Effects of Ps polarization potentials on Ps total cross-sections of the collisions of positrons with alkali metal atoms and alkaline-earth positive ions

Summary The previous form of the coupled-static approximation applied to the inelastic scattering of positrons by alkali metal atoms is improved by switching on the polarization potentials of the positroniums. The effect of these potentials on the total elastic and positronium formation cross-sections of three different scattering problems (namely the collisions of positrons with lithium, sodium and potassium atoms) is discussed in details. Particularly, we devote our interest to the comparison between the resultant positronium formation cross-sections and those of the inelastic collisions of positrons with berylium, magnesium and calcium positive ions.     

Quenching cross-sections of metastable Ar,Kr and Xe atoms by halogen molecules

Summary The quenching of metastable Ar, Kr and Xe atoms by halogen molecules via an electron jump mechanism is examined by using the multiple curve-crossing formalism of Bauer, Fisher and Gilmore. Cross-sections for the quenching process are calculated which are found in good agreement with experiment. Finally, the model presented here is compared with other quenching mechanisms.

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Riassunto Si esamina la tempra di atomi metastabili di Ar, Kr, e Xe mediante molecole di alogeni attraverso un meccanismo di salto di elettroni, usando il meccanismo d'incrocio multiplo di curve di Bauer, Fisher e Gilmore. Si calcolano le sezioni d'urto per il processo di tempra che appaiono in buon accordo coi dati sperimentali. Infine il modello qui presentato è confrontato con altri meccanismi di tempra.

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Резюме Используя формализм Бауэра, Фишера и Джилмора, исследуется гашение метастабильности атомов Ar, Kr и Xe молекулами галогенов через механизм электронного перескока. Вычисляются поперечные сечения для процесса гашения, которые согласуются с экспериментом. В заключение предложенная модель сравнивается с другими механизмами гашения.

     

Elaborate treatment of the inelastic collisions of positrons with alkali atoms and alkaline-earth positive ions using a restricted coupled-static approximation

Summary We present the results of a very elaborate investigation of the inelastic collisions of positrons with the ground states of the first four alkali atoms (³Li,¹¹Na,¹⁹K and³⁸Rb) and their electronically resemblant alkaline-earth positive ions,i.e. ⁴Be⁺,¹²Mg⁺,²⁰Ca⁺ and³⁹Sr⁺, respectively, using a restricted coupled-static approximation in which the reactance matrices are adjusted to be symmetrical. Our interest is concentrated on the calculation of the total elastic and positronium formation cross-sections and their behaviour in the low-energy region (below 10 eV for the atoms) or the regions above the positronium formation thresholds (when the ions are considered). Our results are the most accurate cross-sections expected from the approximation employed. They reveal, within their limits, the most important characteristics of these cross-sections and provide us with a satisfactory comparison between the main features of positron-atom and positron-ion collisions. Present address.     

Three-body collisions between laser-excited Na and K atoms in the presence of buffer gas

Summary Energy pooling in alkali vapour mixtures has been previously investigated by us in an amalgam of sodium and potassium. In this paper we describe the peculiar features appearing when some buffer gas is introduced in the cell. We demonstrate that the buffer gas enhances the energy transfer between Na(3P) and K(4S), which is detected through the effects induced on the highly excited states populated by the Na(3P)/Na(3P) and K(4P)/K(4P) collisions.

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Riassunto Abbiamo in precedenza studiato il processo di collisione di energy pooling in una miscela di sodio e potassio. In questo lavoro si analizzano i particolari effetti indotti dalla presenza di alcuni Torr di gas tampone introdotti nella cella. Viene dimostrato che il gas tampone favorisce il processo di trasferimento di energia fra Na(3P) e K(4S), che viene rivelato attraverso gli effetti indotti sugli stati altamente eccitati popolati dalle collisioni Na(3P)/Na(3P) e K(4P)/K(4P).

