Measurements of anomalous dispersion in rayleigh scattering of characteristic X-ray fluorescence

Summary When using characteristic fluorescence X-rays or γ-rays of energy under 100 keV to study the Rayleigh scattering of photons from atoms the effect of anomalous dispersion is not in general negligible. On the contrary, it may cause the atomic cross-section values to vary with respect to the tabulated values—usually calculated in nonresonant approximation—of an order of magnitude in the most critical cases. In this work we present experimental results of Rayleigh scattering where this effect is well evident. We used as sources of excitation a radioisotope (²⁴¹Am,E _γ 59.6 keV) and a X-ray tube with a series of different secondary anodes and solid-state detectors to detect the scattered radiation. We reviewed the atomic form factor formalism which describes the phenomenon in a nonrelativistic treatment and we show its strict analogy with QEDS-matrix theory. We obtained numerical values for the cross-sections by using the tabulated values of the photoelectric cross-sections and we made a comparison with the experimental values. The error of the measurements (∼3%) derives from the superposition of the Rayleigh scattering with the characteristic X-ray fluorescence lines. Within the error limits the agreement with the values from the form factor approximation is very satisfactory.

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Riassunto Quando si utilizza la radiazione X di fluorescenza caratteristica o la radiazione γ di energia minore di 100 keV per studiare la diffusione elastica da parte degli atomi (diffusione di Rayleigh), l'effetto della diffusione anomala non è in generale trascurabile. Al contrario, essa può causare variazioni anche di un ordine di grandezza dei valori delle sezioni d'urto atomiche rispetto ai valori tabulati che sono usualmente calcolati nell'approssimazione non risonante. In questo lavoro si presentano i risultati sperimentali di alcune misure di diffusione di Rayleigh nei quali questo effetto è ben evidente. Come sorgenti di radiazione sono stati utilizzati un isotopo radioattivo (²⁴¹Am,E _γ=59.6 keV) e una serie di anodi secondari associati a un tubo X. Per la rivelazione della radiazione X sono stati utilizzati rivelatori a stato solido. Si è eseguita una revisione del formalismo dei fattori di forma atomici per la descrizione del fenomeno in una trattazione non relativistica e si mostra la stretta analogia con la teoria della matriceS dell'elettrodinamica quantistica. Si sono ottenuti i risultati numerici per le sezioni d'urto di diffusione elastica utilizzando i valori tabulati per le sezioni d'urto fotoelettriche e si sono confrontati con i valori ottenuti sperimentalmente. L'errore delle misure (∼3%) deriva dalla sovrapposizione della diffusione di Rayleigh con le righe di fluorescenza caratteristica. Entro questi limiti di errore l'accordo con i valori ricavati nell'approssimazione dei fattori di forma è molto soddisfacente.

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Резюме При использовании характеристической рентгеновской или γ-лучевой флуоресценции с энергией до 100 кэВ для исследования рэлеевского рассеяния фотонов на атомах, влияние аномальной дисперсии не является в общем случае пренебрежимо малым. Наоборот, эффект аномальной дисперсии может изменить значения атомных поперечных сечений по сравнению с табулированными значениями, обычно вычисляемыми в нерезонансном приближении, в наиболее критических случаях на порядок величины. В этой работе приводятся резулятаты для рэлеевского рассеяния, где эффект аномальной дисперсии является существенным. Мы используем в качестве источников возбуждения радиоизотоп (²⁴¹Am,E _γ=59.6 кэВ) и рентгеновскую трубку с различными вторичными анодами и твердотельные детекторы для регистрации рассеянного излучения. Мы используем формализм атомного форм-фактора, который описывает явление в нерелятивистском приближении. Мы отмечаем точную аналогию с теориейS-матрицы в квантовой электродинамике. Получаются численные значения для поперечных сечений, используя табулированные величины для фотоэлектрических поперечных сечений. Проводится сравнение полученных значений с экспериментальными данными. Погрешность измерений (∼3%) определяется из суперпозиции рэлеевского рассеяния с линиями характеристической рентгеновской Флуоресценции. В пределах погрешностей получается удовлетворительное согласие с результатами, полученными из приближения атомного форм-фактора.

