Theory of multiphoton and tunnel ionization in a bichromatic field

The imaginary-time method [6, 7] is used to calculate the multiphoton and tunnel ionization probabilities for atoms in a laser radiation field part of which is converted into the second harmonic. We assume that the first harmonic has a linear or elliptical polarization and the second harmonic is polarized linearly, with its polarization vector making an arbitrary angle with that of the first harmonic. The mean momentum of the photoelectrons knocked out from atoms is shown to depend on the phase shift between the first and second harmonics and their mutual polarization and to be identically equal to zero for a monochromatic field. An important difference between the case of elliptical polarization and the case of linear polarization of both harmonics is the absence of conditions under which the conditions for dominance of one of the two generation mechanisms considered here can be identified during the generation of terahertz radiation from the region of optical breakdown in a gas.     

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)     

飞秒激光惰性气体Ar原子多次电离过程的空间分辨研究

使用100飞秒、1014 W/cm2的强激光与惰性气体Ar原子相互作用,我们利用飞行时间质谱仪结合Z扫描技术获得了各次电离的Ar原子在激光焦点附近不同位置处强度分布.测量得到的各个不同电离态Ar n+(n=1～4)的产额随激光焦点位置变化关系与不同电离态的空间分布理论预言相一致.     

Lithium ionization by a strong laser field

We study ab initio computations of the interaction of lithium with a strong laser field. Numerical solutions of the time-dependent fully correlated three-particle Schrodinger equation restricted to the one-dimensional soft-core approximation are presented. Our results show a clear transition from nonsequential to sequential double ionization for increasing intensities. Nonsequential double ionization is found to be sensitive to the spin configuration of the ionized pair. This asymmetry, also found in experiments of photoionization of Li with synchrotron radiation, shows evidence of the influence of the exclusion principle on the underlying rescattering mechanism.     

Spontaneous emission of atoms in a strong laser field

The spontaneous emission of an atomic system in the field of a high-intensity femtosecond laser pulse is considered within the framework of a consistent quantum-mechanical approach based on an examination of the interaction of a quantum system with a set of quantized field modes in a vacuum state. Both even and odd harmonics of the driving field are shown to appear in the atomic emission spectrum, and the mechanisms of their generation are elucidated. A comparison with the semiclassical theory of laser harmonic generation is made. The semiclassical approach in describing the spontaneous emission in strong laser fields, especially under conditions of significant depletion of the ground (initial) state in a laser field, is shown to be limited.     

Tunneling and above-barrier ionization of atoms in a laser radiation field

Calculations are made of the energy and angular distributions of photoelectrons during tunneling ionization of an atom or an ion under the action of high-power laser radiation (for all values of the Keldysh parameter γ). Cases of linear, circular, and elliptic polarizations of the electromagnetic wave are considered. The probability of above-barrier ionization of hydrogen atoms in a low-frequency laser field is calculated. Formulas are given for the momentum spectrum of the electrons when an atomic level is ionized by a general type of alternating electric field (for the case of linear polarization). An analysis is made of tunneling interference in the energy spectrum of the photoelectrons. Analytic approximations are discussed for the asymptotic coefficient C _κ of the atomic wave function at infinity (for s-wave electrons).     

Mechanism of delayed double ionization in a strong laser field

When intense laser pulses release electrons nonsequentially, the time delay between the last recollision and the subsequent ionization may last longer than what is expected from a direct impact scenario [recollision excitation with subsequent ionization (RESI)]. We show that the resulting delayed ionization stems from the inner electron being promoted to a sticky region. We identify the mechanism that traps and releases the electron from this region. As a signature of this mechanism, we predict oscillations in the ratio of RESI to double ionization yields versus laser intensity.     

Lamb shift in atoms interacting with a strong laser field

Radiative corrections to the energy levels of an atom interacting with a strong laser field have been investigated. A formula for the Lamb shift of the laser-dressed states is derived, which makes it possible to calculate this shift with an error up to α². Original Russian Text ? R.Kh. Gainutdinov, A.A. Mutygullina, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 2, pp. 292–293.     

