ALD of ZnO using diethylzinc as metal-precursor and oxygen as oxidizing agent

We report on ZnO atomic layer deposition (ALD) with a precursor combination of diethylzinc as metal-precursor and pure oxygen (O₂) as oxidant as an alternative to H₂O as oxygen precursor. The temperature region of self-limiting ALD growth (ALD window) is determined and shows an increase in growth rate of about 60% compared to water as oxygen-precursor. Finally, in situ X-ray photoelectron spectroscopy (XPS) and synchrotron-radiation photoelectron spectroscopy (SR-PES) have been used to analyze the initial growth and film properties of ALD-ZnO deposited in monolayer steps using both precursor combinations.     

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.     

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.     

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.     

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.     

Determination of three-photon ionization cross sections of xenon atoms from comparison with electron ionization

Multiphoton and electron-impact ionization of a gas-dynamic beam of xenon atoms are compared in order to determine the effective multiphoton ionization cross sections. The three-photon ionization cross section of the xenon atom is determined.     

Resonance ionization spectroscopy of thorium

Here we describe experiments aimed at developing an element-selective ion source for thorium (Th). The technique applied is resonance ionization spectroscopy (RIS) with a thermal atom beam. Ionization schemes for isotopically nonselective ionization of Th as well as for isotopically selective ionization of ²³⁰Th are proposed. The RIS-scheme used is two-photon two-colour ionization with excitation in the ultraviolet spectral range between 244 nm and 267 nm or in the visible spectral range between 485 nm and 529 nm. Ionization of the excited atoms is performed either by ultraviolet photons or by visible photons, depending on the energy required for this process.     

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.