
Institute of Space Science
AtomiČ™tilor 409, RO077125 Phone: +(40)214574471

Email: 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 electronatom
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 boundfree and freefree 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 electronatom scattering in an electromagnetic field (freefree transitions) (in coll. with A. Cionga and F. Ehlotzky)
a) In the low intensity domain of the electromagnetic field

I
have studied freefree transitions in laserassisted electronHydrogen
scattering in a bichromatic electromagnetic field. I have derived
within the timedependent perturbation theory an analytical formula for
the elastic differential cross sections (DCS) for freefree 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 twophoton 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.
I have studied freefree transitions in laserassisted electronHydrogen scattering in a bichromatic electromagnetic field. I have derived within the timedependent perturbation theory an analytical formula for the elastic differential cross sections (DCS) for freefree 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 twophoton 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 freefree transitions in laserassisted
electronHydrogen scattering in a bichromatic field of frequencies w and 2w. A semiperturbative
approach was used, in which the projectilefield interaction is
described exactly but the fieldtarget 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 freefree transitions involving photon energies
connected to atomic resonances.
I have studied freefree transitions in laserassisted electronHydrogen scattering in a bichromatic field of frequencies w and 2w. A semiperturbative approach was used, in which the projectilefield interaction is described exactly but the fieldtarget 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 freefree 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
targetdressing can be treated within firstorder 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 onephoton absorption revealed that
the scattering process is greatly influenced by the dressing of the
target in particular at small scattering angles.
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 targetdressing can be treated within firstorder 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 onephoton 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 twovalenceelectron atom (in coll. with P. Lambropoulos, L. Nikoloupoulos and T. Nakajima)

I
have studied the multiphoton ionization of a twovalence
electron atom in a strong ultrashort laser field using a
nonperturbative method in order to solve the timedependent Schrodinger
equation. In this context I have investigated the total and partial
ionization yields, and the abovethreshold ionization spectra (ATI) of
Mg in a Ti:Sapphire laser field when multiple ionization thresholds are
involved in the ionization process.
I have studied the multiphoton ionization of a twovalence electron atom in a strong ultrashort laser field using a nonperturbative method in order to solve the timedependent Schrodinger equation. In this context I have investigated the total and partial ionization yields, and the abovethreshold 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 fourphoton ionization of Mg in its
singlet and triplet ground states 3s^{2 } ^{1}S and 3s3p^{ }^{3}P,
respectively, by
an ultrashort laser pulse. We have calculated the two, three, and
fourphoton ionization cross sections by a linearly and circularly
polarized laser fields. Both a frozen core HartreeFock 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 ^{1}P (n=3,4,5,...
etc)
bound excited
states of Mg.

I
have studied twophoton ionization of Ca in its ground state 4s^{2}^{ }^{1}S
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 ^{1}P
(n=4,5,6...
etc)
bound excited
states of Ca.
I have studied two, three and fourphoton ionization of Mg in its singlet and triplet ground states 3s^{2 } ^{1}S and 3s3p^{ }^{3}P, respectively, by an ultrashort laser pulse. We have calculated the two, three, and fourphoton ionization cross sections by a linearly and circularly polarized laser fields. Both a frozen core HartreeFock 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 ^{1}P (n=3,4,5,... etc) bound excited states of Mg.
I have studied twophoton ionization of Ca in its ground state 4s^{2}^{ }^{1}S 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 ^{1}P (n=4,5,6... etc) bound excited states of Ca.
III) Control of physical processes in electromagnetic fields
a) Coherent control for the electronH 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 3p^{2} 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 4p_{1/2}6p_{1/2}, and 4p_{3/2}6p_{3/2 }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 Ltype atom interacting with two short laser pulse trains
I have studied the quantum interference between the excitation pathways in a threelevel Ltype 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 lambdatype 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 Ltype 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 threelevel Ltype atoms by numerically solving the MaxwellSchrodinger 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.

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 http://www.researchgate.net/profile/Gabriela_Buica?ev=hdr_xprf
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Last update: 30 August 2014