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Резюме Ранее авторы в амальгаме натрия и калия исследовали распределение энергии в смесях щелочных паров. В этой статье мы описываем особенности, возникающие при введении буферного газа в ячейку. Мы поквзываем, что буферный газ усиливает обмен энергией между Na(3P) и K(4S), который детектируется через эффекты, связанные с высоко возбужденными состояниями, заселенными при Na(3P)/Na(3P) и K(4P)/K(4P) соударенияш.

     

Application of the density matrix multipole expansion to stark quantum beats from beam-foil-excited hydrogen atoms

Summary The density matrix multipolar-expansion theory, as applied to radiation from polarized atoms, is outlined. A calculation is performed for Stark quantum beats from beam-foil-excited hydrogen atoms, and comparison is good with experimental results.

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Riassunto Si sottolinea la teoria dello sviluppo multipolare della matrice di densità applicata alla radiazinone da atomi polarizzati. Si effettua un calcolo per i colpi quantici di Stark da atomi di idrogeno eccitati dal ?beam-foil? e il confronto con i risultati sperimentali è buono.

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Резюме Обсуждается применение теории мультипольного разложения матрицы потности к излчению поляризованных атомов. Вычсления проводятся для квантовых биений Штарка у атомов водорода, возбужденных при пролете через фольгу. Получается хорошее сравнение с экспериметальными результатами.

     

Collisional transfer of roto-vibrational energy from quantum calculations: The He-HF system

Summary The problem of simultaneous vibrational and rotational excitations of HF molecules in collision with helium has been approached via a quantum-mechanical treatment of the full dynamics and by employing a very accurate potential-energy surface suggested earlier in the literature. The complicated coupling of rotational and vibrational channels has been partly simplified by taking advantage of their different time scales, thus allowing a reduction of dimensionality for the corresponding multichannel scattering couple equations (IOS approximation). The relative influence of the various modes on preferential energy depositions upon collision is discussedvis à vis specific features of the interaction and specific regions of the range of collision energies employed. The essential inefficiency of (R, T) processes as apposed to (V, T) or (V, R, T) processes is once more pointed out for the present system. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Dynamic shadow effect in the energy loss of protons

Summary The nonrelativistic quantum-mechanical approach for the stopping power has been developed by means of the Glauber-Franco approximation for the inelastic-scattering amplitude. It takes into account excitation and ionization processes in the amorphous target by heavy charged particles. The calculations of the stopping power of protons in H and He targets are in excellent agreement with the available experimental data due to the effect of nuclear dynamic blocking (shadow) and the rescattering processes by the electrons of the target atom. Formulae of the stopping-power calculations, taking into account individual atom shell effects, have been derived. On this basis, the case of violation of Bragg’s additivity rule is discussed.     

Elastic and inelastic scattering of positrons from hydrogenlike ions

Summary The total elastic cross-sections of the collisions of positrons with various hydrogenlike targets (with nuclear chargeZ=1, 2, 3, 4, 5 and 11) are determined at 13 values of the incident energy lying below 6.8 e V(the positronium formation threshold of e⁺-H scattering). The inelastic scattering of positrons by the same targets is investigated for the first time at energies above the positronium formation thresholds. Both scattering processes are treated using a coupled-static formalism and an iterative Green's function partial wave expansion technique. The resulting cross-sections demonstrate the stability of this technique and emphasize the importance of exploring the aspects of positron-ion collisions more fundamentally on both theoretical and experimental levels.     

Elastic and vibrational excitation cross-sections in the Ps-H2 scattering: a perturbative approach

Summary A detailed account is reported of a perturbative calculation of the elastic and vibrational excitation cross-sections for the positronium-H₂ molecule scattering. The problem is studied in the context of the multichannel scattering theory for identical particles; the first-order exchange amplitude and the second-order direct amplitude are evaluated for energy from zero to the positronium excitation threshold. The lowest-order Padé approximant is also constructed and this suggests the occurrence of a strong enhancement in the vibrational cross-section and in the related energy loss in matter at the positronium excitation threshold. In comparison with a short preliminary account presented elsewhere, various improvements are introduced both in the physical approximations and in the numerical calculations.

