Electron-Helium Scattering in Free-Free Transitions in a Bichromatic Laser Field

Theoretical calculation of the differential cross section （DCS） for elastic electron-helium scattering in the presence of a bichromatic CO2 laser field is carried out in the first Born approximation with a simple screening electric potential. The two components of the laser field have the frequencies w and 2w, which are out of phase by an arbitrary scale φ. The variations of the differential cross section as a function of the phase angle φ in the domain 0°≤ φ ≤360° are presented. We discuss the influence of the number of photons exchanged on the phase-dependee effect. Moreover, for different scattering angles and incident electron energies, the DCS has outstanding ditferences. These illustrate that the two parameters have important effects on the differential cross section and the screening electric potential is effective.     

Classical Effects of Carrier--Envelope Phase on Nonsequential Double Ionization

A classical microcanonical 1＋1-dimensional model is used to investigate the ion momentum distributions in nonsequential double ionization with linearly polarized few-cycle pulses. We find that the ion momentum distribution has a strong dependence on the carrier-envelope phase of the few-cycle pulse, which is consistent with the experimental results qualitatively. Back analysis shows that the ionization probability of the first electron at different phases and its returning kinetic energy play the main role on the ion momentum distributions.     

Laser polarization effects on K-shell Compton scattering

We study Compton scattering on the ground state of the hydrogen atom in the presence of an intense low-frequency electric field (the laser) of arbitrary polarization, for incident and scattered photons of energies around 15 keV. The adopted formalism is the nonrelativistic one developed by Voitkiv et al. [J. Phys. B: At. Mol. Opt. Phys. 36, 1907 (2003)] and applied by them for a circularly polarized laser. We explore the spectrum and the electron energy distribution in their dependence on the incident photon energy or electric field intensity, for different polarizations.     

Electron Scattering by C4H10 and C6H6 in the Energy Range 100--1000eV

We investigate the applicability of the independent atom model （IAM） to elastic electron scattering from complex polyatomic molecules, namely C4H10 and C6H6, in the energy range 100-1000eV. The cross sections of the elastic electron scattering are calculated by employing the IAM together with the relativistic partial waves. The incorporation of both the modified absorption potential and the extended structural factor in the IAM makes the elastic differential cross sections and momentum transfer cross sections have a good agreement with the available experimental data. The present simple model seems to be insensitive to the complexity of the target molecules so that the proposed procedure can be quite useful for calculation of electron scattering from bio-molecules.     

erent Phase Control in e-Ar Scattering in a Bichromatic Laser Field in the Second Born Approximation

We study the elastic scattering of atomic argon by an electron in the presence of a bichromatic laser field in the second Born approximation. The target atom is approximated by a simple screening potential. We explore the dependences of the differential cross sections on the relative phase φ between the two components of the radiation field and discuss the influence of the number of photons exchanged on the phase-dependence effect. Moreover, for different scattering angles and incident electron energies, the differential cross sections are notably different.     

Light-induced effect on the density distribution in a sample of cold trapped atoms

As it is known [1] an intense laser field can induce atom-atom interaction according to a dipole-dipole R ^–3 law. Such an interaction depends on the angle between light polarization and interatomic vector-position R. This angular dependence may produce an anisotropy in the spatial density distribution of the confined sample of cold atoms. We develop the main relations and apply them to the case of an atomic cloud of cold trapped neutral atoms with the density higher than or of the order of ^–3, where is the wavelength of light. The results presented here show the effect of such an interaction in a density regime of high experimental interest.     

Laser influence on positron-antiproton radiative capture collision

The laser-assisted radiative capture between a positron and an antiproton is studied in detail. The theoretical results show that the cross-section for antihydrogen formation is significantly reduced with the application of a laser background. This effect is most marked when the laser polarization is parallelto the incident direction. Received: 13 November 1998 / Received in final form: 16 March 1999     

On the theory of the relativistic cross-sections for stimulated bremsstrahlung on an arbitrary electrostatic potential in the strong electromagnetic field

On the base of relativistic generalized eikonal approximation wave function the multiphoton cross-sections of a Dirac particle bremsstrahlung on an arbitrary electrostatic potential and strong laser radiation field are presented. In the limit of the Born approximation the ultimate analytical formulas for arbitrary polarization of electromagnetic wave have been obtained and numerically analyzed. Received 5 April 2001 / Received in final form 18 March 2002 Published online 28 June 2002     

Electron-Helium Atom Collisions in the Presence of a Bichromatic Laser Field

The differential cross section （DCS） for electron-helium atom collisions in the presence of a bichromatic CO2 laser field is investigated as a function of the scattering angle θ by employing first-Born approximation （FBA） with a simple screening electric potential. We discuss in detail the influence of the scattering geometry, the photon energy and the number of photons exchanged on the DCSs. These illustrate that the three factors have important effects on the elastic scattering and the screening electric potential is effective.     

Bichromatic and trichromatic manipulation of spontaneous emission in a three-level Λ system

Bichromatic and trichromatic manipulation of spontaneous emission in a three-level system in Λ configuration is studied on the basis of density matrix equation and quantum regression theorem. The spontaneous emission spectrum is numerically calculated by using harmonic expansion and matrix inversion. Two characteristic features are shown. Firstly, the central resonance peak, which is absent in the case of monochromatic excitation, is recovered for the bichromatic or trichromatic excitation. Secondly, selective elimination of the spectral lines is obtained by varying the amplitudes and phases of the trichromatic components. For the phase dependence, it is the sum of the relative phases of the two sideband components to the central component that plays a crucial role. The spontaneous emission spectrum is drastically modified once the sum phase is changed, but is kept unchanged regardless of the respective phases when the sum phase is fixed.     

