Isotope-selective ionization utilizing molecular alignment and non-resonant multiphoton ionization

We demonstrate laser nitrogen isotope separation, which is based on field-free alignment and angular-dependent ionization of ¹⁴N₂ and ¹⁵N₂ isotopologues. A linearly polarized short laser pulse (???~?795?nm, ?????~?60?fs) creates rotational wave packets in the isotopologues, which periodically revive with different revival times as a result of different moments of inertia. Another linearly polarized short laser pulse (???~?795?nm, ?????~?60?fs) ionizes one of the isotopologues selectively as a result of their different angular distributions. In the present experiments, the ion yield ratio R [=I(¹⁵N₂ ⁺)/I(¹⁴N₂ ⁺)] can be changed in the range from 0.85 to 1.22, depending on the time delay between the two laser pulses.     

Phase-dependent excitation and ionization in the multiphoton ionization regime

We theoretically study the dependence of atomic excitation and ionization on the carrier envelope phase of few-cycle laser pulses in the multiphoton ionization regime. Our theoretical results for the hydrogen atom based on the solution of the 3D time-dependent Schr?dinger equation show that the strong phase dependence can be seen in not only total ionization, but also bound-state population under the weak laser intensity regime.     

Photoelectron angular distributions from two-photon ionization of atoms near ionization threshold

Photoelectron angular distributions(PADs) from two-photon ionization of atoms in linearly polarized strong laser fields are obtained in accordance with the nonperturbative quantum scattering theory.We also study the influence of laser wavelength on PADs.For two-photon ionization very close to the ionization threshold,most of the ionized electrons are vertically ejected to the laser polarization.PADs from twophoton ionization of atoms are determined by the second order generalized phased Bessel function at which the ponderomotive parameter plays a key role.In terms of dependence of PADs on laser wavelength,corresponding variations for the ponderomotive parameter are demonstrated.     

Electron-impact ionization and dissociative ionization of sulfur in the gas phase

We describe the methods and the results of investigation of the yield of positive ions formed as a result of electron-impact ionization of sulfur. The ionization energy for the basic molecule and the energies corresponding to the emergence of fragment ions are obtained from the ionization efficiency curves. The dynamics of formation of molecular sulfur ions in the temperature range 320–700 K is investigated. The energy dependences of efficiency S _n of the ion formation for n = 1–6 are analyzed, and their appearance energies are determined. The total cross section of sulfur ionization by a monochromatic electron beam is also investigated. Using the linear approximation method, we marked out features on the ionization function curve, which correspond to the ionization and excitation energies for multiply charged ions. The total cross section of the formation of negative sulfur ions is measured in the energy range 0–9 eV.     

Electron impact ionization. The probability of occupying states after ionization

The probability function of occupying states with a definite energy in the conduction band after collision in the process of electron impact ionization is calculated. The valence band is assumed of finite width. Several cases of the primary energy and the effective mass in the valence band are examined.

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This work was begun at the Institute of Technical Physics of the Czechoslovak Academy of Sciences in Prague. The author wishes to thank the Director of that Institute for the possibility of working there and especially Dr E. Antoník for suggesting the problem and for helpful discussions.     

Field ionization of ozone

Experimental results on the field ionization of ozone at an iridium surface using a magnetic sector atom-probe FIM show that at the lowest field of ~1.5 V/Å only 25% of all ions produced are O⁺₃, while O⁺₂ and a small percentage of O⁺ indicate field induced dissociation of ozone and recombination of atomic oxygen into O₂. At high fields and free space ionization, O⁺₂ is predominant, while the atomic oxygen as a dissociation product escapes without being ionized and cannot be detected. Using pure oxygen as an imaging gas in the FIM, O⁺₂ ions do not dissociate under the same conditions.     

钡准分子共振电离光谱及离化机制研究

本文报道用激光共振电离法获得一个新的钡准分子结构.在染料激光波长扫描范围内观察到钡原子双光子共振跃迁、双光子混合跃迁和钡准分子谱带.我们提出一种用热离子二极管信号特性分析原子和分子里德伯态的离化机制的方法,并以此对钡原子和钡分子的离化通道进行讨论.     

Self-chemical ionization of diethylzinc

The self-chemical ionization of diethylzinc is examined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry and semiempirical molecular orbital calculations. Electron impact of diethylzinc neutral produces the radical cation, C(4)H(15)Zn(+) (m/z x 122), which reacts further with the neutral (C(2)H(5))(2)Zn to give the following product ions: Zn(+) (m/z x 64), C(2)H(5)Zn(+) (m/z x 93), C(4)H(9)Zn(+) (m/z x 121), C(4)H(11)Zn(2)(+) (m/z x 187), and C(6)H(15)Zn(2)(+) (m/z x 215). To determine the structure and pathways for production of these ions, monoisotopic (12)C(4)H(15)(64)Zn(+), (64)Zn(+) and (12)C(2)H(5)(64)Zn(+) were individually isolated and reacted with the neutral background. We also performed semiempirical molecular orbital calculations (ZINDO/1). The molecular orbital calculations and experimental data are consistent in predicting that the ethyl group on the diethylzinc cation carries the positive charge. Copyright 1999 John Wiley & Sons, Ltd.     

