On the observation of anomalous electron emission from a thin film under the influence of an outside electric field

Summary It is established that accelerated secondary electrons with very low energies emitted from a thin surface layer where M?ssbauer absorption and normal photoeffect are taking place show two distinctly separated energy lines. The maximum of the first line corresponds to the applied accelerating voltage while that of the second one corresponds to about 30% higher energy. The investigations are performed with a magnetic beta-spectrometer and with an electrostatic analyser. It is found that the intensity of the second, ?anomalous? line depends on the nature of the emitting layer. A number of experiments confirming the reality of the observed effects are reported. This authors of this paper have agreed to not receive the proofs for correction.     

The integral approach to the sputtering

Summary The emission-oriented integral form of Boltzmann transport equation is applied to the sputtering phenomenon. It is shown that the central quantity in the formulation, namely the characteristic mean free path of the phenomenon, can be the low-energy transport mean free path. The same mean free path gives a measure of the mean escape depth as derived in experiments and computer simulation studies. Finally, the sputtering yields, calculated by using the same mean free path, when compared with the experimental results, show an evident improvement with respect to the Sigmund theory.     

Energy distribution of reflected ions at normal incidence on the target surface

Summary The energy distribution of reflected ions at normal incidence of the primary beam on the target surface is obtained as a result of approximate solution of the Boltzmann equation by the method of discrete streams. This method gives the opportunity to avoid the problem of negative distribution function, arising in the generally applied method of spherical harmonics, and to present the final result in analytical form. The analysis is performed for a power scattering cross-section and the theoretical energy distribution demonstrates one maximum when the ion mass is equal to the mass of a target atom, and two maxima when the ion mass is much less than the mass of a target atom.     

Inelastic ion interaction in solid hydrocarbons

Summary Erosion and decomposition of solid hydrocarbons under bombardment with proton, helium and argon ions is studied as a function of ion energy in the keV÷MeV range. Methane (10K), benzene (77K) and polystyrene (300 K) show a strong molecular hydrogen emission with a complex behaviour as a function of ion fluence (10¹²÷10¹⁷ions/cm²). Dimer formation is observed in the irradiated volume and high yield is measured (200 dimers/ion) for all the samples. At high fluence a three-dimensional carbon-hydrogen network is formed whose thickness is proportional to the ion range.

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Riassunto L'erosione e la decomposizione di idrocarburi solidi indotte da irraggiamento con fasci ionici quali protoni, elio e argo sono state studiate in funzione dell'energia dello ione incidente nel range keV÷MeV. Metano (10 K), benzene (77 K) e polistirene (300 K) mostrano una forte emissione molecolare con un comportamento complesso in funzione della fluenza nel range (10¹²÷10¹⁷) ioni/cm². Dal volume irradiato è osservata una alta emissione di dimeri (200 dimeri/ione). Ad alta fluenza si ha formazione di un composto carbone-idrogeno il cui spessore è proporzionale al range dello ione incidente.

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Реэюме ИсследуУтся эроэия и распад углеводородов при бомбардировке протонами и ионами гелия и аргона в эависимости от энергии ионов в кэВ-МэВной области. Метан (10 K), бенэол (77 K) и полистирол (300 K) обнаруживают сильную эмиссию молекулярного водорода, которая слоьной функцией флуенса ионов ((10¹²÷10¹⁷)ион/см²). Наблюдается образование димеров в облученном объеме и для всех образцов измеряется большой выход (200 димеров/ион). При высоком флуенсе образуется трехмерная углеводородная структура, толщина которой пропорциональна пробегу ионов.

     

