Viorica Florescu

Two-photon bremsstrahlung in the coulomb field

Abstract We present compact integral representations for the invariant amplitudes of the two-photon bremsstrahlung matrix element using the nonrelativistic dipole approximation in the Coulomb field case. The numerical evaluations for Z =79 lead to values smaller than the experimental cross sections determined by Altman and Quarles for Au targets.

New developments in the theory of bremsstrahlung

Abstract We discuss new progress in bremsstrahlung, beyond the predictions previously available. A relativistic code for the calculation of bremsstrahlung in a screened central potential, utilizing an expansion in electron partial waves and photon multipoles, was developed some time ago by Tseng and Pratt [Phys. Rev. A 3 (1971) 100]. For electron bremsstrahlung, unlike for bremsstrahlung in ion-atom collisions, this potential bremsstrahlung contribution generally dominates the atomic or polarizational bremsstrahlung contribution associated with atomic structure, except for radiation of energies comparable to transition energies [M.Y. Amusia and R.H. Pratt, Commun. At. Mol. Phys. 28 (1992) 247]. Predictions from the potential bremsstrahlung calculations are in reasonably good agreement with experiment [R.H. Pratt and I.-J. Feng, in: Atomic Inner Shell Physics, ed. B. Crasemann (Plenum, New York, 1985) p.]. Tabulations are available for both the spectrum and the bremsstrahlung angular distribution [R.H. Pratt, Atom. Data Nucl. Data Tables 20 (1977) 175; L.D. Kissel et al., ibid. 28 (1983) 381]. A major shortcoming has been that the code only obtains the doubly differential cross section, summing over all states of the outgoing scattered electron. No adequate predictions have been available to compare with experimental results for the triply differential cross section, which show substantial disagreement with simpler theory [W. Nakel, in press, 1994]. This problem has recently been remedied [X.M. Tong, C.D. Shaffer and R.H. Pratt, to be published], directly summing numerically the partial wave contributions to the matrix element. In the first cases studied good agreement has been achieved with the experiments. A second shortcoming of the code was its inability to deal with incident electron energies below about 1 keV, needed to study the transition between classical and quantum behavior [L. Kim and R.H. Pratt, Phys. Rev. A 36 (1987) 45]. We discuss recent results in this classical domain. We also review new developments in polarizational bremsstrahlung and two photon bremsstrahlung.

Phase dependence of three-colour two-photon ionization of hydrogen

We consider a ground-state hydrogen atom interacting with a three-colour electromagnetic field, taken as the superposition of a fundamental frequency ω and two consecutive odd harmonics of order 2p-1 and 2p + 1 (p≥2 and integer), with constant relative phase differences. We study in the lowest-order perturbation theory the process of ionization due to the net absorption of energy 2pω. The ionization rate is controlled by the relative phase difference δ between the harmonics, which modifies the interference terms. We present an analysis of both the angular electron distribution and the total rate, which shows that the interference effects are influenced by the order of the harmonics and their intensity ratio.

Inelastic electron scattering from a Coulomb potential in a laser field

Abstract We present a theoretical study of inelastic electron scattering from a fixed Coulomb potential in the presence of a laser field, based on analytic expressions for the two-photon continuum-continuum transition matrix element. We present here the particular case of incident and scattered light polarizations orthogonal to each other and to the incident electron momentum. Results are given for the cross sections corresponding to the detection of electrons and polarized photons in coincidence, and for the detection of polarized photons only.

The linear modification of Coulomb scattering states by a uniform harmonic electric field

Abstract For an electron in the Coulomb field of a fixed nucleus and in a weak uniform electric field (introduced adiabatically), we present a compact analytic form for the solution of the Schrodinger equation which in the remote past coincides with the continuum outgoing Coulomb solution.

Analytic inner shell internal conversion coefficients for screened Coulomb potentials

An analytic method is presented for evalution of internal conversion coefficients (ICC) in inner shells of neutral or ionized atoms. Analytic formulas for ICC are given which include electron screening and static nuclear size effects. Predictions have also been obtained from these formulas for point Coulomb ICC or for screening effects included only in bound state normalization. Very good agreement with full numerical calculations is found, except in those cases where the formation region of the ICC is not well inside the atom. While the discussion in this paper concerns mediumZ elements (30<Z<80) and not too low energy, the method can be extended to other situations. The method is suited also for ICC of the inner shells of ionized atoms, even for very high degree of ionization.

Electron distributions in nonlinear Compton scattering

Our calculation of the final electron distribution in nonlinear Compton scattering with very energetic electrons is performed for an electromagnetic pulse with finite duration. In contrast to the monochromatic case, this gives access to the the asymptotically free electrons and not to those dressed by the laser field.

Two-colour three-photon ionization of hydrogen

Based on analytical equations valid in perturbation theory, we study the process of ionization of the ground state of the hydrogen atom by the absorption of three photons, two of them being of the same frequency. Results are given for the total ionization rate and for the angular distribution of the ejected electrons, in a regime of frequencies in which two-colour two-photon ionization is not possible. The case of orthogonal photon polarizations, investigated in some detail, displays a strong dependence on the electron direction.

Internal conversion in inner shells of atomic ions and relativistic ionic potentials

Abstract Internal conversion coefficients for inner shells of atomic ions of Sn are calculated for various degrees of ionization and compared with neutral atom predictions. The relativistic version of an analytic perturbation theory is used to include screening effects.

Above-threshold ionization of hydrogen and hydrogen-like ions by X-ray pulses

We clarify a strong link between general nonlinear Fokker-Planck equations with gauge fields associated with nonequilibrium dynamics and the Fisher information of the system. The notion of Abelian gauge theory for the non-equilibrium Fokker-Planck equation has proposed in the literature, in which the associated curvature represents internal geometry. We present the fluctuation of the gauge field can be decomposed into three parts. We further show that if we define the Fisher information matrix by using a covariant derivative then it gives correlation of the flux components but it is not gauge invariant.This paper adresses the problem of above-threshold ionization (ATI) of hydrogen interacting with an intense X-ray electromagnetic field. Two approaches have been used. In the first approach, we calculate generalized differential and total cross sections based on second-order perturbation theory for the electron interaction with a monochromatic plane wave, with the A2 and A · P contributions from the nonrelativistic Hamiltonian (including retardation) treated exactly. In the second approach, we solve the time-dependent Schrödinger equation (TDSE) for a pulsed plane wave using a spectral approach with a basis of oneelectron orbitals, calculated with L2-integrable B-spline functions for the radial coordinate and spherical harmonics Ylm for the angular part. Retardation effects are included up to O(1/c), they induce extra terms forcing the resolution of the TDSE in a three dimensional space. Relativistic effects [of O (1/c2)] are fully neglected. The isoelectronic series of hydrogen is explored in the range Z = 1 − 5 in both TDSE and perturbative approaches. Photoelectron angular distributions are obtained for photon energies of 1 keV and 3 keV for hydrogen, and photon energy of 25 keV for the hydrogenic ion B4+. Perturbative and TDSE calculations are compared.