     

The effect of the chemical environment on theKβ/Kα X-ray intensity ratio

Summary Chemical effects on theKβ/Kα X-ray intensity ratios were investigated for different constitutions of Ti, V, Fe, Se, Br, Zr and Ce by a Ge(Li) X-ray detector. The vacancies were produced by heavily filtered²⁴¹Am gamma-rays. It is found that theKβ/Kα X-ray intensity ratios measured with compounds deviated up to 12% from the corresponding values of the pure elements (Ti, V, Fe) are larger than for the others (Se, Br, Zr and Ce).     

Positron bound systems in benzene and in naphthalene

Summary Positron decay in liquid naphthalene and benzene was investigated through measurements of lifetimes and magnetic quenching. The obtained results when compared with those already known in solid phase suggest that positrons become bound in two different systems. The first has a lifetime τ₃≈1 ns which is unaffected by the melting, while the second (τ₄≈3 ns) arises above melting and behaves as a relaxed positronium. This work was supported by CISM (Centro Interuniversitario di Struttura della Materia) del Ministero della Pubblica Istruzione, and by GNSM (Gruppo Nazionale di Struttura della Materia) del CNR.     

Local coordination and dynamics in liquid antimony trichloride/water mixture

Summary EXAFS Rayleigh wing and Raman-scattering data in SbCl₃/H₂O mixture are presented. The measurements, carried out at the PULS facility on the ClK-edge and on theL _{I, II, III}-edges as a function of temperature, allow us to define the local order around the absorber. In particular, clear evidence exists that intramolecular Sb−Cl and intermolecular Sb...Cl distances and coordination, similar to the ones existing in pure SbCl₃, are present. The data analysis reveals also that the water molecules are linked to the chlorine, giving rise to a cross-link between the antimony trichloride chains. The light scattering response confirms these structural hypotheses, indicating that the primitiveC _3v symmetry of the SbCl₃ molecule is lowered by the water presence. Evidence also exists that new vibrational bands are originated by the chlorine-water bonds. Finally we detected a phononlike contribution, in the low-frequency region, that gives an effective Raman vibrational density of states.

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Riassunto Nel presente lavoro si presentano e si discutono i risultati di misure EXAFS, di Rayleigh wing e di scattering Raman nella miscela SbCl₃/H₂O. Le misure EXAFS, eseguite presso i laboratori PULS di Frascati, sia sullo spigoloK del cloro che sugli spigoliL _{I, II, III} dell’antimonio, permettono di ottenere informazioni sull’ordine locale del sistema. In particolare si evidenziano formazioni di legami intramolecolari Sb−Cl ed intermolecolari Sb...Cl che sono simili nelle distanze e nella coordinazione a quelli esistenti nel fuso. Si definisce anche il ruolo dell’acqua che, coordinandosi con i clori, agisce da ponte tra catene di tricloruro di antimonio. Le misure di scattering di luce confermano le ipotesi strutturali emerse dai risultati di spettroscopia EXAFS: la simmetria originaleC _3v della molecola SbCl₃ si abbassa nell’interazione con l’acqua. Inoltre nuove bande vibrazionali appaiono negli spettri Raman, legati all’interazione cloro-acqua. Infine nella regione spettrale a bassa frequenza si evidenziano modi cooperativi connessi con la presenza di una densità di stati vibrazionali.

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Резюме В работе приводятся данные для рзлеевского крыла EXAFS и комбинационного рассеяния в смеси SbCl₃/H₂O. Измерения наK-кра Cl иL _{I, II, III}-краях, как функция температуры, позволяют нам определить локальный порядок около поглотителя. В частности, отмечается, что внутримолекулярные Sb−Cl и межмолекулярные Sb...Cl расстояния и координация оказываются сходными с величинами, существующими в чистом SbCl₃. Анализ данных также показывает, что молекулы воды связаны с хлором, что приводит к кросс-связи межлу цепочками SbCl₃. Рассеяние света подтверждает эти структурые гипотезы, указывая на то, что примитивнаяC _3v симметрия молекулы SbCl₃ уменьшается из-за присутствия воды. Также подтверждается, что образуются новые колебательные зоны благодаря связям хлора с водой. В заключение, мы детектируем фононно-подобный вклад в низкочастотной области, который дает эффективную плотность рамановских колебательных состояний.