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Riassunto Si dà un resoconto particolareggiato di un calcolo perturbativo della sezione d'urto elastica e di eccitazione vibrazionale per la collisione positronio su molecola d'idrogeno. Il problema è studiato nel contesto della teoria dello scattering a molti canali per sistemi di particelle identiche; sono calcolate l'ampiezza di scambio al primo ordine e l'ampiezza diretta al secondo ordine per energie da zero alla soglia di eccitazione del positronio. è anche valutata l'approssimante di Padé piú bassa e questo suggerisce la comparsa di una forte esaltazione della sezione d'urto vibrazionale e della perdita di energia nella materia attorno alla soglia di eccitazione del positronio. Rispetto ad un breve resoconto preliminare presentato in precedenza si sono apportati diversi miglioramenti sia nelle approssimazioni di carattere fisico che nelle valutazioni numeriche.

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Резюме Проводится пертурбационное вычисление поперечных сечений упругого рассеяния и вибрационного возбуждения в процессе рассеяния позитрония на молекуле H₂. Рассматриваемая проблема исследуется в связи с теорией многоканального рассеяния тождественных частиц. Вычисляются обменная амплитуда первого порядка и прямая амплитуда второго порядка в области энергий от нуля до порога возбуждения позитрония. Также определяется низший порядка Падэ приближения, предполагая наличие сильного увеличения в вибрационном поперечном сечении и в энергетических потерях в веществе на пороге возбуждения позитрония. По сравнению с существующими подходами, предлагаются различные улучшения в физических приблизениях и в численных расчетах.

     

Electrons from target and projectile ionization in simple collision systems

Summary Absolute double differential cross-sections for target ionization and projectile electron loss were determined in collisions of H⁺, H₂ ⁺,³He⁺ and³He⁺⁺ ions (0.4 to 2.0 Me V/u) with He and Ar gas targets at electron emission angles from 0° up to 60°. The experimental results are qualitatively well described by plane-wave Born approximations including screening of the projectile in target ionization and of the target atom in the case of electron loss.

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Riassunto Sono state determinate le sezioni d’urto doppie differenziali assolute per la ionizzazione del bersaglio e la perdita dell’elettrone proiettile in collisioni di ioni H⁺, H ₂ ⁺ ,³He⁺ e³He⁺⁺ (da 0.4 a 2.0 Me V/u) con bersagli di gas He e Ar ad angoli di emissione degli elettroni da 0° a 60°. I risultati sperimentali sono qualitativamente ben descritti dalle approssimazioni di Born per onde piane che comprendono la schermatura del proiettile nella ionizzazione del bersaglio e dell’atomo bersaglio nel caso di perdita dell’elettrone.

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Резюме Определяются абсолютные дважды дифференшиальные поперечные сечения для ионизации мишени и потери электровов налетающими частишами при соударениях ионов H⁺, H ₂ ⁺ ,³He⁺ и³He⁺⁺ (от 0.4 до 2.0 Me V/u) в случае газовых Не и Ar мишеней при углах эмиссии электронов от 0° до 60°. Экспериментальные результаты качественно хорощо описываются с помощью плоско-волнового борновского приближения, включая экранирование налетающей частицы при ионизации мишени в случае потери электрона.

     

Collisional transfer of rotovibrational energy from quantum calculations

Summary A newly obtained anisotropic interaction for the He−N₂ system is here employed to analyse in detail the inelastic flux distribution between the many rotational states of the target molecule and the few of its vibrational states that are accessible at near-thermal ranges of collisional energy. Both integral and differential inelastic cross-sections are obtained from a quantal formulation of the energy transfer process, within which the orientational dependence of rotational and vibrational couplings is examined for its bearing on the interference structures that are exhibited by the dynamical observables as functions of final rotational states and of centre-of-mass scattering angles. Similarities and differences with previously studied polar molecules also interacting with helium are underlined and explained.

