The cross section for ejection of high-energy electrons by He{sup +} is considerably larger than that for H{sup +} at the same velocity. This is attributed mostly to electron emission from the projectile. The presence of prominent Doppler-shifted helium autoionization peaks in the spectra indicates an appreciable probability for electron capture by the helium ions with simultaneous double excitation. {copyright} {ital 1996 The American Physical Society.}

Apparatus and procedures are described for the measurement of absolute cross sections, differential in ejected electron energy and angle, for ionization of atomic and molecular hydrogen by ion impact. A hemispherical electrostatic energy analyzer, rotatable from 15[degree] to 165[degree] with respect to the direction of the incident ion beam, was used to measure energy spectra of secondary electrons from 1.5 to 300 eV. Cross sections at ten angles (nine at some energies) and five incident-ion energies from 20 to 114 keV for H[sup +]+H[sub 2] collisions are given. The doubly differential cross sections were integrated over angle and electron energy to obtain singly differential and total-ionization cross sections. The uncertainty in the doubly differential cross sections is 21% at a secondary energy of 1.5 eV decreasing to 18% at 10 eV and above. The total cross sections have a rms deviation of 12% from recommended values. A broad peak at 6 eV in the energy spectrum of electrons from low-energy H[sup +]+H[sub 2] collisions is attributed to autoionization.

Results of crossed-beam measurements of cross sections differential in ejected electron energy and angle for ionization of atomic hydrogen by 20--114-keV protons are reported. Secondary electrons were measured over an energy range of 1.5--300 eV and an angular range of 15[degree]--165[degree]. Atomic-hydrogen targets were produced in a radio-frequency discharge source with a dissociation fraction of about 74%. Ratios of cross sections for H targets to those for H[sub 2] targets were obtained from measurements on the mixed target. From these ratios, the measured dissociation fractions, and the absolute cross sections measured for H[sub 2] targets, the cross sections for H targets were determined. These measurements are compared with the results of the first-order Born approximation, the continuum-distorted-wave eikonal-initial-state approximation, and the classical trajectory Monte Carlo (CTMC) methods. Good overall agreement is found with the CTMC results, except for slow, backward electron emission. The addition of the classically suppressed dipole transitions from the Born approximation to the CTMC results yields a good estimate of the ejected electron spectrum.

The fundamental process of ionization in the collisions of protons with atomic hydrogen is considered at an impact energy of 70 keV by comparing experimental and theoretical doubly differential cross sections for electron ejection. This collision system provides a critical test of intermediate energy theories since effects such as electron-electron interaction (correlation) are not present and the roles of the ionization mechanism and subsequent evolution of the ejected electron in the two-centre Coulomb field are isolated. Results of the classical-trajectory Monte Carlo technique, the continuum distorted wave-eikonal initial state method and the plane wave Born approximation are compared with the present experimental data which are the first differential measurements that have been performed for the ionization of atomic hydrogen by proton impact.

A large, broad peak has been found in the electron energy spectrum from 30 to 100-keV H{sup +}+H collisions. The peak, which is centered at 31 eV, appears only in the backward directions. It is suggested that while the peak is unlikely to come from collisions with atoms in the ground or metastable states, it could arise from collisions with atoms in highly excited states or from autoionizing states of the residual H{sub 2} in the target.

Discrepancies between previous experimental values of differential cross sections for electron-impact ionization of water vapor and recent model calculations have been largely resolved. A new measurement with improved suppression of spurious electrons has removed most of the discrepancy in the midrange of detected electron energies. A second discrepancy at secondary energies just below the primary energy has been explained by a more accurate accounting for electrons scattered at angles between zero and the minimum angle of the experimental apparatus. The improved data show more clearly the oxygen K-shell edge in the spectra at small angles and the Bethe ridge at angles up to 90/sup 0/. The forward peak seen in the earlier data is no longer present.