Highly selective multistep ionization of atoms by laser radiation

The possibility of dramatic selectivity increase during the stepwise ionization of atoms due to selective excitation at every step has been examined. Evaluations have been made to estimate the selectivity of excitation and ionization by laser radiation of some atoms, their isotopic shifts of absorption lines being observed at several excitation steps, for instance those of U, Yb, Gd. During three-step ionization of Yb atoms in atomic beam one may obtain an excitation selectivity equal to 4.4 × 10¹⁶ cm².     

Multiphoton and tunneling ionization of atoms in an intense laser field

We study the ionization probabilities of atoms by a short laser pulse with three different theoretical methods,i.e.,the numerical solution of the time-dependent Schro¨dinger equation(TDSE),the Perelomov-Popov-Terent’ev(PPT) theory,and the Ammosov-Delone-Krainov(ADK) theory.Our results show that laser intensity dependent ionization probabilities of several atoms(i.e.,H,He,and Ne) obtained from the PPT theory accord quite well with the TDSE results both in the multiphoton and tunneling ionization regimes,while the ADK results fit well to the TDSE data only in the tunneling ionization regime.Our calculations also show that laser intensity dependent ionization probabilities of a H atom at three different laser wavelengths of 600 nm,800 nm,and 1200 nm obtained from the PPT theory are also in good agreement with those from the TDSE,while the ADK theory fails to give the wavelength dependence of ionization probability.Only when the laser wavelength is long enough,will the results of ADK be close to those of TDSE.     

Relativistic theory of tunnel and multiphoton ionization of atoms in a strong laser field

Relativistic generalization is developed for the semiclassical theory of tunnel and multiphoton ionization of atoms and ions in the field of an intense electromagnetic wave (Keldysh theory). The cases of linear, circular, and elliptic polarizations of radiation are considered. For arbitrary values of the adiabaticity parameter γ, the exponential factor in the ionization rate for a relativistic bound state is calculated. For low-frequency laser radiation , an asymptotically exact formula for the tunnel ionization rate for the atomic s level is obtained including the Coulomb, spin, and adiabatic corrections and the preexponential factor. The ionization rate for the ground level of a hydrogen-like atom (ion) with Z ? 100 is calculated as a function of the laser radiation intensity. The range of applicability is determined for nonrelativistic ionization theory. The imaginary time method is used in the calculations.     

Tunneling ionization of air in the strong field of femtosecond laser pulses

The effect of the tunneling ionization of air excited by individual intense femtosecond pulses with subcritical peak powers has been investigated by the optoacoustic method. With an increase in the laser radiation intensity in the range of 0.5–20 PW/cm², the saturation of the amplitude of acoustic pressure with the corresponding decrease in the nonlinearity index of this dependence is observed. The analysis of the ionization of air molecules in the Ammosov-Delone-Krainov model of tunneling ionization predicts a weakly nonlinear increase and saturation of the yield of singly charged ions at an air ionization degree close to 50% under these conditions of the effect of femtosecond laser radiation.     

Interference effects in the scattering of identical atoms in a laser radiation field

The properties of the collision integral in a quantum Boltzmann-type kinetic equation are studied under the conditions of spatially nonuniform distributions of colliding particles interacting with an external electromagnetic field. The components of the nonlinear resonances and the velocity distribution of the excited atoms, which are due to polarization transitions, are determined on the basis of the Kazantsev collision integral.     

Dynamics of moving interacting atoms in a laser radiation field and optical size resonances

The forces acting on interacting moving atoms exposed to resonant laser radiation are calculated. It is shown that the forces acting on the atoms include the radiation pressure forces as well as the external and internal bias forces. The dependences of the forces on the atomic spacing, polarization, and laser radiation frequency are given. It is found that the internal bias force associated with the interaction of atomic dipoles via the reemitted field may play an important role in the dynamics of dense atomic ensembles in a light field. It is shown that optical size resonances appear in the system of interacting atoms at frequencies differing substantially from transition frequencies in the spectrum of atoms. It is noted that optical size resonances as well as the Doppler frequency shift in the spectrum of interacting atoms play a significant role in the processes of laser-radiation-controlled motion of the atoms.     

