The ionization of atoms in an intense nonclassical electromagnetic field

The ionization of atoms in a strong nonclassical unimodal electromagnetic field was considered. It was shown that the probability of ionization could substantially change depending on the quantum state of the field even at a constant mean number of quanta in the radiation mode. The difference of ionization rates was especially large for multiphoton ionization processes. It was, in particular, shown that a nonclassical field could be much more effective from the point of view of the ionization of atoms than a classical field of the same intensity. The characteristics of the decay of a bound atomic system state in a strong nonclassical field were studied without invoking perturbation theory.     

Entanglement of atoms by a resonance classical electromagnetic field

It is shown that the interaction of a system of two two-level atoms with a resonance classical electromagnetic field under conditions of collective relaxation gives rise to quantum correlations and controlled entanglement. Various models of the bath are examined, such as the unidirectional, one-dimensional, and three-dimensional models, as well as a squeezed bath. As a measure of the entanglement, the minimum negative eigenvalue of the Peres-Horodecki matrix is used.     

New computational method for atoms in an intense electromagnetic field

A new representation is found for the interaction of intense circularly polarized light with an atom. A stationary centrosymmetric part, which depends on the field parameter a ₀=F/ω², is separated out from the atom-field interaction. The time-dependent part of the interaction is represented in the form of a multipole expansion with a ₀ taken into account. The application of this representation for calculating the nonlinear dynamic polarizability of a complicated atom in the random-phase approximation with exchange is studied. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 189–193 (10 August 1998)     

Numerical modeling of the photoionization of Rydberg atoms by the field of an electromagnetic wave

The ionization of excited hydrogen-like atoms in a femtosecond laser pulse is studied by direct numerical integration of the time-dependent Schrödinger equation for a quantum system in the field of an electromagnetic wave. Expressions are obtained for the ionization probability of the system as a function of the parameters of the laser pulse and the initial state of the atom. Ionization suppression is found, confirming the basic premises of the theory of interference stabilization of Rydberg atoms.     

Quantum theory of electromagnetic fields interacting with atoms and molecules

This paper reviews the recent achievements in nonrelativistic quantum electrodynamics, especially nonlinear and coherent phenomena. The general properties of coupled radiation and matter are presented within simple models in section 1. The following sections treat in some detail three main aspects of the system and can be read independently of each other. Section 2 discusses some experiments with long-wave-length radiation (r.f.) and atoms. Section 3 presents the quantum theory of a laser and the ensuing photon distributions. Section 4 treats the case of strongly correlated emission of radiation called superradiance. The use of statistical ensembles is briefly discussed in Appendix A, whereas Appendices B, C and D present some technical details of the text.     

Cooling and trapping of atoms and molecules by a resonant laser field

A method is suggested for simultaneous cooling and trapping of atoms and molecules in a low-pressure gas under forces caused by recoil during spontaneous or induced transitions of the particles in the resonance field of a three-dimensional standing light wave. It is shown that at light field intensities ～0.01–0.1 W/cm² it is possible to cool atoms and molecules to the resonance field photon momentum and to hold the particles in the light field volume during a long period of time. The proposed approach opens the way for high-resolution Doppler-free spectroscopy of a small number of atoms and molecules.     

Scattering of atoms by light

This review discusses the latest theoretical and experimental achievements in the resonant light pressure acting on the translational motion of atoms. Alongside with the effects due to the spontaneous light pressure (atomic deflection, velocity bunching, cooling), various manifestations of the effects of induced light pressure are considered in detail. This paper provides the theory and experiments of atoms scattering by a standing light wave under the conditions of coherent and non-coherent interaction, diffraction and interference of atomic beams. The problems where atomic motion along two trajectories and Landau-Zener transitions between them are essential, are studied. The kinetic phenomena (scattering, cooling, channeling) due to the motion of the particles exposed to gradient force and also friction and diffusion caused by spontaneous emission are considered. The influence of the recoil effect under spontaneous emission of atoms on non-linear polarization phenomena is discussed.     

Scattering of electrons by atoms

The theoretical procedures of current interest in the calculation of electron atom scattering are discussed with emphasis on the intermediate energy range. The essential features of the mathematical methods are described. Some specific results are compared with experiment to illustrate the accomplishments and the limitations of the theory.     

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.     

