R. Gayet

Coulomb-Volkov approach of ionization by extreme-ultraviolet laser pulses in the subfemtosecond regime

In conditions where the interaction between an atom and a short high-frequency extreme ultraviolet laser pulse is a perturbation, we show that a simple theoretical approach, based on Coulomb-Volkov-type states, can make reliable predictions for ionization. To avoid any additional approximation, we consider here a standard case: the ionization of hydrogen atoms initially in their ground state. For any field parameter, we show that the method provides accurate energy spectra of ejected electrons, including many above threshold ionization peaks, as long as the two following conditions are simultaneously fulfilled: (i) the photon energy is greater than or equal to the ionization potential, (ii) the ionization process is not saturated. Thus, ionization of atoms or molecules by the high order harmonic laser pulses which are generated at present may be addressed through this Coulomb-Volkov treatment.

Cross sections for electron capture from atomic hydrogen by fully stripped ions

We have calculated electron-capture cross sections from atomic hydrogen with the Continuum Distorted Wave method for impact energies between 40 and 10,000 keV/amu. The projectile ions are H+, He2+, Li3+, Be4+, B5+, C6+, N7+, and O8+. The cross sections are given for each value of the final quantum numbers n, l, m, summed over m for given n and l and over l and m for given n. The maximum n runs from n = 5 for H+ to n = 10 for O8+.

The first Born approximation for charge transfer collisions

The authors evaluate the first-order Born perturbation theory for electron capture from atoms by swift ions, with proper allowance for the asymptotic conditions for this three-body Coulomb problem as derived by Belkic et al. (1979). The authors show that the results differ strongly from those of the conventional Oppenheimer-Brinkman-Kramers approximation. Hence the magnitude of the higher terms in the Born series have to be re-evaluated. Some information on this topic is obtained by a comparison between the authors results and those of higher order approximations.

Target nuclear charge dependence of 1s2 to 1s2p and 1s2 to 1s3p excitation cross sections of Fe24+ projectiles in the intermediate velocity range

Excitation cross sections into the 1s2p and 1s3p states of 400 MeVFe24+ projectiles colliding with He, N2, Ar and Kr targets have been extracted from the intensities of helium-like 2P to 1S and 3P to 1S transitions de-exciting the ion after one-electron 1s to 2p and 1s to 3p excitation. The exciting partner being neutral, the capture channel is closed, and these experiments thus give an opportunity to study the pure distortion effect even in the case of quasisymmetric systems at intermediate velocity (Vp/Ve approximately 0.6). A striking feature of the results is the experimental evidence of saturation in the excitation cross section as the nuclear charge of the exciting partner increases. This trend is supported by theoretical predictions of a new method based on the Schwinger variational principle that was developed for that purpose.

Double electron capture by fast bare ions in helium atoms: production of singly and doubly excited states

Double electron capture by fast bare ions in helium is studied in the framework of the Four-Body Continuum Distorted Wave theory. It is shown that experimental data is well reproduced when the theoretical total cross-section include both contributions from the ground state and from the first singly excited states. In addition, cross-sections for the production of the first doubly excited states are also calculated. Present results call for additional experimental data at higher impact energies than presently available.

State-to-state charge exchange cross sections in high-velocity asymmetric and near-symmetric collisions of 400 MeV Fe26+ ions

Absolute 1s to np(n<or=4) and 1s to nd(n<or=5) charge exchange cross sections have been measured for the first time for collisions of bare ions other than protons. The asymmetric case with the projectile nuclear charge higher than the target one has been explored and tests are presented of recently developed multiple scattering theories (continuum distorted-wave methods, restricted strong-potential Born, peaking impulse approximation). Target-induced distortion effects are clearly displayed in the case of heavy target atoms.

Ionization dynamics in interactions of atoms with ultra-short and intense laser pulses

Atom ionization by intense laser pulses, whose electric field performs less than two oscillations during the pulse, is investigated theoretically using both quantum and classical approaches. We show that, under these conditions, the ionization process exhibits a classical aspect. Further, up to laser field amplitudes comparable to the Coulomb field of the nucleus, which is experienced by the active electron on its initial target orbital, the nuclear field is shown to play a significant role in the dynamics of ionization. For higher laser fields, a simple approach based on Coulomb-Volkov states appears much more convenient than full numerical treatments.

New investigation of saturation effect in ion-atom excitation

Excitation cross sections of Kr34+ ions colliding with various target atoms, (Zt between 6 and 40) at nu =35 au have been investigated both experimentally and theoretically. The experimental cross sections were obtained from those for production of projectile He-like and H-like Lyman X-rays. The combined experimental and theoretical analysis of H-like emission allowed for a determination of the competitive capture-ionization process, whose contribution to the Lyman production cross sections is important for near-symmetric collision systems and has to be subtracted. A saturation effect of the experimental excitation cross sections with Zt is analysed from the theoretical point of view through atomic and molecular coupled-state calculations.

Transfer and excitation in ion-atom collisions at high impact velocities: a unified continuum distorted wave treatment of resonant and non-resonant modes in a four-body approach. II. Application to the collision S15+(1s)+H(1s)

For pt.I see ibid., vol.25, no.4, p.825-37 (1992). Resonant (RTE) and non-resonant (NTE) transfer and excitation processes which occur in collisions of fast S15+ (1s) with H atomic targets, are investigated through a four-body continuum distorted wave approximation (CDW-4B) derived by Gayet and Hanssen (1992). The present application is a test of this new theoretical unified approach. Results are compared with previous theoretical and experimental data obtained with molecular targets for impact energies close to resonances. A first quantitative study of the interference between the two coherent processes is made using a model in which both the projectile and the target are hydrogen-like ions. For inner-shell capture from targets whose nuclear charge is comparable with the charge of the projectile, it is shown that interference effects might be significant, even at impact energies close to resonances.

Transfer and excitation in ion-atom collisions at high impact velocities. III. Application of the CDW-4B theory to an almost symmetrical system: He+-He

For pt. II see Nucl. Instrum. Meth. Phys. Res. B, vol. 86, p. 52-61 (1994). Transfer and excitation (TE) processes which occur in collisions between He+ ions and He atoms are investigated through the continuum distorted wave approximation with four bodies (CDW-4B) proposed by Gayet and Hanssen (1994). State selective CDW-4B cross sections have been calculated for this symmetrical collision system. A particular attention is paid to possible interference effects between resonant (RTF) and non-resonant (NTE) modes. CDW-4B results are in reasonable agreement with previous experimental data.