Tunneling ionization of atoms with excitation of the core

A general formula is obtained for the probability of tunneling ionization of an atom accompanied by excitation of the core. This formula is a generalization of the Carlson formula for the probability of a single-photon two-electron transition in atoms. The limiting case of this formula, just as that of the Carlson formula, is the well-known random-perturbations approximation. Numerical results are presented for Zn, Sr, and Cd atoms. For these atoms the contribution of the excited states of singly charged ions to the probability of the formation of doubly charged ions is a nonmonotonic function of the laser radiation intensity. Analysis of the tunneling ionization of molecules shows that with overwhelming probability an ion is formed in the ground vibrational state, while for the standard photoionization the distribution over vibrational states is determined by the Franck-Condon factors.     

Lorentz ionization of atoms in a strong magnetic field

Lorentz ionization emerges due to the motion of atoms or ions in a strong magnetic field. We use the semiclassical approximation to calculate the probability w _L of Lorentz ionization. We also find the stabilization factor S, which takes into account the reduction by the magnetic field of the probability of ionization decay of the bound s state. We estimate the probabilities w _L in magnetic-cumulation experiments and in astrophysics. We also qualitatively examine the dynamics of the magnetic cumulation process with allowance for the conductivity of the shell. Finally, we discuss a paradox related to the use of the quasistationary solution at the shell expansion stage. Zh. éksp. Teor. Fiz. 115, 1642–1663 (May 1999)     

Tunneling ionization of a hydrogen atom in short and ultrashort laser pulses

The ionization of a hydrogen atom in a linearly polarized low-frequency electromagnetic field is investigated by direct numerical integration of the time-dependent Schrödinger equation. The data obtained for various ionization regimes and various initial atomic states are compared with the Keldysh and Perelomov-Popov-Terent’ev (PPT) theories. The validity ranges for the quasi-static model of tunneling ionization and the PPT theory in laser intensity and frequency are determined. The tunneling ionization of the excited 2s and 2p states is discussed. The ionization of a hydrogen atom in an ultrashort (on the order of one optical period) pulse is investigated.     

Production of photoionic gallium beams through stepwise ionization of atoms by laser radiation

Production of intensive (up to 1 mA) ionic beams through stepwise photoionization of atoms induced by laser radiation is investigated. Optimum photoionization schemes are presented. A photoionic gallium beam is obtained with an average current value of 2.5 μA.     

Multiphoton ionization of atoms by a two-color laser pulse

The probability of multiphoton ionization of atoms in a laser radiation field containing an additional second harmonic is calculated by the imaginary time method [9, 10]. The conditions are found when the second-harmonic contribution to the ionization of atoms dominates over that of the first harmonic. It is shown that the average momentum of photoelectrons ejected from atoms depends on the phase shift between the first and second harmonics and their mutual polarization. The obtained asymptotic expressions can be used for the qualitative explanation of experiments on generation of terahertz radiation from the optical breakdown region in gas in the focus of a femtosecond laser.     

Trapping and storage of atoms in a laser field

Analysis of the possibility of spatial trapping of cold atoms in a standing-wave laser field is presented. It is shown that cold atoms can be trapped for a long time in region ≈ λ in a nonresonant standing-wave field. In a resonant standing-wave field cold atoms can be stored for a long time in the region determined by the cross-section of the laser beam. Trapping and storage together with cooling of atoms that has been suggested earlier allow us in specific cases to increase the sensitivity and the resolution of spectroscopic studies.     

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.     

Optical radiation and ionization of hydrogen atoms in heterogeneous exothermal reactions proceeding in an electric field

Optical radiation related to the Balmer series (H_α, H_β, H_γ) of hydrogen atoms is discovered when studying the isothermal reaction of trimeric acetone peroxide decomposition on the surface of oxidized tungsten in a static electric field with a strength of up to 4 × 10⁶ V/cm at T = 300 K. The distance from the surface over which desorbing excited hydrogen atoms radiate is determined from the Stark splitting of the lines. Electronically excited atoms remaining on the surface ionize according to the surface ionization mechanism.     

Quantum-electrodynamic effects for atoms in a laser field

Quantum-electrodynamic radiation corrections to atomic energy levels in the presence of monochromatic laser field are considered as radiation shifts of quasienergies. General expressions are obtained for self-energy part of radiation shifts for an arbitrary multilevel atomic system. As an appendix, the radiation shifts of spectral lines of atomic resonance transitions are investigated.     

Controlled suppression of ionization of Rydberg atoms in a microwave field

A computational procedure for controlling the stochastic ionization of one-dimensional single electron Rydberg atoms in the correspondence principle regime is developed. Using our procedure it is possible to suppress excitation and ionization of the one-dimensional Rydberg atom even in strong microwave fields for which ionization would otherwise be instantaneous.