     

Valence and conduction bands in MPS3 layered compounds studied by synchrotron radiation

Summary With the resonant photomeission technique we investigated the valence bands of FePS₃ and NiPS₃. The experimental results, support the ionic picture of the compounds and our previous identification of the valence band structures. The structures rapidly varying in intensity when the excitation energy is scanned across the Fe and NiM _2,3 absorption edge are associated to the transition metal 3d states; the nonresonating features are ascribed to the (P₂P₆)⁴⁻ cluster states. With the yield technique we measured the high-resolution absorption spectra of the phosphorus and sulphur inner-core levels in Mn, Fe and Ni thiophosphates. TheL _2,3(P) andL _2,3(S) spectra are similar to each other in all the compounds and are interpreted in terms of the projected density of states of the conduction bands derived from the (P₂S₆)⁴⁻ cluster states. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Study of the glass transition region in amorphous selenium by EXAFS

Summary EXAFS spectra of amorphous bulk selenium and Se₈₀Te₂₀ alloy have been analysed in the temperature range of the glass transition region (−15 to 37°C). α-Se shows an unexpected behaviour because the modulated part of the X absorption increases with the temperature, irrespectively of the action of the EXAFS Debye-Waller factor from which one expects an opposite behaviour. Analogous features do not appear in the Se₈₀Te₂₀ alloy, probably because of the role played by the intramolecular covalent Se−Te bond strength, weaker than the corresponding Se−Se. In the absence of suitable models for the temperature dependence of the microscopic processes occurring in glasses nearT _g, a tentative explanation is proposed in terms of the temperature dependence of the backscattering amplitude functionF(k).

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Riassunto Il selenio amorfo e lega Se₈₀Te₂₀ sono stati studiati con tecnica EXAFS nell'intervallo delle temperature noto come regione della transizione vetrosaT _g (da −15 a 37°C). Il selenio amorfo mostra un comportamento inaspettato per effetto dell'aumento, con la temperatura sottoT _g, della parte modulata dell'assorbimento dei raggi X, in contrasto con quanto ci si aspetterebbe per l'effetto di smorzamento del fattore di Debye-Waller. Un comportamento analogo non si riscontra invece nella lega Se₈₀Te₂₀ dove, probabilmente, il legame covalente intracatena Se−Te, piú debole del corrispondente Se−Se, rende piú efficace l'azione di smorzamento del fattore di Debye-Waller, smascherando altri effetti. In assenza di modelli adeguati che descrivono la dipendenza dalla temperatura dei processi microscopici nei materiali vetrosi nella regione della transizione vetrosa, si ipotizza che il comportamento ≪anomalo≫ del selenio sia attribuibile ad una dipendenza dalla temperatura della funzione ampiezza di backscatteringF(k).

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Резюме Анализируются спектры EXAFS аморфного объемного селена и сплава Se₈₀Te₂₀ в области температур переходной области стекла (от 15° до 37°C). а-Se обнаруживает неожиданное поведение, потому что модулированная часть Х поглощения увеличивается с температурой, независимо от действия фактора дебая-Уоллера EXAFS, который предполагает противоположное поведение. Аналогичные особенности не возникают в сплаве Se₈₀Te₂₀, так как сила межмолекулярной ковалентной связи Se−Te, по-видимому, слабее, чем соответствующвя связь Se−Se. В отсутствии соответствующих моделей для температурной зависимости микроскопических процессов, происходящих в стеклах вблизиT _g,a, предлагается предварительное объяснение в терминах температурной зависимости функцииF(k) амплитуды обратного рассеяния.

     

Effect of an electric field on Ps formation in gaseous CO2

Summary The yield of O-Ps has been studied in moderately dense CO₂ with a static electric field applied across the gas. The results are shown to be consistent with recent models of Ps formation which have considered that Ps is formed in molecular gases by both Ore and spur processes.

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Riassunto La produzione di O-Ps è stata studiata in CO₂ moderatamente denso con un campo elettrico statico applicato attorono al gas. Si mostra che i risultati sono consistenti con recenti modelli di formazione di Ps che considerano che Ps sia formato in gas molecolari per mezzo dei processi di Ore e spur.