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Riassunto Una superficie di potenziale recentemente ottenuta da noi per il sistema He−N₂ è utilizzata per studiare il processo quantico di transferimento d'energia interna, per via collisionale, fra i numerosi livelli rotazionali della molecola bersaglio ed alcuni dei livelli vibrazionali che risultano energeticamente accessibili alle velocità traslazionali qui esaminate. La natura anisotropa dell'accoppiamento (V, T) è così analizzata dal punto di vista del suo effettodiretto su osservabili dinamiche come sezioni d'urto inelastiche, parziali integrali e differenziali. Le strutture interferenziali che si originano in tale sistema sono scarsamente influenzate dalla presenza o meno di accoppiamento vibrazionale e sono qui discusse in relazione alla loro diversità con altri sistemi polari (LH, KF) precedentemente da noi studiati con tecniche teoriche simili.

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Резюме Недавно полученное анизотропное взаимодействие для He−N₂ систем испоиьзуется в зтой работе для поробного анализа квантового переноса внутренней знергии меЗду многочисленными ротационными состояниями молекул мишени и несколькими колебатэльными состояниями. Анализируется природа анизотропии вращательіх и колебательньіх связей с точки зрения влияния зтой анизотропии на динамичэскиэ наблюдаемье величины, такиэ как интэграиьіе и дифференциалъные неупругие поперечные сеч∈ния. Интерферендионные структуры, которы∈ возникют в таких системах, слабо зависят от вибрадионной связх. Проводится сравнениэ полученных резульіх резулвтатоб с резулвтатми для других полярных молекул, ранее рассмотренных в рамках того Зе теоретическочо подхода.

     

Inelastic scattering of positrons by lithium-isoelectronic ions

Summary The inelastic scattering of positrons by 11 lithium-isoelectronic ions, (with nuclear chargesZ=4,5,… 14) is investigated for the first time within the framework of the coupled-static and frozen-core approximations. It is assumed that the elastic and positronium channels, in each process, are open and all excitation channels of the target are closed. The calculations of the total cross-sections are carried out by employing eight partial waves, (corresponding to 0≤l≤7, wherel is the total angular momentum of the scattering problem considered). Also, the effect of switching on the polarization potentials of the positroniums on the rearrangement cross-sections is investigated in details. Particular attention is devoted to the relation between the positronium formation cross-sections and the increase in the nuclear charge of the targets. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Equilibrium charge state distributions of beam foil excited molecular fragments exiting carbon foils

Summary Equilibrium charge state distributions of ions emerging from solids have been measured. As incident particles were used both atomic (C⁺, N⁺, O⁺) and molecular (N ₂ ⁺ , CO⁺) projectile ions (0.025<E/M<0.108 MeV/u). The data of atomic projectile ions agree well with the data of other authors in a range in which they overlap. Charge state fractions of emerging molecular-fragment ions behind a carbon foil are strongly influenced by the Coulomb explosion and possibly by the wake potential. Supported by BMFT/Bonn.     

On the Ps-H2 scattering

Summary We perform a perturbative calculation of the elastic, first and second vibrational excitation cross-sections for the positronium-H₂ molecule scattering for an energy from zero to the positronium excitation threshold. We calculate also the ratio between the averaged vibration threshold. We calculate also the ratio between the averaged vibrational-and elastic-energy loss of the positronium in matter. In comparison with a previous similar calculation, we take now into account the contribution of the continuum of the positronium to the second Born approximation to the direct amplitude and we add the calculation of the second vibrational excitation cross-section.

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Riassunto Si presenta un calcolo perturbativo della sezione d'urto elastica, di prima e seconda eccitazione vibrazionale per la collisione di positronio su molecola d'idrogeno per energie da zero alla soglia di prima eccitazione del positronio. Si calcola il rapporto tra la perdita di energia vibrazionale ed elastica del positronio nella materia. Rispetto ad un analogo calcolo fatto precedentemente si tiene conto del contributo del continuo del positronio all'ampiezza diretta al second'ordine e si valuta la sezione d'urto di seconda eccitazione vibrazionale.

