Dr. Gabriela Buicagabriela buica

Research Scientist I


    Institute of Space Science
    Atomiștilor 409, RO-077125                  Phone: +(40)214574471
      Bucharest-Magurele, Romania               Fax:      +(40)214575840 
    E-mail:    buica [at] spacescience.ro

research domains AND Scientific reSults


The behavior of mater interacting with electromagnetic fields in new conditions has multiple applications: in determining of atomic data of astrophysical interest such as: oscillator strengths, ionization cross sections, scattering cross sections, etc., in studying the dynamic of atomic and molecular processes that take place in a very short time, in controlling of atomic and molecular processes through laser parameters such as: the relative phase between the laser components, the pulse durations and intensities, polarizations, etc., in developing of high frequency lasers, in investigating of plasma and condensed matter properties under new conditions. During the last few decades the study of electron-atom collisions in the presence of an electromagnetic field has been the subject of intense research activities, because of the importance  in applied domains such as astrophysics (it is well known that the principal mechanisms for stelar opacity are bound-free and free-free transitions) [S. Chandrasekhar, An Introduction to the Study of Stellar Structure  (Dover Publications, New York, 1967) ; M. J. Seaton, in Advances in Atomic, Molecular and Optical Physics (Academic Press, New York, 1994)],  laser and plasma physics [Y. Shima and H. Yatom,  Phys. Rev. A 12, 2106 (1975); M. B. S. Lima, C. A. S. Lima, and L. C. M. Miranda, Phys. Rev. A 19, 1796 (1979)], or fundamental atomic collision theory, etc. The analysis of observations made in the ultraviolet wavelength domain by the Hubble Space Telescope (HST), Hopkins Ultraviolet Telescope (HUT), Far Ultraviolet Spectroscopic Explorer (FUSE), Interstellar Medium Absorption Spectrograph (IMAPS) and Extreme Ultraviolet Explorer (EUVE) missions require accurate laboratory measurements of electron collision cross sections, ionization cross sections, energy levels and wavelength positions and oscillator strengths of the abundant species.

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 and extensive calculations were performed in the domain of small scattering angles, where the dressing of the target is important.

  • For the case of circularly and elliptically polarized fields I have analyzed the dependence of DCS's on the helicity of photons. For a superposition of a linearly and a circularly polarized lasers I found out 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 was used, in which the projectile-field interaction is described exactly but the field-target one is described within the second order perturbation theory.

  • I have analyzed 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.

    •

d) Inelastic electron scattering by hydrogen atoms in a laser field. Investigation of polarization effects

  • We analyzed the influence of laser polarization in electron-impact excitation of hydrogen atoms in a circularly polarized (CP) laser field. Using a semiperturbative method new closed form formulas have been derived for the DCS in laser-assisted inelastic scattering for 1s-nl excitation. The detailed numerical data obtained for the excitation of the n=4 levels by CP fields indicate that the atomic dressing effects for inelastic processes are important and we found a significant increase in the DCSs at small scattering angles for the s-s, s-d, and s-f optically forbidden transitions due to the simultaneous electron–photon excitation. We have clarified the origin of the peaks in the resonance structure of DCSs as occurring due to the dipole coupling of the initial ground state with n' p states and of the final excited state with n' l ' (l '= l ± 1) states. we studied the polarization effects on the DCSs in inelastic laser-assisted electron-hydrogen collisions inelastic for the 1s-nl excitation, and analyzed the influence of laser polarization for a circularly polarized laser field. New closed form formulas were derived for DCSs in laser-assisted inelastic scattering for 1s-nl excitation, which are valid for both linear and circular polarization, with the kinematic part that depends on the scattering geometry and the polarization vector of the photon clearly separated.

  • We have analyzed the dichroic effect in elastic electron-hydrogen scattering by a two-color bicircular laser fields of commensurate frequencies and moderate intensities and have investigated the circular dichroism (CD) where the monochromatic components of the two-color CP field have identical or opposite helicities. Our analytical results of circular dichroism shows the dependence of the DCS on the transition amplitudes, in a closed form, that allows further investigations of the dressing as well polarization effects due to the asymmetries of the DCSs for co- and counterrotating CP fields.

    •

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 using 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 singlet and triplet ground states 3s2 1S and 3s3p 3P, respectively, by an ultrashort laser pulse. We have calculated the two-, three-, and four-photon ionization cross sections by a linearly and circularly polarized laser fields. Both 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 analyzed. 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 of Mg.

  • I have studied two-photon ionization of Ca in its ground state 4s2 1S by ultrashort linearly and circularly polarized laser fields. The dependence of the photolectron energy spectrum on the temporal profile of the pulse was investigated and 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 of Ca.

    •

III) Control of physical processes in electromagnetic fields

a) Coherent control for the electron-H atom scattering in an electromagnetic field (in coll. with A. Cionga and F. Ehlotzky)

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 analyzed.

b) Coherent control for autoionizing states of Mg (in coll. with L. Nikoloupoulos)

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 takes into account a realistic atomic structure and transition amplitudes calculation.

c) Control of laser induced continuum structure of K (in coll. with T. Nakajima)

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.

d) Quantum coherent effects in a Λ-type atom interacting with two short laser pulse trains

I have studied the quantum interference between the excitation pathways in a three-level Λ-type atom interacting with short probe and coupling laser pulse trains, beyond the steady state approximation, under the electromagnetically induced transparency conditions. We have investigated the modification induced by the laser pulse trains in a lambda-type atom in terms of upper excited state population for different pulse areas and different detunings. For resonant laser pulse trains with a rectangular temporal profile we have derived analytical formulas for the population of the upper excited state at the end of the pulse. We have showed that we can control the interaction of a Λ-type atom with two laser pulse trains under the EIT conditions, for small probe pulse area while that of the coupling is moderate, by manipulating certain parameters of the lasers .

IV) Interaction between atoms and frequency combs in time and frequency domain (in coll. with T. Nakajima)

We investigated the space and time dynamics of a pair of short laser pulse trains propagating in a medium consisting of three-level atoms by numerically solving the Maxwell-Schrodinger equations for atoms and fields. By performing propagation calculations with different parameters, under conditions of electromagnetically induced transparency, we compare the propagation dynamics by a single pair of probe and coupling laser pulses and by probe and coupling laser pulse trains. We discuss the influence of the coupling pulse area,number of pulses, and detunings on the probe laser propagation and realization of electromagnetically induced transparency conditions, as well on the formation of a dark state.