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Реэюме Исследуется выход O-Ps в умеренно плотном CO₂ в случае статического электрического поля, приложенного к гаэу. Покаэывается, что реэулътаты согласуются с недавними моделями обраэования Ps которые предполагают, что Ps обраэуется в молекулярных гаэах.

     

EXAFS and Raman studies of ion-ion and ion-water interactions in strong II-I electrolytic solutions

Summary EXAFS and Raman scattering data in ZnCl₂ aqueous solutions and in their mixture with LiCl, CaCl₂ and SrCl₂ are presented. The measurements, carried out at the PULS facility on the Zn and CaK-edges, allow us to define the local order around the absorber in these multi-component systems. The aqueous ZnCl₂ solutions exhibit inner-shell ion complexing with the formation of tetrahedral complexes about the Zn²⁺. In contrast, Li⁺, Ca²⁺ and Sr²⁺ cations prefer an inner-shell water complexation. In particular clear evidence exists that tetrahedral ZnCl ₄ ²⁻ units, which are the main cation-anion complexes in such solutions, become less and less interacting for increasing values of the ratioR between Cl⁻ and the Zn²⁺ ions. The Raman response confirms these structural hypotheses, indicating that the vibrational bands become more and more localized for values ofR higher than 2.

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Riassunto Vengono presentati e discussi risultati di EXAFS e scattering Raman in soluzioni acquose di ZnCl₂ ed in soluzioni miste di ZnCl₂ con LiCl, CaCl₂ e SrCl₂. Le misure EXAFS, effettuate mediante la facility PULS di Frascati (Italia), riguardano lo studio della struttura fine dello spettro di assorbimento sullo spigoloK dello zinco e del calcio. Le soluzioni acquose di ZnCl₂ mostrano effetti di complessazione intorno allo ione zinco con un ordine locale di tipo tetraedrico. In contrasto i cationi Li⁺, Ca²⁺ e Sr²⁺ preferiscono una complessazione con l’acqua, con la conseguente formazione di una sfera di idratazione primaria. In eccesso di ioni cloro, le unità tetraedriche tendono a diventare sempre piú isolate e gli ioni cloro non agiscono da ponte tra le varie unità. I relativi spettri Raman confermano queste ipotesi topologiche: viene osservato, in particolare, uno spostamento a piú alte frequenze della vibrazione Raman total-simmetrica, associato alle unità tetraedriche ZnCl ₄ ²⁻ , quando il rapportoR tra ioni zinco e ioni cloro è maggiore di due.

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Резюме Приводятся данные EXAFS и данные по рамановскому рассеянию в Zn Cl₂ водных растворах и в их смесях с LiCl, CaCl₂ и SrCl₂. Измерения, проведенные на аппаратуре PULS, на К-краяхцинка и кальция позволяют нам определить локалькый порядок около поглотителя в этих многокомпонентных системах. Водные растворы ZnCl₂ обнаруживают структуру с образованием тетраэдрических комплексов около Zn²⁺. В противоположность этому, катионы Li⁺, Ca²⁺ и Sr²⁺ предпочитают комплексообразование с участием воды. В частности, получено явное подтверждение, что тетраэдрические агрегаты ZnCl ₄ ²⁻ , которые представляют основные катион-анионные комплексы в таких растворах, становятся менее и менее взаимодействующими при увеличении величины отношенияR между ионами Cl⁻ и Zn²⁺. Рамановский отклик подтверждает эти структурные гипотезы, указывающие, что колебательные зоны становятся более и более локализованными для величинR, больших двух.

     

XANES studies of unoccupied electronic states and local real structure of some antimony chalcogenides

Summary The X-rayL absorption edges of antimony and iodine in SbSI, Sb₂S₃, Sb₂S₅ have been measured with high resolution using synchrotron radiation. It is shown that the density of the unoccupied levels in the conduction band is adequate to interpret the edge structures between 0 and 10 eV. These structures and the chemical shifts of the three compounds are compared: a striking similarity of the electric charge of antimony and of the electronic structure in the bottom of the conduction band is found. The structures between 10 and 50 eV are analysed and interpreted in terms of stereochemical co-ordination. The nonexistence of Sb₂S₅ as a stoichiometric compound is confirmed.

