Numerical simulation of the ionization dynamics of a two-electron quantum system in a femtosecond pulse

The ionization of a simple two-electron model system, viz., the one-dimensional negative hydrogen ion, is investigated using direct numerical integration of the time-dependent Schrödinger equation. The one-and two-electron ionization probabilities as functions of frequency and radiation intensity are obtained. It is shown that two-electron ionization is mediated by both direct and sequential mechanisms. The stabilization of the two-electron system against the ionization process is investigated. The data obtained are compared with calculations performed within the one-dimensional single-particle model of H^?. The photoelectron spectrum is analyzed in the region of parameters corresponding to the single-electron ionization regime.     

Ionization of a hydrogen atom from its ground state by a coherent bichromatic laser field

We study the “coherent phase control” between the three-photon ionization by a fundamental laser field and the one-photon ionization by its third harmonic for a hydrogen atom in its ground state. The relative phase δ of the harmonic field with respect to the fundamental laser radiation “modulates” the interference between the two ionization channels, which is important near the crossing points between the ionization rates of the two individual processes. Numerical results for the total ionization rate and for the angular distribution of the photoelectrons as a function of the phase δ are presented for frequencies located in the vicinity of the atomic resonances corresponding to the absorption of two laser photons. Received 31 August 2000 and Received in final form 6 February 2001     

Many-body ionization in a frozen Rydberg gas

In a dense gas of 300 microK 85Rb atoms of n approximately 50 ionization occurs on a 100 ns time scale, far too fast to be explained by the motion of the atoms or photoionization by 300 K blackbody radiation. Rapid ionization is accompanied by spectral broadening, with the spectrum becoming continuous at n=88 at a density of 5x10(10)cm(-3). The atomic transitions broaden both smoothly and by the emergence of new features, which we attribute to multiple atom absorptions. We attribute the rapid ionization to a sequence of near resonant dipole-dipole transitions through virtual states in this intrinsically many-body system, culminating in the ionization of some of the atoms.     

Ionization focusing of a short intense laser pulse and generation of wake plasma waves

The propagation of a short intense laser pulse is studied in a gas taking into account the ionization of gas atoms by the high-frequency electromagnetic field of the pulse. The conditions are found under which the ionization structures produced by the laser pulse cause the pulse focusing accompanied by a substantial increase in its intensity. It is shown that the leading edge of the pulse is subjected to ionization refraction at the ionization front, the temporal profile of the pulse becoming steeper. This results in the efficient generation of a wake wave at the ionization front, which is amplified during the development of self-modulation instability. The amplitude of the wake plasma wave achieves a substantial value already at small paths of the pulse in matter (smaller than the diffraction length of the pulse).     

Time-dependent scattering of a standing surface plasmon by rapid ionization in a semiconductor

Scattering of a standing surface plasmon by rapid ionization in a semiconductor is investigated. We show that, for a standing plasmon, in contrast with a traveling plasmon, the scattering depends on the plasmon phase at the moment of ionization. By changing the moment of ionization, we can control the energy that is transferred into newly excited modes, which include a frequency-upshifted standing surface plasmon, transient outgoing radiation, and free-streaming currents with a static magnetic field in the semiconductor. The phenomena that are described open new possibilities for probing the dynamics of surface excitations in semiconductors on an ultrashort time scale.     

Ionization by a pulsed plasma surface wave

The ionization mechanism is studied of a pulsed surface wave generating a microwave discharge. When the plasma is dominated by collisions, it is found that the velocity of the ionization front depends on the ponderomotive force due to the field gradient in the front.     

Theoretical and experimental investigations on the threshold of a laser-induced breakdown of air

The threshold of a laser-induced breakdown of air is determined experimentally and theoretically. We find that the ionization of air has two steps: the first step is a multi-photon ionization process, which provides enough"seed electrons" to initiate the next step, and the second one is predominated by cascade ionization, which continues to produce free electrons geometrically until the critical free-electron density for breakdown is reached. So a two-step model based on the Morgan ionization model is established to describe the breakdown process. It is found that the time node dividing the two steps is about 9.8 ns in atmospheric air, and the threshold derived from the two-step model proposed here is more consistent with the experimental results than traditional ionization model.     