Nonadiabatic effects in auger-induced ionic desorption

Summary The Knotek-Feibelman mechanism of desorption is analysed in the context of a microscopic dynamical theory that has been proposed recently for closed-band systems. To understand the physical contents of the general equations, the probabilityP of ionic desorption is derived analytically in several selected model situations as a function of the characteristic desorption timet _o. Although typicallyt _o is quite long compared to the inverse level widthW _B ⁻¹, the results show very significant deviations from the limit of small velocities. This is due to the fact thatP is not primarly determined by the level width, but by the structures, like peaks and singularities, that are contained in the two-hole continuum. Such features, that are completely missed in the traditional approach that represents the continuum by a Lorentzian, are naturally enbodied in the present theory. They may originate from the position of the adatom energy level ɛ relative to the substrate band and/or by hole-hole correlation. When split-off localized states arise,P may become comparable with unity even in the limit of small velocities, giving raise to the Knoteck-Feibelman reneutralization bottleneck; however, this is not the case when the localized states are readsorbed by the continuum in the course of desorption. An explicit example of this behaviour is given. On the other hand, when ions desorb from ionic crystals with a large Madelung energy, the bond-breaking process may approach the sudden limit. A strongly nonadiabatic behaviour is then predicted, including a correlation-induced enhancement ofP by several orders of magnitude, even in bandlike situations. This rapid variation leads to suggestions for a new experiment that should enable us to see the effects of a fine tuning of the electronic parameters.

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Riassunto Il meccanismo di Knotek e Feibelman di desorbimento è studiato nel contesto di una teoria quantistica dinamica che è stata recentemente proposta ed è applicabile al caso di sistemi con bande chiuse. Al fine di mettere in luce il contenuto fisico delle equazioni generali, si sono scelte alcune situazioni modello per le quali si è ottenuta analiticamente la dipendenza della probabilitàP di desorbimento ionico dal tempo caratteristico di desorbimentot _o. Benchè tipicamentet _o risulti molto piú lungo della larghezza inversaW _B ⁻¹ del livello, i risultati dimostrano cheP può deviare in modo molto significativo dal limite di velocità basse. Infatti,P è determinato principalmente non dalla larghezza del livello, ma dalle strutture (picchi e singolarità) che sono contenute nel continuo. Questi aspetti sono ignorati completamente nell'approccio consueto, che rappresenta il continuo con una lorentziana, ma sono inclusi in questa teoria in modo naturale. Una possibile origine di strutture nel continuo è rappresentata da una posizione del livello eccentrica rispetto alla banda del substrato; un'altra, dalla correlazione buca-buca. Se ne risultano stati disereti isolati dal continuo,P può essere dell'ordine dell'unità anche nel limite di velocità basse, e questo è appunto il ≪collo di bottiglia≫ che impedisce la rineutralizzazione secondo Knotek e Feibelman; questo non avviene, però, se lo stato discreto è riassorbito dal continuo nel corso del desorbimento. Di tale fenomeno è fornito un esempio esplicito. D'altra parte, quando gli ioni desorbono da cristalli ionici con un'elevata energia di Madelung, il processo di rottura del legame può avvicinarsi al limite repentino. La teoria allora predice un comportamento fortemente non adiabatico, in cuiP è accresciuta di ordini di grandezza dagli effetti di correlazione, anche in situazioni in cui lo spettro è di tipo banda. Questa rapida variabilità diP porta a suggerimenti utili per verificare, con un nuovo esperimento, gli effetti di piccoli aggiustamenti dei parametri elettronici.

     

Contributions of different generations of recoils to sputtering by backscattered light ions

Summary Neumann's expansion of the integral form of the Boltzmann transport equation is used to estimate the contribution of different generations of recoils to sputtering. By considering the flux generated by backscattered ions it is shown that primary recoils are, in the region of low energy (few keV), the main contribution to sputtering.     

Theory of stimulated desorption of ions and neutrals

Summary To understand the electronic processes that occur during desorption, we must allow simultaneously for the intrinsic nonadiabatic character of electron states and for correlation-induced features like two-hole resonances. A recently proposed theory describes the Menzel-Gomer-Redhead and Knotek-Fiebelman mechanisms on equal footing and reveals a complex interplay of electronic and kinematic effects on the ion yield. Here, I wish to show how in the same scheme we can also obtain the yield of ions that have suffered (partial or total) re-neutralization in the course of desorption. Closed-form expressions for the relevant escape probabilities are given in terms of one-hole and two-hole Green’s functions. The surface molecule limit of the theory is worked out in detail assuming that the hopping parameters decay exponentially in time during desorption. It is shown that, in general, when a given species can be desorbed by different mechanisms, we can discriminate them experimentally by the energy and angular dependence of the yield.

