N Andersen

Excitation of Li and Na in collisions with He and Ne: measurements of Li I and Na I resonance-line emission and polarization

Total emission cross sections and polarizations have been measured for the 22S-22P, 22P-32D and 22P-32S Li I multiplets in Li-He, Ne collisions, and the 32S-32P and 32P-32D Na I multiplets in Na-He, Ne collisions in the 0.6-60 keV energy range. The excitation of the alkali resonance multiplet is found to be the dominant inelastic process for all four collision systems. The results are compared with recent calculations by Manique et al. (1977) based on a quasi-one-electron description using various model potentials. Good agreement is found for Li-He with a simple Hartree-Fock frozen-core potential. The agreement is fair for Li, Na-He calculations with a Baylis-type potential but poor for calculations based on Bottcher-model potentials. For Ne as target all three potentials overestimate by far the size of the cross section.

Differential-excitation studies of quasi-one-electron systems. I. Na 32P excitation in Na-He, Ne, Ar collisions

The impact-parameter dependent probabilities for Na(3s from 3p) emission in 1.5-10 keV Na-He, Ne and Ar collisions have been determined experimentally in the range from 0.4 to 3.0 au by means of scattered-particle, photon coincidence measurements. Two mechanisms are proposed to account for the observed emission. At low collision energies, the Na(3s to 3p) excitation takes place at molecular curve crossings, whereas direct excitation dominates at higher energies, around the maximum in the total emission cross section. The direct mechanism is essentially the same for all three collision systems, whereas the quasi-molecular mechanism depends strongly on the asymmetry of the collision.

Electron detachment processes in keV H-, Li-, Na-, K--rare-gas collisions

Electron detachment processes have been measured for the (quasi-) two-electron negative ions with ns2 structures (n=1, 2, 3, 4), namely H-, Li-, Na- and K- in collisions with He, Ne and Ar at energies of 0.5-100 keV amu-1. Total detachment cross sections have been determined for n=1-4. Detachment with simultaneous ns-np excitation has been measured for n=2 and 3. Finally, cross sections for target excitation and/or ionisation have been estimated for 50 keV. For energies above 1 keV amu-1, the total detachment cross sections are found to scale with the cross section for H- at the same velocity as the inverse square of the detachment energies. Simultaneous ns-nsp excitation is found to be important ( approximately 30%) and exhibits a considerable alignment. Target excitation at these energies is relatively small in all cases except for Na--Ne, due to strong quasi-molecular effects in this system.

Coherence study of S→D excitation: Li(2s→3d) excitation of Li-He collisions

Collision-induced coherent S to D excitation is investigated. A polarisation analysis of the subsequent D to P transition by the photon-particle coincidence method is not sufficient to completely determine the excitation amplitudes for the D state. Two distinct solutions are obtained. The electron distributions associated with these two are mirror images of one another in the plane determined by the normal to the scattering plane and the direction of maximum intensity of linearly polarised light, as measured from the direction of the normal. The Li(2s to 3d) excitation induced by collisions with helium is studied both theoretically and experimentally in the keV energy range. The measurements confirm the existence of a theoretically predicted low-energy structure in the circular polarisation. The results are interpreted by generalising concepts previously developed for S to P excitation in quasi-one-electron systems.

Differential excitation studies of quasi-one-electron systems. III. Be II 22P excitation, alignment and orientation in Be+-He, Ne, Ar collisions

For pt.II see ibid., vol.12, no.15, p.2541 (1979). The impact parameter dependence of the Be II 22S-22P emission probability and polarisation has been studied for 1.5-15 keV Be+-He, Ne, Ar collisions by detection of photons in two directions perpendicular to the incident beam, one orthogonal and to one in the scattering plane, in coincidence with ions scattered through a selected angle theta . The studied range of reduced scattering angles tau =E theta corresponds to impact parameters b in a range of 0.3-1.7 au for helium, 0.5-2.2 au for neon and 1.2-2.3 au for argon. The emission probabilities show that at small impact parameters the excitation takes place at quasi-molecular curve crossings for the Be+-Ne, Ar systems. Direct excitation dominates for the Be+-He system and for the large impact parameter collisions of the Be+-Ne, Ar systems. In the region where the direct excitation mechanism dominates, the coherence parameters lambda (b,E) and chi (b,E) describing the relative size and phase of the quantum-mechanical excitation amplitudes for the ML=0 and ML=+or-1 magnetic sublevels have been derived for the Be+-He and Be+-Ne collisions. The lambda and chi parameters vary little with b or E for He as the target, while strong variations are found for Ne. Loss of coherence due to internal forces and cascade effects are treated in detail.