Strong-field molecular ionization: determination of ionization probabilities calibrated with field-free alignment

We report an original optical method providing the probability of molecular ionization induced by femtosecond laser pulses. The approach consists of exploiting molecular alignment to extract reliable information about ionization. The cross defocusing technique implemented for this purpose reveals a sensitivity with respect to postpulse alignment, as well as to the free electron density induced by the ultrashort laser pulse. The analysis of the resulting signal thus gives access to absolute single-ionization probabilities calibrated through the degree of alignment, provided that free electrons are produced mainly by single ionization. The relevance of the method is assessed in N2.     

Study of the ionization rate variations in ionization waves in plasma I

The value of the phase of the maximum of ionization rate variations is calculated by using the relations determined in the present paper and the experimental results obtained by ícha and Drouet (Can. J. Phys.46 (1968), 2491). A possible interpretation of the results obtained has been proposed, taking into account the variation of the electron energy distribution function.     

Near-threshold positron-hydrogen ionization

Ionization of hydrogen by positron impact near the breakup threshold is considered. The two-centre convergent close-coupling method is used. We find that convergence in the cross section is only practically obtained if two near-complete expansions are used, one centred around H and the other around Ps. The contribution to the breakup cross section becomes the same for both centres as the threshold is approached. The calculations are found to be in good agreement with the threshold law predicted by Ihra et al. (1997) Phys Rev Lett 78:4027.     

Comparison study of atomic and molecular single ionization in the multiphoton ionization regime

In this Letter, we report, for the first time in the multiphoton ionization regime, a comparison study of single-electron ionization of diatomic molecules versus rare gas atoms with virtually the same ionization potentials. In comparing N2+ to Ar+, a higher ion signal is seen in N2+ compared to Ar+ for linear polarization but the difference vanishes in circularly polarized light. In comparing O2+ to Xe+, we observe a suppression in O2+ compared to Xe+ for both linear and circular polarization but this suppression exhibits an intensity dependence; i.e., there is little suppression for O2+ at the lowest intensity range, but the suppression becomes increasingly stronger as the laser intensity increases. The multielectron screening model is used to discuss possible mechanisms of this intensity dependent suppression in O2+ in the multiphoton ionization regime.     

Study of the ionization rate variations in the ionization waves in plasmas II

Measurements of the plasma potential variation in fast r type ionization waves will be presented. The phase of the ionization rate variations is calculated on the basis of this experiment and experimental results obtained later. The advantage of the calculation presented is that it does not contain the variation of the electron mean energy. Therefore, no preliminary assumptions about the nature of the electron distribution function and about its variation are necessary.The experimental work was performed by two of the authors (M. . and M. G. D.) at the Laboratoire de Physique des Plasmas, Université de Montréal.The authors wish to thank Dr. G. G. Cloutier for his support of the experimental work and for valuable discussions during the course of the experiment.     

Near-threshold electron-impact ionization of magnesium

The effective electron-impact ionization cross section of the magnesium atom is studied in detail in the energy range from the ionization threshold to 16 eV. A large number of particular features caused both by atomic autoionization states and by the formation and decay of a short-lived state of the negative ion of the magnesium atom are revealed. These particular features are identified using experimental and theoretical data on the photoionization (photoabsorption) and on the spectra of ejected electrons.     

啁啾激光场中氢分子离子电离过程的理论研究

本文在非玻恩-奥本海默(Non Born-Oppenheimer)近似条件下,通过数值求解一维含时薛定谔方程,探究了氢分子离子体系在超短啁啾激光条件下的电离过程.数值结果表明:有效选取啁啾参数可显著增强氢分子离子的电离过程,并且激光强度越大,电离过程对啁啾参数越敏感.结合不同强度下平均核间距及电子波包密度随时间的变化,分别从核与电子运动的角度,有效的解释了啁啾脉冲对氢分子离子电离过程的影响.     

Theory of laser-induced gas ionization

The mechanism of laser-induced gas ionization is analyzed in the context of the theories presently available, namely multiphoton and cascade theory, and their predictions are shown to be in serious divergence from experimentation. A novel hypothesis is then formulated which considers the classical photon energy expression ε=hν as the limit of a general expression ε=hν/[1?β_ν f(I)],f(I) being a function of light intensity and β_ν a coefficient such that β_ν f(I) significantly differs from zero only for high-intensity laser light. Starting from the new formulation of the photon energy expression, a theory is developed for the laser ionization phenomenon and a few simple relations are found. The successful use of these relations in the verification of the experimental results suggests that the new photon energy expression has a real physical meaning rather than an empirical significance, and some considerations on the physical meaning are presented.