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Riassunto Per comprendere i processi elettronici che avvengono nel desorbimento, dobbiamo tener conto che gli stati hanno un carattere intrinsecamente non adiabatico. Allo stesso tempo, occorre includere gli effetti di correlazione, come, ad esempio, le risonanze a due buche. Una teoria recente consente una descrizione unificata dei meccanismi di Menzel-Gomer-Redhead e di Knotek-Feibelman; essa mostra che effetti elettronici ed effetti cinematici concorrono in modo complesso a determinare la resa di ioni. Qui si vuol far vedere come tale teoria consenta di ottenere la resa anche per le specie che subiscono una rineutralizzazione parziale o totale nel corso del desorbimento. Le probabilità che lo ione desorba nei vari stati di carica sono espresse in forma chiusa in termini delle funzioni di Green a una e due buche. Il limite della molecola superficiale è risolto in dettaglio nel caso in cui i parametri di salto decadono esponenzialmente nel tempo durante il desorbimento. Si trova che, quando diversi meccanismi portano al desorbimento di una stessa specie, è generalmente possibile distinguerli per le diverse dipendenze dall’energia cinetica e dagli angoli.

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Резюме Чтобы понять электронные процессы, происходящие при десорбции, мы должны одновременно учитывать внутренний неадиабатический характер электронных состояний и особенности, подобные двухдырочным резонансам. Недавно предложенная теория описывает механизмы Менцеля-Гомера-Редхеда и Нотека-Фейбелмана и обнаруживает комплексное влияние электронного и кинематического эффектов на выход ионов. В рамках этой схемы мы хотим показать, как можно получить выход ионов, которые испытывают (частичную и полную) ренейтрализацию в процессе десорбции. Приводятся замкнутые выражения для соответствующих вероятностей вылета в терминах одно-дырочных и двух-дырочных функций Грина. Разрабатывается предел теории для молекулы на поверхности, предполагая, что параметры ?перепыгивания? экспоненциально затухают во времени в процессе десорбции. Показывается, что, когда заданный тип частиц может быть десорбирован за счет различных механизмов, мы можем дискриминировать их экспериментально, используя энергетическую и утловую зависимости выхода.

     

On the existence of an universal energy function for sputtering yields by low-energy light-ion bombardment

Summary The mechanism wherenism whereby heavy solids are sputtered by lowenergy light ions can be explained by a linear-transport theory, by assuming only primary recoils as candidate for sputtering. It is shown that, from a theoretical point of view, scaling properties of the sputtering yield can be introduced if the appropriate dimensionless energy variable is assumed. In fact, the yield expression can be reduced to a formulation in which an energy function common to all the target-ion combinations is multiplied by a constant factor different for each combination. This expression, free from any kind of fitting parameter, gives a good approximation to the experimental data. Further developments are discussed in relation to the spreading of experimental points which do not exibit strict scaling properties.

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Riassunto Il meccanismo di erosione di solidi durante il bombardamento di ioni leggeri di bassa energia può essere spiegato nell'ambito della teoria del transporto assumendo che solo i rinculanti primari contribuiscono all'erosione. L'approccio teorico mostra che il coefficiente di erosione può esprimersi in termini di una funzione universale assumendo un'opportuna variabile energetica adimensionale. Infatti il coefficiente di erosione può essere ricondotto ad un' espressione in cui una funzione della sola energia, comune a tutte le combinazioni ione-bersaglio, è moltiplicata per una fattore costante differente per ogni combinazione. Questa espressione, priva di parametri aggiustabili, fornisco una buona approssimazione per i dati sperimentali. Ulteriori sviluppi sono discussi in relazione al fatto ai dati sperimentali non sono strettamente riconducibili ad una fuzione universale.

     

Crystal-glass transition in ion-bombarded alloys

Summary A model for glass formation and crystal structure retainment in binary alloys undergoing irradiation by energetic ion beams is presented. The major physical features provided by experimental conditions, namely spike formation and evolution of bombardment-induced surface compositional variations, are taken into account. At spike-lattice interface interatomic interactions with charge transfer occur, which induce formation of amorphization, or crystallization nuclei. Qualitative differences are found between glass forming and crystalline systems. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Influence of the deposited-energy function on the sputtering yield

Summary An explicit spatial dependence (a Gaussian) of the depositedenergy function has been considered in order to compare the sputtering yield obtained by taking into account all the energy deposited inside the target with that obtained by assuming the deposited-energy function throughout constant, namely equal to its value at the surface. The results indicate that the two yields differ sensibly for high incident-ion atomic number and/or for small target/ion mass ratio. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Heavy ion induced cluster formation and fragmentation phenomena in amorphous graphite

