Laser-assisted Electron-ion Recombination Research Articles

Atomic processes in bicircular fields

Various laser-induced and laser-assisted processes in atoms exposed to bicircular fields are investigated. The strong-field ionization process is exemplified by the direct and by the high-order above-threshold detachment of negative fluorine ions both for corotating and counterrotating bicircular fields. The symmetries of the electron distributions in the momentum plane are analyzed in detail. The results for laser-assisted electron-ion recombination and for high-order harmonic generation are presented for the bicircular field with counterrotating components.

Atomic processes in bicircular fields

We investigate laser-assisted electron-ion recombination (LAR), high-order harmonic generation (HHG) and above-threshold ionization (ATI) of argon atoms by a bicircular laser field, which consists of two coplanar counter-rotating circularly polarized fields of frequencies rω and sω. The energy of soft x rays generated in the LAR process is analyzed as a function of the incident electron angle and numerical results of direct recombination of electrons with Ar+ ions are presented. We also present the results of HHG by a bicircular field and confirm the selection rules derived earlier for inert-gas atoms in a p ground state. We show that the photoelectron spectra in the ATI process, presented in the momentum plane, as well as the LAR spectra exhibit the same discrete rotational symmetry as the applied field.

Resonant electron-atom bremsstrahlung in an intense laser field

We analyze a resonant mechanism for spontaneous laser-assisted electron bremsstrahlung (BrS) involving the resonant transition (via either laser-assisted electron-ion recombination or electron-atom attachment) into a laser-dressed intermediate quasibound state (corresponding, respectively, to either a field-free neutral atom or a negative-ion bound state) accompanied by ionization or detachment of this state by the laser field. This mechanism leads to resonant enhancement (by orders of magnitude) of the BrS spectral density for emitted photon energies corresponding to those for laser-assisted recombination or attachment. We present an accurate parametrization of the resonant BrS amplitude in terms of the amplitudes for nonresonant BrS, for recombination or attachment to the intermediate state, and for ionization or detachment of this state. The high accuracy of our general analytic parametrization of the resonant BrS cross section is shown by comparison with exact numerical results for laser-assisted BrS spectra obtained within time-dependent effective range theory. Numerical estimates of resonant BrS in electron scattering from a Coulomb potential are also presented.

Quantum-orbit theory of high-order atomic processes in intense laser fields

The quantum-orbit formalism of high-order atomic processes in a strong laser field is presented starting from the strong-field approximation generalized to include higher-order effects. It is shown how to apply the quantum-orbit theory to various processes such as high-order above-threshold ionization and detachment, high-order harmonic generation, laser-assisted X-ray–atom scattering, laser-assisted electron–ion recombination, laser-assisted electron–atom scattering and non-sequential double ionization. Particular attention is devoted to high-order above-threshold ionization by few-cycle laser pulses. The results obtained using the strong-field approximation and the theory of quantum orbits are compared with the ab initio solution of the time-dependent Schrödinger equation. It is shown that Coulomb effects are important for low-energy electron spectra.

Generation of attosecond pulses in electron-ion recombination processes

Laser-assisted electron-ion recombination is considered. For a very short radiation pulse we analyse the generated x-ray spectrum. The development in time of the x-ray spectrum within the laser pulse duration is of particular interest. Comparison is made of our results with the spectra presented in our earlier work on this process, where the laser pulse was described by a plane wave. Moreover, we compare our results with the predictions of our classical theory of this process, addressing Bohr's correspondence principle. This shows to what extent the classical model of recombination in a laser field is still successfully applicable, if the laser pulse is composed of a few cycles only. In addition, the influence of ‘windowing' on the x-ray spectrum is discussed and finally, we shall comment on the macroscopic efficiency of this process for the x-ray generation in a plasma.

Time-frequency analysis of x-ray generation by recombination in short laser pulses

We consider electron-ion recombination in the presence of a very short laser pulse and we analyze the spectrum of x rays generated during this process. In particular, we investigate the development of the $x$-ray spectrum as a function of time within the duration of the laser pulse. We compare our results with the spectra we obtained in our earlier work on this topic, describing the laser pulse by an electromagnetic plane wave and we also relate our results to our classical investigation of this process based on Bohr's correspondence principle. As we can show, our classical model of laser-assisted electron-ion recombination can still be successfully applied in the case of a very short laser pulse of a few cycles.

Laser-assisted electron-ion recombination: Emitted photons' spectra and recollision effects

We investigate laser-assisted electron-ion recombination with an emphasis on the spectrum of the emitted high-energy photons. A theory that includes the modification of this spectrum due to the recollision of the incident electron and the ion is presented. Numerical results for the soft X-ray differential power spectra both for a monochromatic and a bichromatic laser field are presented and explained using a semi-classical model. For the bichromatic field with frequencies ω and 2ω, the emitted X-ray spectrum is analysed as a function of the relative phase and the possibility of the coherent phase control is shown. We also show that the laser-assisted electron-ion recombination, which includes the rescattering of the electron at the ion before the recombination, is a process complementary to the well-known processes of high-harmonic generation and high-order above-threshold ionization. For low incoming electron energies, the cut-off of the emitted soft X-ray photon energies, including the process of rescattering, is higher than in the case of the direct recombination process. The results obtained are explained using the semi-classical analysis and the three-step scenario.

Electron-atom scattering and electron-ion recombination in a powerful laser field

Electron-atom (or -ion) scattering and electron-ion recombination are two of the very important processes in the laser heating of plasmas. With the increasing power of laser fields available, it becomes of interest to investigate on the one hand the influence of relativistic and spin effects in electron-atom (or -ion) scattering and on the other hand to evaluate the spectrum of high frequency radiation that can be attained by the recombination process in a powerful laser field. Therefore both the above processes will be investigated in the present work to find out the importance of relativistic and spin corrections to the differential cross-sections and energy conservation relations in free-free transitions in a laser field and, similarly, to investigate the efficiency of the high frequency generation in laser-assisted electron-ion recombination.

Laser-assisted electron-ion recombination: emitted photons' spectra and recollision effects

Laser-assisted electron-ion recombination: emitted photons' spectra and recollision effects

Rescattering effects in soft-x-ray generation by laser-assisted electron-ion recombination

Laser-assisted electron-ion recombination is investigated with an emphasis on the spectrum of the emitted high-energy photons and its modification due to the recollision of the incident electron and the ion. Numerical results for the soft-x-ray power spectra, added up over all intermediate laser photon channels, are presented as a function of the incident electron energy for different laser field intensities. For strong laser fields, maxima, and additional structures are found in these spectra for incident electron energies of the order of magnitude of the ponderomotive energy. We show that the laser-assisted electron-ion recombination, that includes the rescattering of the electron at the ion before the recombination, is a process complementary to the well-known processes of high-order harmonic generation and high-order above-threshold ionization. All these processes can be explained, using the three-step scenario. A semiclassical analysis is presented which shows that for the laser-assisted electron-ion recombination real solutions of the saddle-point equations exist, contrary to what is found with high-order harmonic generation and high-order above-threshold ionization when only complex solutions are permitted. For low incoming electron energies, the cutoff of the emitted soft-x-ray photon energies, including the process of rescattering, is higher than in the case of the direct recombination process. The height of the rescattering plateau is 6--7 orders of magnitude lower than that of the direct process. However, for higher incident electron energies we obtain the unexpected result that the difference between the height of the rescattering plateau and the height of the direct plateau can be less than one order of magnitude.