H. Madeheim

Coherent excitation of H(n=2) in H+, H - He collisions

The coherent excitation of H(n=2) in H+, H - He collisions was investigated at incident energies of 5–25 keV. From a polarization analysis of the emitted Lyman-α radiation as a function of an external electric field, the partial cross sections for excitation to the H(2s) and the H(2pm) magnetic substates and the real part of thes −p0-coherence were extracted. For H+-He collisions, the measured partial cross sections are in fair agreement with previous two-electron calculations by Kimura and Lin; the agreement with one-electron calculations of Jain et al. is, particularly at the lower incident energies, less satisfactory. For both collision systems, an energy-dependent forward-backward asymmetry corresponding to a shift of the center-of-charge relative to the center-of-mass (dipole moment) was observed. In H+ - He collisions, the measured dipole moment was positive; it thus corresponds to an electron trailing behind the proton. The same analysis applied to the H - He system showed the electron riding in front of the proton.

The collision-induced electric dipole moment of H(n=2) in H-He, Ne and Ar collisions

The collision-induced electric dipole moment of H(n=2) in H(1s)-Ne and H(1s)-Ar collisions was measured in an energy range of 1 to 25 keV. For these systems we observe a positive electric dipole moment which corresponds to an electron lagging behind the proton. This behaviour is in contrast to recent measurements for the H-He systems, where a negative dipole moment corresponding to an electron moving in front of the proton was observed. A simple explanation for this difference is given.

Polarization studies of Lyman-α radiation emitted in atom-H2-molecule collisions

A polarization study of Lyman-α radiation emitted in collisions of H+, H, He+, and He projectiles with H2 molecules has been performed at projectile energies ranging from 1–25 keV. In H+−H2 and H−H2 collisions, the measured linear polarization is negative at low incident velocities, indicating a preferred alignment of the excited electron charge cloud perpendicular to the incident projectile direction. This is taken as significance for a rotational coupling which in these two collision systems predominantly populates the H(2p±1) substates. In He+−H2 and He−H2 collisions, the measured linear polarization is about zero and independent of the projectile energy. In these more asymmetric systems,a′−a′ and a′−a″ couplings are now of comparable magnitude.

Alignment of H(2p) in H+H to H+H(2p) collisions

The integral alignment A20 of H(2p) was measured for 3-25 keV H-H collisions. The measured A20 is at low incident energies close to zero; indicating roughly equal population of H(2p0) and H(2p+or-1) magnetic substates. This behaviour is in disagreement with recent calculations by Shingal et al. (1987-9).

Coherent excitation of H(n=2) induced in H+-Ne and H+-Ar collisions

The collision-induced excitation of atomic hydrogen to the H(n=2) states in H+-Ne and H+-Ar collisions was measured in an energy range of 1 to 25 keV. From a polarization analysis of the emitted Lyman- alpha radiation as a function of an external electric field the partial cross sections sigma lm, for excitation to the H(2s) and the H(2pm) magnetic substates (m=0,+or-1) and the real part of the s0-p0 coherence were extracted. For both collision systems an energy dependent forward-backward asymmetry corresponding to a shift of the centre-of-charge relative to the centre-of-mass (electric dipole moment) was observed. The collision-induced electric dipole moment (Dz) is generally small or negative at incident energies below 10 keV and rises steeply towards higher energies. The present results are in reasonable agreement with close-coupling calculations employing a molecular representation.

Alignment of H(2p) in H-He, Ne, Ar collisions

The integral alignmentA20 was investigated for H(2p) excitation in H-He, Ne, Ar collisions at incident energies of 1–25 keV. The experimental results are compared with theoretical calculations based on different theoretical models. Calculations which do not account for the quasi-molecular aspect of the collision process are at variance with the experimental data below incident energies of 10 keV. Above 15 keV, fair agreement is obtained with calculations which include simultaneous excitation of both projectile and target.