Andrey Lugovskoy

Electron relaxation and excitation processes in metals

The energy dependence of the electron-electron scattering rate in copper is numerically calculated with the use of Monte Carlo integration. The screening by d-band electrons, exchange interaction and the non-equilibrium electron distribution are taken into account. We also compute the rates of laser-stimulated electron-phonon collisions with the consideration of umklapp processes. It is shown that in the absence of direct interband transitions, and for energies higher than the Fermi energy of the metal, laser absorption is determined by electron-phonon umklapp processes.

Angular distribution of photoelectrons emitted at pulse laser irradiation

The analysis of electron-distribution formation in a metal irradiated with a laser pulse is given on the basis of the explicit solution of the quantum kinetic equation. The angular distributions of electrons emitted under laser irradiation of a metal surface are associated with the non-Fermi electron distribution generated by the laser action. The anisotropy of photon absorption in an electron–phonon collision causes a two-leafed distribution structure, which qualitatively distinguishes this mechanism of laser-stimulated electron emission.

Positron scattering on atoms and molecules

An overview is given of recent progress in the calculation of positron scattering on atoms and molecules using the convergent close-coupling method. Particular emphasis is given to those cases where positronium formation is one of the reaction channels, as well as the importance of demonstrating convergence with increasing orbital angular momentum of the bases used. Targets considered are atomic hydrogen, lithium, and molecular hydrogen. The last two decades have seen extraordinary progress in the field of electron, positron and photon scattering on atoms and ions. The problems of electron and photon scattering on atoms are very closely related. In the typical case of single photon absorption, the interaction proceeds by the resulting photo-electron scattering on the residual ion. For example, photon-helium scattering is essentially electron scattering on the singly charged helium ion. Positron-atom scattering is a little more interesting due to the possibility of positronium (Ps) formation. This is a rearrangement collision that considerably increases the complexity of the problem. Though it has been often claimed that positron-atom scattering is simpler than the corresponding electron-scattering problem due to the absence of exchange, in practice the introduction of the Ps-formation channel creates considerably more significant challenges. Historically, computational approaches to the problems have been subdivided into the low-, intermediate- and high-energy regimes. In addition, excitation and ionisation processes have also received different treatments. However, our interest in developing the convergent close-coupling (CCC) method has been to unify the approach to all such problems to be valid for the three projectiles across all energies and for the major excitation and ionisation processes. In developing the CCC method for excitation we took note of the techniques used specifically in their regimes of validity. At low energies the R-matrix close-coupling approach (1) has yielded outstanding results. At the higher energies the perturbative approach (2) has been particularly successful. The CCC method (3) combines the two techniques because it formulates close- coupling as coupled Lippmann-Schwinger equations in momentum space, which may be readily expanded in a perturbative series. Furthermore, the coupled equations may be solved in a distorted-wave formalism (4). However, unlike distorted-wave approximations the CCC results are independent of the choice of the distorting potential. In this sense the usage of such a potential is solely for numerical ease of solution. Following the pioneering implementation of the close-coupling method to ionisation processes (5), we developed an even simpler CCC approach (6). Rather than reconstructing the total wavefunction of the electron-atom system, we associated ionisation amplitudes with excitation of the positive-energy pseudostates. In other words, we extracted the required

Convergent close-coupling approach to positron and antiproton collisions with atoms

Recent developments in application of the convergent close-coupling approach to antimatter-matter scattering are outlined. These include positron collisions with alkalis and helium, in the ground or metastable states, as well as extension of the method to heavy projectiles, such as antiprotons.

Electron excitation in thin metal films due to the magnetic field of ultrashort laser pulses

The behavior of the conduction-band electrons in a metal film subjected to the action of an ultrashort laser pulse at normal incidence is analyzed quantum-mechanically. The pulse duration is assumed to be shorter than the electron mean free time. It is shown for the first time that electrons in the film can be excited resonantly to the higher-energy quantum well levels due to the action of the magnetic field of the wave. The numerical analysis is conducted for an electron which is initially in the lowest quantum well and has a quasi-momentum directed along the electric field. It is shown that the excitation probability due to the magnetic-field excitation mechanism can be even larger than the excitation probability due to the inverse bremsstrahlung in electron–phonon collisions.

Convergent close coupling calculations for positron-magnesium scattering

While a two-centre convergent close-coupling approach to positron-magnesium scattering is developed, a single-centre method has been used to calculate total cross sections up to incident energies of 100 eV. The results agree very well with the measurements of Stein et al. [1] for positron energies above the ionisation threshold (7.6 eV). Similar accuracy is expected for energies below the positronium formation threshold (0.8 eV) where presently there are no experimental data to compare to. In this energy region we find a large p-wave resonance at 0.17 eV. Similar resonance behaviour was found in calculations by Mitroy and Bromley [2] at an energy of 0.1 eV.

Two-center convergent close-coupling calculations for positron-lithium and positron-sodium collisions

The convergent close-coupling calculations of e+-Li and e+-Na collisions are reported. The target is treated as one active electron interacting with an inert ion core. The positronium formation channels are taken into account explicitly utilizing both negative- and positive-energy Laguerre-based states. A large number of channels and high partial waves are used to ensure the convergence of the cross sections.

Acceleration of electrons in thin metal films by strong ultra-short laser pulses

The possibility of effective electron acceleration in a thin solid film exposed to irradiation by two power ultra-short laser pulses is discussed. The acceleration effect is caused by resonant action of the radiation on electrons oscillating across the film with frequencies close to the laser frequency. The relativistic equations of electron motion are numerically analyzed.

Electron gas kinetics in a field of ultrashort laser pulses

Using the quasienergy set of wave functions: the equation for one-particle density matrix (kinetic equation) describing electron gas kinetics in high-power laser field with consideration for electron-electron and electron- lattice collision processes was derived. The solutions of this equation have been found for two limiting cases: (1) when the electron-electron collision frequency greatly surpasses the frequency of electron-lattice collisions and (2) in the opposite case.

Acceleration of electrons in thin metal films by strong ultrashort laser pulses

The possibility of effective electron acceleration in a thin solid film exposed to irradiation by two power ultrashort laser pulses are discussed. The acceleration effect is caused by resonant action of the radiation on electrons oscillating across the film with frequencies close to the laser frequency. The numerical analysis of the relativistic equations of electron motion is conducted.