W. Claeys

Crossed Beam Measurement of Absolute Cross-sections - An Alternative Method and its Application To the Electron-impact Ionization of He+

A new experimental method is reported for the crossed beam measurement of absolute cross sections. It essentially consists in sweeping one of the beams across the other in a linear see-saw motion. The method is applied to one case of electron-ion collisions. A description is given of an electron gun designed to realise a linear and distortionless motion of a ribbon electron beam. Measurements are reported regarding the electron impact ionisation of He+ in the energy range 55-74 eV.

H2+ formation in low energy H+-H- collisions

H++H- associative ionisation cross sections over the relative energy interval 0.001-3 eV have been measured in a merged beam experiment. The energy dependence below 1 eV is found to be E-0.9 while the cross section at 0.003 eV is 1.5*10-14 cm2.

Electron-impact Ionization of Metastable Atomic-hydrogen

The authors report on measurements of the electron impact ionisation cross section of metastable atomic hydrogen in the energy range 6.3-998.3 eV. Crossed electron and atom beams are used in a new method where the more usual intensity modulation is replaced by a see-saw motion of the electron beam across the atom beam. The main advantage of the method is that it obviates the problem of the density distribution in the beams. Absolute results, with 10% accuracy, are obtained for the difference ( sigma 2s- sigma 1s) between the cross sections of the atom in the metastable and ground states. At high energies, the results are consistent with the Bethe formula, in contrast with earlier experimental data. At low energies large discrepancies subsist with all theoretical predictions.

The Formation of H+ From H- Ions By Electron-impact

Cross sections of the reaction H-+e to H++3e have been measured for interaction energies between 17.8 and 1000 eV. Comparison with the only other measurement shows present results to be much smaller. The discrepancy ranges from a factor four at low energies to a factor 12 at high energies. The influence of positive ion trapping in the electron beam has been carefully investigated in the course of this study as it could explain the large discrepancy.

Rabi oscillations and adiabatic rapid passage measured in the 2s-3p transition of atomic hydrogen

The transitions to the 3p/sub 1/2/ and 3p/sub 3/2/ levels of H have been observed by transverse CW laser excitation of the metastable component of a hydrogen atomic beam. The velocity of the atomic beam ( nu ~4*10/sup 4/ cm/s) was chosen to avoid spontaneous decay during the interaction time. The energy separation between the 1/2 and 3/4 levels is larger than the Doppler and transit time broadening and allows measurements of the two excitation processes separately, according to their transitions dipole moments. The authors report measurements of Rabi oscillations and adiabatic following created by spatial modulation of the laser field.

H(3s) Beam Production By Laser Excitation of Metastable Hydrogen-atoms in An Electric-field .2. Theoretical Approach

For pt.I see ibid., vol.18, no.18, p.3557-37 (1985). A quantitative description of H(3s) atoms produced by laser excitation of a metastable hydrogen beam in an electric field is performed and compared with the recent experimental results of Claeys et al. (1985). The Stark effect for n=3 is calculated to obtain the relative intensities of the five Stark transitions as a function of the field strength. Good agreement with the experimental values is found. The dynamical behaviour of the 3s and 3p Stark states when the atoms leave the field is then analysed. Due to the adiabatic evolution we found that only one Stark state can produce H(3s) atoms.

Electron detachment in H+-H- collisions

This letter reports a two-state model calculation of the cross section of the electron detachment reaction HA++HB- to HA++HB+e. Centre B is chosen as electron coordinate reference and the asymptotic parts of both the interaction and the energy difference are considered. Agreement with experiment entails the interpretation of the detachment process, as a dipole transition to the continuum under the Stark effect induced by the proton.

Double charge transfer in H/sup +/-H/sup -/ collisions

The cross section for double charge transfer in H/sup +/-H /sup -/ collisions has been measured, using merged beams, over the energy range 30-200 eV. The cross section shows an oscillatory behaviour. A semiclassical calculation has been completed, the results of which show the same behaviour but overestimate the magnitude of the cross section.

Ion pair production in the collisional dissociation of H/sub 2/

Discusses the ion-pair formation in collisional dissociation of H /sub 2//sup +/ and follows earlier work on H/sub 2/ (Brouillard et al. (1975)). Cross sections for ion-pair formation and the energy distribution of the fragments have been measured for 6 keV H /sub 2//sup +/ ions impinging on He, Kr, and Xe targets. In addition, cross sections for negative-ion production have also been determined. The results indicate that ion-pair formation from H/sub 2 //sup +/ in rare gases could be described as a two-step process involving electron capture into a predissociated Rydberg state. This contrasts with the case of H/sub 2/. Moreover, it is shown that ion-pair formation cannot account for the total yield of negative ions.

H(3s) Beam Production By Laser Excitation of Metastable Hydrogen-atoms in An Electric-field .1. Experiment

An experimental method to produce an intense fast beam of H(3s) atoms is described. H(3s) atoms are formed in a two-step process, first laser excitation of a beam of H(2s) atoms is achieved in an electric field where transitions occur between Stark states; in the second step, the atoms travel to a zero-field region and the Stark states formed evolve adiabatically to non-perturbed atomic states. By adjusting the laser frequency, 2s-3s excitation can be achieved. A neutral hydrogen beam with 2.7% of the atoms in the 3s state has been produced at an energy of 3 keV with 400 mW laser power.