A coherence study of 2pσ-2pπ rotational coupling: Li(22P) and He(21P) orientation and alignment in 1-25 keV Li+-He collisions

The authors have studied the alignment and orientation of the electron cloud of the Li(22P) and He(21P) states excited by 2p sigma -2p pi rotational coupling for impact parameters between 0.2 and 1.1 au in 1-25 keV Li+-He collisions by coherence and correlation analysis techniques. It is found that for collision energies below 5 keV the shape of the excited electron cloud is very nearly that of a p orbital, aligned perpendicular to the asymptotic internuclear axis, independent of impact parameter and of whether the electron stays on the He core or is transferred to the Li centre during the collision. These findings agree with the predictions based on an analysis of the simple diabetic MO diagram for the Li-He system. At energies above 5 keV, the shape changes significantly. Also, the alignment angle deviates from the perpendicular direction, being larger than 90 degrees for Li and smaller for He. The angular momentum perpendicular to the collision plane shows a pronounced variation with collision energy but is only weakly dependent on impact parameter. Possible origins for this behaviour are discussed.

Differential-excitation studies of quasi-one-electron systems. II. Mg II 32P alignment and orientation in Mg+-He, Ne, Ar collisions

For pt.I see ibid., vol.12, no.15, p.2529 (1979). The impact-parameter dependence of the Mg II 32S-32P emission probability and polarisation has been measured for 3-15 keV Mg+-He, Ne and Ar collisions by detection of the photons emitted perpendicular to the scattering plane in coincidence with ions scattered over a selected angle theta . From the four measured Stokes parameters, the emission probability and the degree of polarisation P have been determined. The two parameters lambda and chi describing the relative size and phase of the quantal excitation amplitudes for the magnetic sublevels were evaluated in the cases where the excitation is coherent. The degree of polarisation is high in all cases except for the Mg+-Ne system for distances of closest approach below 1.3 au, where molecular curve crossings due to promotion of neon 2p electrons cause a breakdown of the quasi-one-electron description. The parameters lambda and chi exhibit a strong dependence on the impact parameter, beam energy and target gas.

Impact-parameter dependence of Na(3s→3p) excitation in Na-Ne collisions

A combined experimental and theoretical investigation of the impact-parameter dependence of Na(3s to 3p) excitation in 1-10 keV Na-Ne collisions is presented. Two different mechanisms are shown to account for the features observed in the Na(3p) excitation. At low collision energies, the excitation takes place at well localised molecular curve crossings. At high energies, direct excitation at larger impact parameters dominates.

Polarized-Photon, Scattered-Ion Coincidence Study of Mg+ Collisions with He, Ne, and Ar

The impact-parameter dependence of the Mg II (3s ← 3p) emission has been studied for 3–15 keV Mg+-He, Mg+-Ne, Mg+-Ar collisions by means of photon, scattered-particle coincidence measurements, including polarization analysis. The resonance excitation can be described by means of two mechanisms: At low collision energies, the Mg II (3s → 3p) excitation occurs at molecular curve crossings, whereas direct excitation dominates at higher energies. The direct mechanism is essentially the same for all three collision systems and exhibits a high degree of coherence. The quasimolecular mechanism to a large extent depends on the asymmetry of the collision. The asymmetric Mg+-He, Mg+-Ar systems show substantial coherent excitation, whereas the resonance level is noncoherently excited in the quasisymmetric Mg+-Ne collision.