Photoelectron imaging of resonance-enhanced multiphoton ionization and above-threshold ionization of ammonia molecules in a strong 800-nm laser pulse

In this work, we mainly investigate the NH3 molecular multiphoton ionization process by using the photoelectron velocity map imaging technique. Under the condition of femtosecond laser(wavelength at 800 nm), the photoelectron images are detected. The channel switching and above-threshold ionization(ATI) effect are also confirmed. The kinetic energy spectrum(KES) and the photoelectron angular distributions(PADs) are obtained through the anti-Abel transformation from the original images, and then three ionization channels are confirmed successfully according to the Freeman resonance effect in a relatively low laser intensity region. In the excitation process, the intermediate resonance Rydberg states are C~(~(～1)) A _1(6 + 2 photons process), B~(～1) E(6 + 2 photons process) and C~(～1) A _1(7 + 2 photons process), respectively. At the same time, we also find that the photoelectron angular distributions are independent of laser intensity. In addition, the electrons produced by different processes interfere with each other and they can produce a spider-like structure. We also find ac-Stark movement according to the Stark-shift-induced resonance effect when the laser intensity is relatively high.     

Relativistic corrections to the energy levels of hydrogen atoms in a strong magnetic field

Relativistic corrections to the binding energies of various states of the strongly magnetized hydrogen atom are estimated by a perturbation calculation.     

The above-threshold ionization spectrum in a strong linearly polarized laser field

We use the saddle-point method to derive analytical expressions for the spectral-angular probability distributions of the ionization by a strong linearly polarized laser field in Keldysh’s model that are valid for an arbitrary electron energy and adiabaticity parameter. We obtain asymptotic expansions of the general formulas in the multiphoton and tunneling regimes, analyze their accuracy, and formulate the validity conditions. We provide literal estimates of the parameters that characterize the properties and evolution of the distributions.     

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.     

飞秒激光作用下3-甲基吡啶的多光子电离质谱研究

利用800 nm的飞秒激光作为光源对3-甲基吡啶分子进行了多光子电离解离过程和机制的研究,发现3-甲基吡啶为先电离后解离,并分析了可能的解离通道.在B3LYP/6-311G++(d,p)水平下,计算了各个解离通道所需的能量,与我们实验所得的各离子的光强指数符合较好.     

Relativistic theory of inverse beta-decay of polarized neutron in strong magnetic field

The relativistic theory of the inverse beta-decay of polarized neutron,ν _e +n → > p +e ^-, in strong magnetic field is developed. For the proton wave function we use the exact solution of the Dirac equation in the magnetic filed that enables us to account exactly for effects of the proton momentum quantization in the magnetic field and also for the proton recoil motion. The effect of nucleons anomalous magnetic moments in strong magnetic fields is also discussed. We examine the cross-section for different energies and directions of propagation of the initial neutrino accounting for neutron polarization. It is shown that in the super-strong magnetic field the totally polarized neutron matter is transparent for neutrinos propagating antiparallel to the direction of polarization. The developed relativistic approach can be used for calculations of cross-sections of the other URCA processes in strong magnetic fields.     

Absolute ionization rates of multielectron transition metal atoms in strong infrared laser fields

We report on nonresonant strong field ionization of the multielectron transition metal atoms V, Nb, Ta, Ni, and Pd. Operating in the adiabatic regime (lambda = 1.5 microm), we quantitatively determined both (i) the first charge state saturation intensities and (ii) the absolute ionization rates for intensities ranging from threshold up to 3 x 10(14) W/cm2. We observed a dramatic suppression of ionization relative to single active electron approximation expectations. We suggest that this derives from dynamic polarization or screening effects within the multielectron atom, stressing a need for many-body theories of strong field ionization.