Centrifugal electron-ion recombination

Transient electric-field pulses have been used to stimulate electron/ion recombination in a low density plasma in the presence of a static electric field. The measured recombination rates exhibit a strong dependence on the relative orientation of the pulsed and static fields. For weak pulses, the recombination rate is significantly higher for orthogonal as opposed to parallel or antiparallel field configurations. The enhanced recombination rate is attributed to the dynamic stabilization of high-m Rydberg levels that are populated during the pulse. Classical simulations confirm the importance of angular momentum rather than energy transfer.     

Enhancement of low energy electron-ion recombination in a magnetic field: influence of transient field effects

Electron-ion recombination observed in storage ring experiments shows a strong enhancement relative to what standard radiative recombination rates predict. We simulate the effect of a transient motional electric field induced by the merging of an electron and an ion beam in the electron cooler which opens an additional pathway for free-bound transitions of electrons. We show that the measured rate contains a significant contribution from radiative stabilization of Rydberg states formed by this transient motional electric field. The absolute excess recombination rates obtained are in good agreement with the experimental data. The scaling of the rate with the ion charge and the magnetic guiding field is analyzed.     

Resonant enhancement of dielectronic recombination by a microwave field

We have observed dielectronic recombination of Ba+ and e(-) from a continuum of finite bandwidth in the presence of microwave fields of frequencies 8.08 and 12.05 GHz and amplitudes of up to 2 V/cm. There are sharp resonant enhancements when the microwave frequency matches the Deltan = 1, 2, and 3 resonances of the intermediate autoionizing Rydberg states, and we attribute the enhancements to resonant microwave Stark l mixing. The microwave field provides a simple and powerful way to enhance the recombination rate for incident electrons of a specific energy.     

Effect of target polarization in electron-ion recombination

We present results of a study of the effect of target polarization on electron-ion recombination, and show that coherent radiation by the target electrons gives a large contribution to the recombination rate. It significantly modifies the nonresonant photorecombination background. A procedure has been devised whereby this contribution can be evaluated together with the conventional radiative recombination, independently of the dielectronic recombination component. Numerical results are presented for Zn2+, Cd2+, Sn4+, and Xe8+, showing up to an order-of-magnitude enhancement.     

Correlation effects in electron-ion collisions in a strong laser field

We examine elastic and inelastic scattering of electrons by ions in intense laser light. A method of numerical investigation of the scattering characteristics based on regularizing the Coulomb singularity is proposed. We show that over a broad range of parameter values the transport scattering cross section is weakly dependent on the intensity of the high-frequency field. We detect a significant modification of the dependence of the effective inelastic scattering cross section. We also show that the energy exchange with the field is determined by a fairly small group of electrons, called the representative electrons. Finally, we propose a qualitative model that explains our results by the fact that the leading contribution is provided by inelastic collisions of electrons with relatively small impact parameters traversing the region important for the interaction at large angles. Zh. éksp. Teor. Fiz. 115, 463–478 (February 1999)     

Experimental determination of the electron-avalanche and the electron-ion recombination coefficient

The electron-ion recombination coefficient γ and the avalanche coefficient δ = (α ? a) · v_d, where α and a are the ionizat ion and attachment coefficients respectively and v_d the drift velocity of the electrons, have been experimentally determined in a self-sustained CO₂-laser system (1:1:3 mixture) as a function of the E/N value. For low voltages we found the expected decrease of the recombination coefficient for increasing E/N values. However, it appears that for larger voltage the recombination coefficient increases sharply for increasing E/N values. The measurements of δ show a much smaller value than expected from theoretical calculations. This must be explained by a lower value of the electron-energy distribution function for higher energies, which may be consistent with our measured high recombination probability for electrons having high energy.     

Breakdown in air in a rising microwave field

Results are presented from studies of a high-pressure electrodeless breakdown in air at the focus of a standing wave in a high-Q quasi-optical two-mirror resonator pumped by single microwave pulses. In the experiment, the breakdown occurred at the front of the pulse of the resonator field. The breakdown field substantially exceeded the critical level and, under fixed conditions, showed a scatter from pulse to pulse. It is shown that the experimentally found excess in the threshold breakdown field over the critical level is due to the fact that the resonator field increases as a discharge plasmoid forms during breakdown and that the appearance of an electron initiating breakdown in a gas is a random event.     

Highly charged ions from laser-cluster interactions: local-field-enhanced impact ionization and frustrated electron-ion recombination

Our molecular dynamics analysis of Xe_{147-5083} clusters identifies two mechanisms that contribute to the yet unexplained observation of extremely highly charged ions in intense laser cluster experiments. First, electron impact ionization is enhanced by the local cluster electric field, increasing the highest charge states by up to 40%; a corresponding theoretical method is developed. Second, electron-ion recombination after the laser pulse is frustrated by acceleration electric fields typically used in ion detectors. This increases the highest charge states by up to 90%, as compared to the usual assumption of total recombination of all cluster-bound electrons. Both effects together augment the highest charge states by up to 120%, in reasonable agreement with experiments.     

