We investigate the contribution of Interatomic Coulombic Decay induced by ion impact in neon and argon dimers (Ne 2 and Ar 2 ) to the production of low energy electrons. Our experiments cover a broad range of perturbation strengths and reaction channels. We use 11.37 MeV/u S 14+ , 0.125 MeV/u He 1+ , 0.1625 MeV/u He 1+ and 0.150 MeV/u He 2+ as projectiles and study ionization, single and double electron transfer to the projectile as well as projectile electron loss processes.The application of a COLTRIMS reaction microscope enables us to retrieve the three-dimensional momentum vectors of the ion pairs of the fragmenting dimer into Ne q+ /Ne 1+ and Ar q+ /Ar 1+ (q = 1, 2, 3) in coincidence with at least one emitted electron.

Cross sections for ionization with excitation and for double excitation in helium are evaluated in a full second Born calculation. These full second Born calculations are compared to calculations in the independent electron approximation, where spatial correlation between the electrons is removed. Comparison is also made to calculations in the independent time approximation, where time correlation between the electrons is removed. The two-electron transitions considered here are caused by interactions with incident protons and electrons with velocities ranging between 2 and 10 au. Good agreement is found between our full calculations and experiment, except for the lowest velocities, where higher Born terms are expected to be significant. Spatial electron correlation, arising from internal electron-electron interactions, and time correlation, arising from time ordering of the external interactions, can both give rise to observable effects. Our method may be used for photon impact.

Experimental results are presented on the degree of linear polarization of the extreme-ultraviolet emission of neutral and ionized helium following electron-impact excitation and ionization excitation of helium. The polarization of the photon emission from the decay of He (1snp) 1 P o states with wavelengths of 517 to 584 Å has been extended to electron-impact energies of 1500 eV, and compared with theory for electron-impact excitation of neutral helium. In addition, the polarization of a number of the He ϩ decays with wavelengths between 256 and 1640 Å have been measured. Particular attention has been paid to the (2p) 2 P o˜( 1s) 2 S and (3p) 2 P o˜( 1s) 2 S decays with wavelengths of 304 and 256 Å, respectively. These have been measured from threshold ͑66 and 73 eV, respectively͒ to 1500 eV using a characterized molybdenum/silicon multilayer mirror polarimeter whose reflection and polarization characteristics have been optimized at 304 Å He ϩ (2p) 2 P o results are compared with a recent threshold alignment measurement and distorted-wave Born-approximation calculation for the (e Ϫ ,2e Ϫ ) reaction of ionization excitation. Very good agreement with the threshold theoretical predictions is obtained. These results indicate that near threshold, partial waves with LϾ0 contribute substantially to the two-electron wave function of the escaping electrons. Measurements on the unresolved multiplets at 1215 and 1640 Å, made using a more conventional reflection-type polarization analyzer, indicate that relative fine-structure cross sections are quite different than predicted by some theories.

Experimental magnetic sublevel scattering-angle-integrated cross sections following ionization-excitation of He(1s 2) 1 S to He + (2p) 2 P o in e − + He and H + m +He (m = 1-3) collision systems have been determined using a combination of total cross sections and polarization fraction measurements in the extreme ultraviolet range. The derived magnetic sublevel cross sections, σ 0 and σ 1 , for M L = 0 ± 1 have been studied over a wide range of velocities (2-8.5 au) for electron impact on helium. These results are compared with previous experimental proton cross sections (2-6 au) as well as new measured data for hydrogen molecular impact (1.4-4.0 au) for equi-velocity. In addition, our electron and proton measurements are compared with earlier theoretical predictions, our recent second-Born calculations fully including off-shell energy terms, and present predictions from a hybrid first-order and second-order distorted-wave plus R-matrix (close-coupling) model (DWB1 + RMPS and DWB2 + RMPS). Finally, we have extended our polarization measurements for H + + He collisions and found excellent agreement between theory and experiment at nearly all impact energies. However, the present second-Born and DWB1 + RMPS results deviate slightly from the experimental electron data while the DWB2 + RMPS calculations tend toward the experimental data of Forand et al (1985

The method of high-resolution Auger spectroscopy was used to study mechanisms for double-electron capture producing the projectile configurations 3lnlЈ and 4lnlЈ (nу4) in Ne 10ϩ ϩHe collisions. Emphasis was given to slow collisions with projectile energies near 1 keV. At low collision energies the production of the configurations 3lnlЈ (nу6) of nonequivalent electrons is found to become dominant. It is shown that dielectronic processes produced by the electron-electron interaction play a major role in the creation of the Ne 8ϩ (3lnlЈ) states.