Photoassociation of Atomic BEC within Mean-Field Approximation： Exact Solutions

We propose an exactly solvable method to study the coherent two-colour photoassociation of an atomic Bose- Einstein condensate, by linearizing the bilinear atom-molecule coupling, which allows us to conveniently probe the quantum dynamics and statistics of the system. By preparing different initial states of the atomic condensate, we can observe very different quantum statistical properties of the system by exactly calculating the quadrature- squeezed and mode-correlated functions.     

Positron-Impact Ionization of Atomic Hydrogen in a Bichromatic Laser Field in the Second Born Approximation

The positron impact-ionization of atomic hydrogen in the presence of a linearly polarized bichromatic field is investigated in the second Born approximation. The field is composed of a fundamental frequency and its second harmonic. The state of positron in the field is described by the Volkov wavefunction, and the continuum state of the ejected electron is described by the Coulomb—Volkov wavefunction. The dressed ground state of target is a first order time-dependent perturbative wavefunction. The triple differential cross sections and their dependencies on laser field parameters are discussed. Numerical results demonstrate that the second-order effect plays a crucial role in understanding the laser-assisted positron scattering process.     

Above-barrier ionization and quantum interference in strong laser fields

Classical predictions of field ionization of hydrogen and of Coulomb scattering in strong laser fields are compared with corresponding exact numerical solutions of the time-dependent Schr?dinger equation. In the quasi-static approximation, ionization rates are significantly lower than the quasi-classical tunneling rates in the regime of above-barrier ionization. This reduction is consistent with a simple classical barrier-suppression model. In time-periodic fields, the major differences between the classical and quantum-mechanical treatments arise from quantum-mechanical interference oscillations. These are typical for tunneling barriers in an applied field and for scattering by an oscillating potential. Received: 3 November 1999 / Published online: 30 June 2000     

Enhancement of Bichromatic High-Order Harmonic Generation by a Strong Laser Field and Its Third Harmonic

We investigate high-order harmonic generation （HHG） in a linearly polarized bichromatic field composed of a fundamental laser field with frequency w and an additional laser field with frequency 3w. The numerical results show that it is possible to enhance the intensity of most high harmonics in orders of magnitude. A most striking feature in the enhancement is that the intensity of several special high harmonics is practically impaired as compared with that in the monochromatic case. The qualitative explanation to the great enhancement is that the additional high-frequency field can provide new transition paths for electrons to reach the continuum. The relative phase between the fundamental field and its third harmonic field also affects the intensity of high-order harmonics near the cutoff efficiently.     

Superradiant Rayleigh scattering from a Bose-Einstein condensate with the incident laser along the long axis

We have realized the superradiant Rayleigh scattering in an elongated Bose-Einstein condensate, where the pump laser pulse travels along the long axis. In this incident configuration the spatial asymmetries and the backward scattering pattern are reported. The spatial and temporal evolution of the superradiant process is analyzed by our semiclassical model.     

Induced focusing due to counter-propagation of a pair of bright and dark optical beams in self-defocusing Kerr media

We present two-dimensional numerical simulations of a nonlinear optical system made of a bright probe and a dark pump beam that counter propagate in a thin slice of self-defocusing Kerr media, simultaneously. The numerical results show that although the medium is self-defocusing to both pump and probe beam, a profile compression occurs under some conditions, and the weak bright probe beam is focused due to cross-phase modulation. The influence of the characteristics of beams and medium length on the probe beam compression is also investigated.     

Time-resolved studies of emission excited in sodium vapour by 330 nm laser pulses

Received: 21 June 1996/Accepted: 4 November 1996     

Phase signatures in laser-assisted electron-atom collisions

We study electron-atom scattering in the presence of a laser field with elliptic polarization. We discuss the dependence of the differential cross sections for the cases of circular and linear polarizations as a function of scattering angle. Interesting typical signatures of the phase between the two components of the circular polarization of the laser field appear in the differential cross section.     

Optimizing the production of metastable calcium atoms in a magneto-optical trap

We investigate the production of long-lived metastable (4³ P ₂ ) calcium atoms in a magneto-optical trap (MOT) operating on the 4¹ S ₀ ?4¹ P ₁ transition at 423 nm. For excited 4¹ P ₁ atoms a weak decay channel into the triplet states 4³ P ₂ and 4³ P ₁ exists via the 3¹ D ₂ state. The undesired 4³ P ₁ atoms decay back to the ground state within 0.4 ms and can be fully recaptured if the illuminated trap volume is sufficientlylarge. We obtain a flux of above 10¹⁰ atoms/s into the 4³ P ₂ state. We find that our MOT lifetime of 23 ms is mainly limited by this loss channel, and thus the 4³ P ₂ production is not hampered by inelastic collisions. If we close the loss channel by repumping the 3¹ D ₂ atoms with a 671 nm laser back into the MOT cycling transition, a non-exponential 72 ms trap decay is observed, indicating the presence of inelastic two-body collisions between 4¹ S ₀ and 4¹ P ₁ atoms. Received: 10 July 2001 / Revised version: 22 October 2001 / Published online: 23 November 2001     

Excitation of sodium atoms by 330-nm laser pulses

1/2 ) atoms in a dense sodium vapour irradiated by nanosecond laser pulses tuned near the 3S→4P transition was investigated. It was observed that the population of Na(4P) atoms remained high only within the laser pulse, in spite of the relatively long lifetime of the 4P level (110.ns)The 3P_1/2 level, which is populated as a result of cascade transitions from the higher levels, reached the highest population several nanoseconds after the laser shot. The fast population changes are explained by cascade-stimulated transitions between the excited atomic levels. Received: 16 July 1997/Revised version: 27 October 1997