Phase-dependent effects of a few-cycle laser pulse

The Keldysh theory of above-threshold ionization (ATI) is applied to few-cycle laser pulses in order to explore the potential of a recently published new method to measure "carrier-envelope phase difference" phenomena. In this experiment, the carrier-envelope phase difference dependent left-right asymmetry of few-cycle ATI was measured and investigated with a correlation technique. Here, we explore spectral features of the asymmetry, present a theoretical analysis of the experiment, and establish a method to determine the duration of few-cycle pulses whose carrier-envelope phase differences are not controlled.     

四能级系统中相位控制电磁诱导透明研究

提出了一种利用微波场的相对相位调控电磁诱导透明(EIT)光谱特性的方法. 在外加双微波驱动场的准Λ型四能级原子系统中,通过求解系统的密度矩阵方程得到探测场的吸收谱,分析了电磁诱导透明窗口的位置随微波驱动场相位的变化规律,并借助缀饰态理论给出了准确解释. 结果表明,对于确定的作用场强度,调节微波驱动场的相位可实现电磁诱导透明的频率位置及间隔的准确控制. 关键词： 四能级原子系统 电磁诱导透明(EIT) 相位控制 微波场     

Control of radiative recombination by a strong laser field

Strong-field laser-assisted radiation recombination is investigated: (a) to explore the control possibilities with two laser fields having commensurable frequencies and (b) to get some insight into the influence of the medium (a plasma) in which recombination occurs. It is found that by varying the relative phase of the two fields it is possible to control enhancement, broadening and symmetry properties of the recombination differential emitted power. In the case of an anisotropic two-temperature plasma, an interesting interplay is found between the shape of the laser-modified electron velocity distribution function and the shape of the emitted X-ray photon power spectrum. The novel features of strong laser-assisted radiative recombination are mostly restricted to the region of the process parameters corresponding to the physical situation when the characteristic electron velocities (translation, quiver and thermal) are comparable. A classical picture allows a fairly good understanding of the process. PACS 32.80.Wr; 34.80.Lx; 34.50.Rk     

Quantum Poincare recurrences for a hydrogen atom in a microwave field

We study the time dependence of the ionization probability of Rydberg atoms driven by a microwave field, both in classical and in quantum mechanics. The quantum survival probability follows the classical one up to the Heisenberg time and then decays algebraically as P(t) approximately 1/t. This decay law derives from the exponentially long times required to escape from some region of the phase space, due to tunneling and localization effects. We also provide parameter values which should allow one to observe such decay in laboratory experiments.     

Spin-polarized current in a Rashba ring pumped by a microwave field

We report a theoretical study on generation of a spin polarized charge current with arbitrary spin polarization, including the fully-spin-polarized current. In a two-terminal mesoscopic ring device, the Rashba spin-orbit coupling (RSOC) is considered as well as a microwave field applied on one of arms of the ring. It is shown that at zero external bias a spin current can be produced in addition to the usual charge current pumped by the microwave field, which is attributed to the the quantum interference effect of the RSOC induced spin precession phase. By varying the system parameters such as the microwave frequency and the RSOC strength, not only the magnitude but also the direction of the spin current can be efficiently controlled, moreover, the spin-polarization degree of the charge current can readily be tuned by these system parameters in the range [-1,1]. Since all the parameters can be controlled electrically in our study, the proposed device may shed light on the possibility of an all-electrical generation and tuning of a spin-polarized current in the field of the spintronics.     

Collisions between linear polar molecules trapped in a microwave field

The collisions between linear polar molecules, trapped in a microwave field with circular polarization, are theoretically analyzed. We demonstrate that the collisional dynamics is mostly controlled by two ratios ν/B and x=μ₀E₀/ħ B (ν is the microwave frequency, B is the molecular rotational constant, μ₀ is the dipole moment, and E₀ is the electric field amplitude). We discuss the dependence of collision cross sections on these ratios in order to find an advantageous condition for evaporative cooling.     

Resonant scattering of three-level Rydberg atoms in a microwave field

We examine the theory of potential scattering of Rydberg atoms in a microwave field. The model of a three-level atom is employed to calculate the radiative force emerging in the resonant coherent interaction with the microwave field for the case of a two-photon resonance and high intensities, using the method of quasienergies of the system consisting of the atom and the field. We determine the probabilities of Landau-Zener transitions in the spatial regions where under two-photon resonance conditions the quasienergies of the atoms approach one another by a small quantity. We also study the dynamics of the variation of the spatial profile of a beam of Rydberg atoms caused by resonant scattering. Finally, we give the results of the first experimental observation of the variation of the transverse beam profile when Rydberg atoms pass through a nonuniform microwave field formed in a rectangular waveguide and in resonance with the two-photon 36P–37P transition. Zh. éksp. Teor. Fiz. 111, 796–815 (March 1997)     

The electron-ion interactions in metals

The electron-ion interactions are evaluated exactly over the actual shape of the atomic polyhedron, instead of approximating it by a sphere, by making use of simple coordinate axes transformations and lattice symmetry in the case of f.c.c. and b.c.c. structures. It is shown that there are several alternative ways of expressing the interference factor,S(q) and the different expressions given by Sharan and others and Bross and Bohn are just two of these equivalent expressions. By comparing these expressions in the symmetry directions with those obtained under spherical approximation their apparent differences are discussed.