Ionization of a molecular hydrogen ion by a strong low-frequency electromagnetic field of laser radiation

Simple analytical expressions are obtained for the energy and angular distributions of outgoing electrons in ionization of a molecular hydrogen ion by a strong low-frequency electromagnetic field as well as for the ionization probabilities per unit time. The cases of linear and circular polarization of the laser radiation are studied. It is shown that in contrast to the case of the ionization of atoms oscillations appear in the energy spectra of the photoelectrons as a function of their kinetic energy. The well-known limits for the tunneling ionization probabilities for the hydrogen atom by a strong low-frequency alternating field are obtained in the case of large internuclear separations. Zh. é ksp. Teor. Fiz. 113, 583–592 (February 1998)     

Charge Composition of Ions in a Cluster Plasma Formed under the Action of a High-Power Laser Pulse

A model describing the ionization of atoms and ions in a cluster under irradiation by a short laser pulse has been constructed. It is shown that the electron-impact ionization weakly affects the final charge composition, and the main mechanism of ion formation in the cluster is the over-barrier ionization by an electric field. The electric field acting on atoms and ions is the result of combined action of the external laser field and the intrinsic electric field of the cluster. The key parameters of the cluster beam and the laser pulse, which determine the properties of the charge composition of the cluster plasma, have been established.     

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.     

飞秒强激光场中分子双电离的半经典动力学模拟

文章介绍了超短强激光场中原子、分子双电离的研究现状,提出了能够有效处理双电离过程的半经典模型,成功地重现了双电离率随激光强度变化的实验数据,同时预言了分子取向对双电离率的重要影响.文章还通过分析典型的双电离轨道的演化,给出了理解双电离复杂动力学过程的直观物理图像.     

Semiclassical calculation of ionization rate for Rydberg hydrogen atoms near a metal surface

The ionization of Rydberg hydrogen atoms near a metal surface at different scaled energies above the classical saddle point energy has been discussed by using the semiclassical method. The results show that the atoms ionize by emitting a train of electron pulses. In order to reveal the chaotic and escape dynamical properties of this system in detail, the sensitive dependence of the ionization rate upon the scaled energy is discussed. As the scaled energy is close to the saddle point energy, the ionization process of the hydrogen atom is nearly the same as the case of hydrogen atom in an electric field. There is only a single pulse of electrons, with an exponentially decaying tail. With the increase of the scaled energy, the ionization rates are similar to the case of the hydrogen atom in parallel electric and magnetic field, a series of electron pulses appear in the ionization process. This is caused by classical chaos, which occurs for the metal surface. Our studies also suggest that the metal surface can play the role of both the electric and the magnetic fields. Our theoretical analysis will be useful for guiding experimental studies of the ionization of atoms near the metal surface.     

Generation of homogeneous plasma in a low-pressure glow discharge

The possibility of producing homogeneous plasma in a low-pressure discharge with the use of a hollow anode or hollow cathode is analyzed. It is shown that, in contrast to the high-pressure discharge, where uniform ionization is needed to produce homogeneous plasma, in the low-pressure discharge, nearly uniform radial distribution of the plasma parameters can be achieved under nonuniform ionization conditions by increasing the ionization probability at the system periphery and reducing it near the system axis. It is shown that the magnetic field can facilitate generation of the homogeneous plasma instead of interfering with it.     

Experimental and theoretical analysis of bias ionization by?α-particles in a nitrogen laser

Nitrogen laser performance with TE configuration and wedge electrodes is analyzed with background ionization in the laser discharge channel by α particles at a low exposition rate. With the bias ionization, the laser power presents two peaks as a function of gas pressure, with one at the normal low pressure, without bias ionization, and the other at high pressure generated by bias ionization. A simple theoretical model has been developed in a trial to understand this behavior. This model was first tested in later results for a TE configuration nitrogen laser, with flat electrodes, without and with bias ionization. It has been observed that due to the competition between electrode shielding by positively charged α particles and bulk ionization by impact, the laser energy is suppressed with pressure below 50 Torr and enhanced above it.     

Ionization of a multilevel atom by ultrashort laser pulses

Specific features of ionization of single atoms by laser fields of a near-atomic strength are investigated. Calculations are performed for silver atoms interacting with femtosecond laser pulses with wavelengths λ = 800 nm (Ti:Sapphire) and λ = 1.064 μm (Nd:YAG). The dependences of the probability of ionization and of the form of the photoelectron energy spectra on the field of laser pulses for various values of their duration are considered. It is shown that the behavior of the probability of ionization in the range of subatomic laser pulse fields is in good agreement with the Keldysh formula. However, when the field strength attains values close to the atomic field strength, the discrepancies in these dependences manifested in a decrease in the ionization rate (ionization stabilization effect) or in its increase (accelerated ionization) are observed. These discrepancies are associated with the dependence of the population dynamics of excited discrete energy levels of the atom on the laser pulse field amplitude.     

