V. P. Krainov

Absorption of electromagnetic energy by slow electrons under scattering from Coulomb centers

Simple analytical expressions are obtained for the rate of the inverse stimulated bremsstrahlung absorption under electron scattering from a Coulomb center with charge Z in the presence of the electromagnetic field. The initial and final values of electron energy are assumed to be small compared to the Rydberg energy Z2 (atomic units are used throughout). Single-photon processes of absorption and induced radiation of photon by electron are treated. It is assumed that the electromagnetic field frequency ω is rather low, so that the condition Zω/p3 ≪ 1, where p is the electron momentum, and the condition ħω ≪ p2 are valid. However, this frequency is assumed to be fairly high compared to the electron-Coulomb center collision frequency: ω ≫ vnei. The dependences of the rates of photon absorption and induced radiation on the angle θ between the direction of incident electron and the electromagnetic field polarization vector (assumed to be linearly polarized) are obtained. It is demonstrated that, for any angles θ, the rate of photon absorption is higher than the rate of induced radiation and, therefore, the Marcuse effect for slow electrons (electromagnetic field amplification) is absent. It is further demonstrated that a slow electron on the average absorbs double ponderomotive energy per collision with an ion (Coulomb center) in Maxwellian plasma. This agrees both with the known results calculation for fast electrons and with the known results of the calculation based on the classical Boltzmann kinetic equation for plasma.

Ionization of atoms in strong low-frequency electromagnetic field

The ionization of atoms in a low-frequency linearly polarized electromagnetic field (the photon energy is much lower than the ionization potential of an atom) is considered under new conditions, in which the Coulomb interaction of an electron with the atomic core in the final state of the continuum cannot be considered in perturbation theory in the interaction of the electron with the electromagnetic field. The field is assumed to be much weaker that the atomic field. In these conditions, the classical motion of the electron in the final state of the continuum becomes chaotic (so-called dynamic chaos). Using the well-known Chirikov method of averaging over chaotic variations of the phase of motion, the problem can be reduced to non-linear diffusion on the energy scale. We calculate the classical electron energy in the final state, which is averaged over fast chaotic oscillations and takes into account both the Coulomb field and the electromagnetic field. This energy is used to calculate the probability of ionization from the ground state of the atom to a lower-lying state in the continuum using the Landau-Dykhne approximation (to exponential accuracy). This ionization probability noticeably depends on the field frequency. Upon a decrease in frequency, a transition to the well-known tunnel ionization limit with a probability independent of the field frequency is considered.

Nuclear fusion induced by a super-intense ultrashort laser pulse in a deuterated glass aerogel

A SiO2 aerogel with absorbed deuterium is proposed as a target for the fusion reaction d + d → He3 + n induced by a superintense ultrashort laser pulse. The multiple inner ionization of oxygen and silicon atoms in the aerogel skeleton occurs in the superintense laser field. All the formed free electrons are heated and removed from the aerogel skeleton by the laser field at the front edge of the laser pulse. The subsequent Coulomb explosion of the deuterated charged aerogel skeleton propels the deuterium ions up to kinetic energies of ten keV and higher. The neutron yield is estimated at up to 105 neutrons per laser pulse for ∼200–500 ps if the peak intensity is 1018 W/cm2 and the pulse duration is 35 fs.

Vacuum heating of large atomic clusters by a superintense femtosecond laser pulse

The analytic approach of vacuum (Brunel) heating mechanism is generalized to the case of large atomic clusters irradiated by a superintense femtosecond laser pulse. The hydrodynamic cluster expansion is taken into account in this approach. Simple universal expressions are obtained for the absorbed laser energy by a cluster and for the radius of an expanding cluster. The absorption of laser energy and the cluster expansion are determined by only one dimensionless field parameter.

Multiple-scale analysis for resonance reflection by a one-dimensional rectangular barrier in the Gross-Pitaevskii problem

We consider a quantum above-barrier reflection of a Bose-Einstein condensate by a one-dimensional rectangular potential barrier, or by a potential well, for nonlinear Schroedinger equation (Gross-Pitaevskii equation) with a small nonlinearity. The most interesting case is realized in resonances when the reflection coefficient is equal to zero for the linear Schroedinger equation. Then the reflection is determined only by small nonlinear term in the Gross-Pitaevskii equation. A simple analytic expression has been obtained for the reflection coefficient produced only by the nonlinearity. An analytical condition is found when common action of potential barrier and nonlinearity produces a zero reflection coefficient. The reflection coefficient is derived analytically in the vicinity of resonances which are shifted by nonlinearity.

Laser-induced conductivity of semiconductors at low temperatures

We consider the negative conductivity of electrons in semiconductors excited by a picosecond laser pulse at low temperatures, due to the inelastic electron-phonon collisions. For the first time, the dependence of the deformation potential on the phonon wave number is taken into account. This dependence significantly changes the region of negative electron conductivity as a function of the phonon temperature.

Rotation and alignment of diatomic molecules and their molecular ions in strong laser fields

A theory of classical rotation and alignment of diatomic molecules with and without permanent dipole moments and of their molecular ions in strong laser fields is developed. The actions of both cw and pulsed laser fields is considered. Conditions under which molecular axes are aligned with the field, which is presumed to be linearly polarized, have been determined. The analysis leads to a conclusion that molecules exposed to ultrashort laser pulses continue to rotate even after the end of the laser pulse. The effect of dynamical chaos on the rotational angular velocity in strong laser field is discussed.

Dynamic chaos in the solution of the Gross-Pitaevskii equation for a periodic potential

We analytically and numerically investigate the solution to the stationary Gross-Pitaevskii equation for a one-dimensional potential of the optical lattice in the case of repulsive nonlinearity. From the mathematical viewpoint, this problem is similar to the well-known problem of the classical mathematical Kapitza pendulum perturbed by a weak high-frequency force. At certain values of the parameters, dynamic chaos is produced in the considered problem. It is modeled analytically by a nonlinear diffusion equation.

Recombination processes in atomic clusters irradiated by superintense femtosecond laser pulses

Various mechanisms of recombination of electrons with multiply charged atomic ions in atomic clusters irradiated by superintense femtosecond laser pulses are discussed. All of the recombination mechanisms are shown to take a time considerably longer than the laser pulse duration and, hence, they can develop only in a homogeneous, fairly rarefied cluster plasma after pulse termination. All autoionization states of multiply charged ions in a dense cluster plasma have been found to be destroyed by the Holtsmark electric field.

Generation of harmonics during the interaction of ultrashort superstrong laser pulses with solid targets

Generation of even and odd harmonics in the skin layer formed during the interaction of a short relativistic laser pulse with solid targets is considered. The complex motion of free electrons in the skin layer along the electric field vector and along the direction of propagation of a laser wave is analyzed. The Fourier expansion of the trajectory of this motion is used to obtain the components of the conductivity tensor and of the amplitude of the transverse electromagnetic field of harmonics propagating along the electric field. Even harmonics appear due to relativistic effects. The efficiency of generation of even and odd harmonics at the leading front of a laser pulse is calculated.