Scattering of electromagnetic radiation by an ionized gaseous medium

A statistical model is proposed for the scattering of microwave radiation by a weakly ionized gaseous medium irradiated by hard ionizing radiation. It is shown that owing to the correlation of the positions of the elementary scatterers, the predominant contribution comes from scattering on the tracks of the high-energy ionizing particles. The prospects for radar detection of the tracks of radioactive ionization of air are assessed. The maximum distance for radar detection of a cloud of radioactive contamination is calculated as a function of the second release of nuclide activity. An analysis is made of the dependence of the detection threshold on the type of radioactivity, observation geometry, and wavelength of the probe radiation. Zh. Tekh. Fiz. 67, 76–82 (February 1997)     

Elementary processes involving Rydberg atoms and molecules in an intense laser radiation field

An analytical review of the theory of elementary atomic and molecular processes in the field of intense laser monochromatic radiation is presented. The discussion focuses on near-threshold processes involving Rydberg intermediate complexes, which play an important role in Earth’s upper atmosphere. The possibilities of the stationary method of radiative scattering matrix, based on the formalism of the renormalized Lippmann-Schwinger equations, are demonstrated. The approach is used to describe in a unified way a variety of near-threshold processes, the probability amplitudes of which are elements of the radiative scattering matrix for all possible reaction channels. They also include processes of restructuring of the particles (predissociation, associative ionization, exchange reactions, etc.), which in principle cannot be described using traditional nonstationary theories.     

Accuracy of mean field approximations for atoms and molecules

We estimate the accuracy of the mean field approximation induced by the Thomas-Fermi potential for the ground state energy of atoms and molecules. Taking the Dirac exchange correction into account, we show the error to be of the formO(Z ^5/3– )+D for any <2/231 as the total nuclear chargeZ becomes large.D is an electrostatic energy of the difference density that measures the deviation of the mean field groud state from self-consistency.     

Scattering of charged particles by atoms

A closed variant of the Born approximation for calculating differential scattering cross sections in ion-atom collisions is developed. An expression in terms of the matrix elements J _ij with respect to the single-electron states of the atom is found for the matrix element describing the target atom in the formula for the differential cross section. The matrix elements J _ij are averaged over the relative orientation of the momentum transferred in the collision and the symmetry axis of the electronic orbitals of the target atom, using the single-electron Rutaan-Hartree-Fock wave functions. The algebraic representation of the matrix elements J _ij makes it possible to perform calculations for atoms with any value of Z. The model developed is used to calculate the cross sections σ_Σ and characteristic scattering angles θ_c for the process of electron loss by H^? ions with energy E = 0.1–100 MeV in targets consisting of atoms with Z = 2–54. It is shown that σ_Σ ∝ E ^?1 and θ_c ∝ E ^?1/2 for all Z, and for fixed E the behavior of σ_Σ(Z) and θ_c(Z) is determined by the order of filling of the electronic shells of the target atoms (the ionization potential). The computational results are analyzed and compared with the experimental data and the results of other calculations.     

Reactions of excited atoms and molecules with atoms and molecules

Penning electron distributions arising from the ionization of Na and K by He (1s 2s ^1,3 S)-metastables in thermal collisions, as well as the absolute cross section for Penning ionization of Na by He (2³ S) and relative cross sections for ionization of Na and K by He(2¹ S) and He(2³ S) are measured. It is shown that under fairly general conditions the well depth ε* of the interaction potential between the metastable and the target particle can be obtained directly from the measured electron distributions. ε*-values are reported for the moleules He(1s 2s ^1,3 S)-Na(² S), K(² S) (^2,2Ω), and for He(1s 2s ^1,3 S)-Hg(¹ S)(^1,3Ω). These latter values are obtained from previously published measurements and are to be considered preliminary. Further, additional evidence is given, that Penning ionization with metastables is an electron exchange process.     

Multiphoton transitions in helium atoms in intense electromagnetic field

The T-matrix for the interaction of intense electromagnetic field with helium-like atoms is obtained using a time-dependent unitarity transformation. Essentially the transformation acts as a space transition operation.     

Reactions of excited atoms and molecules with atoms and molecules

From a He-beam excited by electron impact we eliminated the He(2¹ S) component to better than 0.5% by irradiating light from a He discharge. The quenching process ishv(2¹ P→2¹ S)+He(2¹ S)→He(2¹ P)→He(1¹ S +hv) (2¹ P 1)¹ S. By measuring the ions produced in collisions of the He-metastables with various target gases in a mass spectrometer, singlet to triplet Penning-cross section ratios were obtained. These ratios are without exception close to one, which is taken as evidence for the previously proposed electron exchange mechanism of the Penning ionization. In the case that more ions are produced in the collision of He (2¹ S) and He(2³ S) with a target gas, separate relative production cross sections are obtained for the two metastables. For the rare gases the measurements are performed at two temperatures of the He-beam, 320 and 90 °K. It is found that the cross section ratio of associative — to Penning ionization increases considerably as the temperature is decreased for both, He(2¹ S) and He(2³ S), the effect being much more pronounced for He(2¹ S). The results of this work are found to confirm conclusions drawn from measured energy distributions of the electrons ejected in the Penning process.