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Riassunto Le soglie di assorbimentoL dell'antimonio e dello iodio sono state misurate ad alta risoluzione con luce di sincrotrone in SbSI, Sb₂S₃ e Sb₂S₅. La densità degli stati liberi nella banda di conduzione è risultata adeguata per interpretare le strutture di soglia tra 0 e 10 eV. Il confronto delle strutture di soglia e degli spostamenti chimici dei tre composti ha rivelato una notevole somiglianza nella carica dell'atomo di antimonio e nella struttura della parte bassa della banda di conduzione. Le strutture tra 10 e 50 eV sopra la soglia sono state analizzate ed interpretate in relazione alla coordinazione stereochimica. è stata confermata la non esistenza del Sb₂S₅ come composto stechiometrico.

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Резюме Были определны рентгеновскиеL края поглощения суьмы и йода в SbSI, Sb₂S₃, Sb₂S₅ с высоким разрешением, используя синхротронное излучение. Показывается, что плотность незаполненных уровней в зоне проводимости адекватно описывает структуры в области от 0 до 10 эВ. Для трех соединений проводится сравнение полученных структур и химических сдвигов. Обнаружено удивительное подобие электрического заряда сурьмы и электронной структуры на дне зоны проводимости. Анализируются структуры в области от 50 эВ и предлагается их интерпрпретация. Отмечается отсутствие Sb₂S₅, как стехиометрического соединения.

     

Hamiltonian formulation for the positron trapping model

Summary A Hamiltonian formulation is used to build up an adequate Hamiltonian for the positron trapping model. The number of positrons annihilated in the free state,n _f, or the trapped one,n _v, are considered canonical conjugate variables; this point of view provides a route to propose a Hamiltonian which leads to the previously proposed phenomenological master equations.

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Riassunto Si usa una formulazione hamiltoniana per elaborare un'hamiltoniana adeguata per il modello a trappola dei positroni. Il numero di positroni annichilati nello stato libero,n _f, o in quello intrappolato,n _v, è considerato come variabili coniugate canoniche; in questo senso si fornisce una via per proporre un'hamiltoniana che porti alle equazioni principali fenomenologiche proposte precedentemente.

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Резюме Гамильтонова формулировка используется для конструирования адекватного Гамильтониана для модели захвата позитрона. Число аннигилированных позитронов в свободном состоянии,n _f, или число захваченных позитронов,n _v, рассматриваются как канонически сопряженные переменные. Этот подход приводит к Гамильтониану. Который дает ранее предположенные феноменологические ?управляющие? уравнения.

     

Study by Mössbauer spectroscopy of an iron meteorite

Summary An iron meteorite has been studied by backscattering M?ssbauer spectroscopy. The internal magnetic field is found to exceed significantly that of pure iron.     

Developments of solid-state spectroscopy in Pavia: from first steps to newest achievements

Summary In this paper some significant aspects of the evolution of solid-state spectroscopy in Pavia are briefly reviewed in their basic lines starting from the '50s. As an example of historical trends and scientific developments, the authors describe and discuss two experiments on low-dimensional systems which, over a period of nearly three decades, represent a coherent approach to spectroscopic disciplines. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

On the harmonic oscillator with nonzero periodic mass

Summary Following previous papers of Leach, and Wille and Vennik, we consider various new expressions for the nonvanishing periodic mass parameter of the time-dependent harmonic oscillator in a Pérot-Fabry cavity in contact with an atomic reservior. The authors of this paper have agreed to not receive the proofs for correction     

Analysis of the CO vibrational distribution obtained from formaldehyde laser photodecomposition

Summary A simple analysis of the overall CO vibrational distribution obtained from formaldehyde laser photodecomposition has been carried out. Extrapolation to zero vibrational energy transfer to CO gives an energy value of (27770±80) cm⁻¹ for the intermediate assumed in the formaldehyde predissociation. The result obtained turns out to be very reasonable and consistent with several mechanisms suggested in previous studies To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Energy loss of 18 keV electrons in gaseous T and quench condensed D films