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Резюме В рамках теории возмущений вычисляются поперечное сечение упругого рассеяния и поперечное сечение первого и второго колебательных возбуждений при соударении позитрония с молекулой водорода при энергиях в диапазоне от нуля до порога возбуждения позитрония. Мы также вычисляем отношение между потерями энергии на колебательные возбуждения и упругими потерями энергии позитрония в веществе. По сравнению с предыдущими вычислениями мы учитываем вклад непрерывного спектра позитрония в прямую амплитуду во втором борновском приближении. Мы также оцениваем поперечное сечение второго колебательного возбуждения.

     

Collisions of fast electrons with positronium atoms

Summary Elastic and inelastic collisions of fast non-relativistic electrons with positronium atoms have been studies in the Born-Ochkur approximation. It has been shown that exchange plays an important role for transitions between states with the same parity as in this case the direct scattering amplitude vanishes. Numerical results for the total and ortho-para conversion cross-sections for the 1s→1s, 1s→2s and 1s→2p transitions have been presented for projectile energies varying from 0.1 to 10 keV. According to charge symmetry, presented results apply also to positron projectiles.     

Elastic scattering of positrons by xenon atoms

Summary Partial and total cross-sections of positron scattering by Xe atoms are calculated at twenty values of incident energy ranging from 0.01 Ry to 4.00 Ry. The calculations are carried out via model potential (describing the positron target interaction) and a polarization potential introduced previously by Lane and Geltmann (1967). The comparison between our results and those obtained by other authors shows significant agreement and supports our simple model scattering process. The author of this paper has agreed to not receive the proofs for correction.     

Studies on vibrational excitation differential cross-sections of low-energy electron scattering from N2 molecule

The vibrational excitation differential cross-sections (DCS) of low-energy electron-N₂ scattering are studied using vibrational close-coupling (VCC) method and vibrational scattering potentials which include static, exchange and polarization contributions. By including the contributions of 18 partial waves, 20 vibrational states, and 16 molecular symmetries (up to Λ=7), the converged vibrational excitation (0↦2, 0↦3, 0↦4) DCS agree well with experimental results. Also obtained are converged vibrational (1↦0, 1↦1, 1↦2, 1↦3) DCS, with the impact energies being those of the main resonant peaks (1.92 eV, 1.90 eV, 1.62 eV, 1.63 eV).     

Electron scattering from nonlinear molecules using multicentre wavefunction expansions

Summary The scattering of slow electrons by polyatomic molecules is studied within a full quantum formulation of the problem, and the wavefunctions of the target molecular bound electrons are obtained from a multicentre expansion (MCE) of Gaussian-type orbitals (GTOs) within the single-determinant Hartree-Fock (HF)-SCF scheme. It is shown that the scattering calculations require the solution of radial coupled equations where the interaction is obtained as a further multipolar expansion at the target centre of mass (c.o.m.), while the relevant numerical procedures are discussed for systems like SiH₄, GeH₄ and CF₄. The ensuing interaction coefficients are used to carry out convergence studies of the scattering cross-sections, over a broad range of energies, in the case of silane targets. The results are found to be rapidly converging for such a system, while the different behaviour expected for other targets is also analysed and explained.     

Scattering of electrons from gas-phase N2O(1Σ): Computed cross-sections and angular distributions in comparison with experiments

Ab initio calculations are reported for the quantum scattering of electrons from N₂O molecules in the gas phase and for energies which range from near threshold up to about 100 eV. Elastic integral cross-sections and angular distributions are examined in detail and an extensive comparison is made with existing experiments. The agreement found with the latter data is fairly good and results are further discussed to explain the physical mechanisms at work for this polar target.     

碱金属原子和类氢离子束缚态能级的解析解

对于碱金属原子和类氢离子的原子实模型,从物理实质图像和极化与贯穿作用的量化及实验资料人手深入地进行了分析,引入了半经验碱金属和类氢原子内部相互作用势,建立了定态薛定谔方程.运用微扰理论获得能级Enl,并进一步导出了有效主量子数、有效核电荷数等重要参数.这对物质化学特性和原子特性研究是有益的.