Tonks-Langmuir problem for a bi-Maxwellian plasma

An analytical solution of the Tonks-Langmuir (TL) problem with a bi-Maxwellian electron energy distribution function (EEDF) is obtained for a plasma slab. The solution shows that the ambipolar potential, the plasma density distribution, and the ion flux to the wall are mainly governed by the cold electrons, while the ionization rate and voltage drop across the wall sheath are governed by the hot electrons. The ionization rate by direct electron impact is found to be spatially rather uniform, contrary to the T-L solution where it is proportional to the plasma density distribution. The temperature of hot electrons defined by the ionization balance is found to be close to that of the T-L solution for a mono-Maxwellian EEDF, and is in reasonable agreement with experiments carried out in a low pressure capacitance RF discharge. The energy balance for cold electrons in this discharge shows that their heating by hot electrons via Coulomb interaction is equalized by the cold electrons' escape to the RF electrodes during collapse of the RF sheath     

Collison nebulizer as a new soft ionization source for mass spectrometry

We have proposed that a Collison-type nebulizer be used as an ionization source for mass spectrometry with ionization under atmospheric pressure. This source does not require the use of electric voltage, radioactive sources, heaters, or liquid pumps. It has been shown that the number of ions produced by the ⁶³Ni radioactive source is three to four times larger than the number of ions produced by acoustic ionization sources. We have considered the possibility of using a Collison-type nebulizer in combination with a vortex focusing system as an ion source for extractive ionization of compounds under atmospheric pressure. The ionization of volatile substances in crossflows of a charged aerosol and an analyte (for model compounds of the amine class, viz., diethylaniline, triamylamine, and cocaine) has been investigated. It has been shown that the limit of detecting cocaine vapor by this method is on the level of 4.6 × 10^–14 g/cm³.     

吸收谱线Doppler展宽对原子多步光电离的影响

原子法激光同位素分离的核心之一是如何高效地将原子激发电离.本文从原子法激光同位素分离的实际情况出发,研究了原子吸收谱线的Doppler展宽对原子电离率的影响.研究中使用的理论工具是原子激发电离的密度矩阵方程,并利用数值计算方法对方程进行求解.研究结果表明:当吸收谱线有Doppler展宽时,在激光参数不变的条件下原子电离率会降低;且当激光功率固定时,存在使原子电离率为最大的线宽值.这与已发表文献中无Doppler展宽时的计算结果有很大不同.为了追求最佳的原子电离效果,在原子法激光同位素分离系统中激光应该尽可能地工作在最佳线宽条件下.如果激光线宽有不可控的随机波动,在技术上让激光线宽略大于最佳线宽更为有利.无论如何控制激光线宽,尽可能地降低原子吸收谱线的Doppler展宽都有利于原子电离率的提高.     

Ionization potentials of HCN and HNC by a Green's function method

The vertical valence ionization potentials of HCN and HNC have been calculated by a many-body Green's function method using extended basis sets including polarization functions. For HCN the agreement of the computed ionization potentials with experiment is very satisfactory. The ordering is 1π, 3σ, 2σ. The ionization potentials of HNC have not been measured yet. The calculated ordering is 3σ, 1π, 2σ. The electronic structure of the two molecules is seen to differ.     

Charge composition of a cluster plasma upon irradiation of large atomic clusters by the field of a superatomic femtosecond laser pulse

A theory is developed for calculating the charge composition of a cluster plasma produced upon irradiation of large atomic clusters by the field of a superatomic femtosecond laser pulse. The theory is based on the overbarrier process of a successive multiple internal ionization of atomic ions inside a cluster accompanied by the external field ionization. Collision ionization is also taken into account in the calculations. The theory is illustrated by the example of a cluster consisting of 10⁶ xenon atoms irradiated by a 50-fs laser pulse with a peak intensity of 2×10¹⁸ W/cm². In this case, the Xe²⁶⁺ ions dominate. The amounts of atomic xenon ions with multiplicity up to 31 are calculated.