Main research domains and achievements
Scientific reSults
I) Elastic electron-atom scattering in an electromagnetic field (free-free transitions) (in coll. with A. Cionga and F. Ehlotzky)
a) In the low intensity domain of the electromagnetic field
I have studied free-free transitions in laser-assisted electron-Hydrogen scattering in a bichromatic electromagnetic field. I have derived within the time-dependent perturbation theory an analytical formula for the elastic differential cross sections (DCS) for free-free transitions involving two different photons.
The dependence of the DCS on the scattering angle and photon energy was investigated. Extensive calculation were performed in the domain of small scattering angles, where the dressing of the target is important.
In the case of circularly and elliptically polarized field I have studied the dependence of DCS's on the helicity of photons. For a superposition of a linearly and a circularly polarized lasers we shown that circular dichroism in the angular distribution can be predicted for the nonlinear two-photon transitions, if the dressing of the atomic target by the laser field is treated in second order of perturbation theory. Of special interest is that for particular configurations circular dichroism can be encountered not only in the differential but also in the integrated cross sections.
b) In the moderate intensity domain of the electromagnetic field
I have studied free-free transitions in laser-assisted electron-Hydrogen scattering in a bichromatic field of frequencies w and 2w. A semiperturbative approach is used, in which the projectile-field interaction is described exactly but the field-target one is described by second order perturbation theory.
I have studied the dependence of DCS's on the projectile energy, scattering geometry, photon energy, polarization of the field
I have investigated in detail the DCS in the domain of small scattering angles, where the dressing of the target is important. The effect of the intensities of the two components of the bichromatic field and that of their relative phase is investigated, too. Special attention was paid to the study of free-free transitions involving photon energies connected to atomic resonances.
c) Elastic electron scattering by excited atoms in an electromagnetic field
I have investigated the scattering of fast electrons by excited Hydrogen atoms (in particular, in the 2s, 2p or ns states) in the presence of a linearly polarized laser field of moderate power such that target-dressing can be treated within first-order time dependent perturbation theory.
I analyzed the angular dependence of the nonlinear DCS's, inspecting the contributions of the various electronic and atomic terms of the matrix elements. Detailed numerical results are presented for one-photon absorption revealed that the scattering process is greatly influenced by the dressing of the target in particular at small scattering angles.
II) Multiphoton ionization of a two-valence-electron atom (in coll. with P. Lambropoulos, L. Nikoloupoulos and T. Nakajima)
I have studied the multiphoton ionization of a two-valence electron atom in a strong ultrashort laser field employing a nonperturbative method in order to solve the time-dependent Schrodinger equation. In this context I have investigated the total and partial ionization yields, and the above-threshold ionization spectra (ATI) of Mg in a Ti:Sapphire laser field when multiple ionization thresholds are involved in the ionization process.
I have studied two-, three- and four-photon ionization of Mg in its ground state 3s2 1S and 3s3p 3P by an ultrashort laser pulse. We have calculated the two-, three-, and four-photon ionization cross section by a linearly and circularly polarized laser field. A frozen core Hartree-Fock method and a model potential were used in order to describe the interaction between the ionic core and the valence electrons. The dependence of the photolectron energy spectrum on the temporal profile of the pulse was investigated. Since the Mg atom has a dense electronic structure the photolectron energy spectrum exhibits interesting features such as intermediate ATI peaks; our studies showed that the origin of those intermediated ATI peaks is connected to ionization from the 3snp 1P (n=3,4,5,... etc) bound excited states.
I have studied two-photon ionization of Ca in its ground state 4s2 1S by an ultrashort linearly and circularly polarized laser field. The dependence of the photolectron energy spectrum on the temporal profile of the pulse was investigated. Furthermore the photolectron angular distributions were calculated. Since the Ca atom has a dense electronic structure the photolectron energy spectrum exhibits interesting features such as intermediate ATI peaks; our studies showed that the origin of those intermediated ATI peaks is connected to ionization from the 4snp 1P(n=4,5,6... etc) bound excited states.
III) Control of physical processes in electromagnetic fields (in coll. with A. Cionga, F. Ehlotzky, P. Lambropoulos, L. Nikoloupoulos and T. Nakajima)
a) Coherent control for the electron-H atom scattering in an electromagnetic field
I have investigated coherent phase control in electron scattering by Hydrogen atoms in a bichromatic laser field of frequencies w and 2w. The effect of the relative phase between the components of the bichromatic laser field on the scattering process was studied.
b) Coherent control for autoionizing states of Mg
I have studied the coherent phase control of the autoionizing state 3p2 of Mg in a bichromatic laser field of frequencies w and 2w. A motivation for this study was the investigation of the possibility of achieving coherent control of the photoelectron current and the shape of the autoionizing resonance. I have carried out calculation for the multiphoton ionization rate with the laser frequencies chosen such that the ground state of Mg atom is resonantly coupled to the 3s3p and 3s4p levels. The model has taken into account a realistic atomic structure and transition amplitudes calculation.
c) Control of laser induced
continuum structure of K
I have examined how the photoelectron angular distribution (PAD) is altered through laser induced continuum structure (LICS) by the introduction of a dressing laser. We theoretically investigated the effects of LICS on PAD for a specific atomic system: the K atom 4p1/2-6p1/2, and 4p3/2-6p3/2 levels. It turned out that the PAD's are quite different for both systems, and the alteration of PAD by the laser parameters is important, as we expected, and LICS could be used to control the ionization processes.