Measurements of the energy loss of fast electrons at an energy of 18 keV have been performed on molecules of hydrogen isotopes, gaseous T ₂ and frozen D ₂ . Whereas in the case of gaseous T ₂ the values of total inelastic cross-section ( cm² for E = 18.6 keV), average energy loss ( eV) and peak position of the energy loss spectra ( eV) agree well with the expectations, the corresponding values for quench condensed D ₂ differ significantly from the ones for gaseous T ₂ . We observe a significant lower total inelastic cross-section ( cm², for E = 18.6 keV) larger average energy loss ( eV) and higher peak position ( eV). These differences may be interpreted in terms of changes of the final state spectrum. A CI calculation for a D ₂ cluster shows indeed a clear shift of the excited states in agreement with the observation. Received 24 August 1999     

On the question of possible experimental observation of Anderson localization of the neutron

A possible experiment is discussed, for the observation of Anderson localization of the neutron. The localized state may be formed in the process of inelastic downscattering of thermal or cold neutrons in a highly disordered substance with low neutron capture and upscattering cross sections. The lifetime of trapped (localized) neutrons in the sample is measured by counting the upscattered neutrons with a neutron counter surrounding the sample. Estimations of experimental parameters relevant to such an experiment are given. Received: 12 May 1997 / Revised: 12 September 1997 / Accepted: 16 September 1997     

Self-consistent RPA retarded polaron Green function for quantum kinetics

We compute numerically the time dependent retarded Green function of the polaron within the self-consistent RPA approximation. The results show an approximately Gaussian behaviour at t=0 changing at later times its concavity to an exponential decay, as it has been predicted in the approximate form of an inverse hyperbolic cosine function. The result contrasts with the non-selfconsistent RPA, where the exponential decay is only a transitory behaviour and the asymptotics is rather oscillatory. Our conclusions are significant in the context of the quantum kinetics with LO-phonons, where the transition from an intitially coherent scattering kinetics to a Markov kinetics with energy conservation is controlled by the time behaviour of the retarded Green function. Received: 18 September 1997 / Revised: 24 October 1997 / Accepted: 6 November 1997     

High-resolution resonant photoemission study of the 4f states in a typical Kondo system: CePd3

We exploited resonant photoemission at the Ce absorption edge to investigate the Ce 4f states in . High resolution spectra reveal, near the Fermi level, the characteristic fine structure of intermediate valence Ce compounds. The spectral lineshape is consistent with the typical “Kondo” character of CePd, but the prominent ionization peak is found at the unusually low binding energy of 1 eV. We briefly discuss the implications of these observations. Received: 13 October 1997 / Accepted: 21 January 1998     

Radiative frequency shift without ultraviolet divergency in a two-level hydrogenic atom

Summary By taking into account the retardation effects a finite result for the frequency shift of the Lyman α-radiation from a two-level hydrogenic atom is obtained. This result shows a nonnegligible correction as compared to previous results obtained in the dipole approximation with a cut-off frequency.     

Density effects on the spectral intensities of dielectronic satellite line in dense plasmas

Summary In this paper we present a theoretical analysis of the spectral intensity of dielectronic satellite lines in dense plasmas. The ion microfield causes an oscillator strength mixing that alters the spectral intensity of such a transition. Numerical calculations of this high-density effect have been performed for the most intense 2l2l′→1s2l He-like transitions.

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Riassunto In questo articolo si presenta un'analisi teorica dell'intensità spettrale delle linee di satellite dielettroniche in plasmi densi. Il microcampo ionico causa un mescolamento delle forze dell'oscillatore che modifica l'intensità spettrale di una tale transizione. Si sono effettuati calcoli numerici di questo effetto ad alta densità per le transizioni più intense del tipo 2l2l′→1s2l.

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Резюме В этой работе мы предлагаем теоретический анализ спектральной интенсивности диэлектронных линий сателлитов в плотной плазме. Микрополе иона вызывает смешивание сил осцилляторов, что изменяет спектральную интенсивность такого перехода. Проведены численные вычисления эффекта высокой плотности для наиболее интенсивных Не-подобных переходов 2l2l′→1s2l.