A study has been conducted into the mechanisms of evolution of clusters and their subsequent fragmentation under energetic heavy ion bombardment of amorphous graphite. The evolving clusters and their subsequent fragmentation under continuing ion bombardment are revealed by detecting various clusters in the energy spectra of the Direct Recoils (DRs) emitted as a result of collisions between ions and surface constituents. The successive DR spectra reveal that the energetics of bond formation as well as any subsequent fragmentation can be related to the processes of energy dissipation in a cylindrical volume of a few ? surrounding the ion path. The dependence of formation or fragmentation is seen to be a function of the ionic stopping powers in this cylindrical volume. Received: 2 January 1998 / Accepted: 5 March 1998     

Vibrations at 3C-SiC(001)-(32) surfaces

Si-terminated 3C-SiC(001) surfaces with () and ()reconstructions were investigated by high-resolution electron energy-loss spectroscopy (HREELS), low-energy electron diffraction (LEED) and Auger electron spectroscopy. The surfaces were prepared by subsequent annealing steps after cleaning by heating in a Si flux. At ()-reconstructed surfaces, the HREELS intensity increases while the widths of the loss lines decrease with proceeding preparation. Eventually, weak loss structures at 380 and 700 cm^-1 are detected besides the strong Fuchs-Kliewer phonon loss peaks. They are attributed to surface-localized vibrations, i.e., to stretching modes of on-top Si dimers and of C-Si-C groups, respectively. The weak features vanish after exposure to atomic deuterium, but reappear after subsequent annealing. At () reconstructed surfaces the HREELS lines are broadened and no surface-localized modes were resolved. Received 18 January 1999     

HREELS investigation of hydrogenated and deuterated GaN{0001} surfaces

The vibrational properties of clean, H- and D-covered GaN{0001} surfaces were investigated by high-resolution electron energy-loss spectroscopy. Auger electron spectroscopy and low-energy electron diffraction were utilized to monitor the surface cleanliness and structure, respectively. At the clean surface the Fuchs-Kliewer surface phonon frequency was determined to 700 cm^-1 (86.8 meV). For the adsorbate-covered surfaces current structure models predict only Ga-H vibrations for surfaces of either polarity. Despite of this, the HREEL-spectra of the hydrogenated sample show a new loss structure at 3255 cm^-1 (403.6 meV) and a shoulder at 1900 cm^-1 (235.6 meV) which are attributed to N-H and Ga-H stretching vibrations, respectively. After deuterium exposure an isotope shift occurs. Again, a N-adsorbate vibration is clearly resolved. A Ga-D bending mode is observed at 390 cm^-1 (48.4 meV), indicating that vibrations of both species are present. Received 1 February 2000     

Multipolaritons in semiconductor thin layers: Interference effects in the reflectance spectra

Summary We consider the case of a plurality of polariton modes due to the valence band structure, and give a one-dimensional model to compute the optical functions in the exciton frequency range. Interference effects of each mode with itself and with the other modes are identified by a fine structure in the reflectivity spectrum. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Polaronic effects on two-dimensional excitons

Summary We calculate the binding energy of a two-dimensional exciton for a set of states labelled by the quantum numberm associated to the angular momentum in the direction perpendicular to the surface. The Fr?hlich electron-phonon and hole-phonon interactions are taken into account. The statem=0 is more bound with respect to that obtained by the Wannier exciton theory with the screening given by the static dielectric constant and the reduced mass calculated by the electron and hole polaronic masses. The opposite effect is found for the statem=1,2,... Whenm becomes large, the hydrogenic series is recovered with the screening and reduced mass defined above. Our results are compared with the experimental data concerning exciton luminescence GaAs/Al _x Ga_1−x As, CuCl/CaF₂ and CdTe/Cd_1−x Zn _x Te quantum wells. The authors of this paper have agreed to not receive the proofs for correction.     

Electric-field effects on two-photon transitions to Wannier excitons

Summary The effects of an electric field, ε, on the excitonic two-photon absorption is presented for direct-gap semiconductors and insulators. Expressions for the electroabsorption coefficient, α _ex ⁽²⁾ (ω₁, ω₂, ε), have been calculated adopting a three-band model and allowed-transition matrix elements. The final state allows as-excition state. A numerical application to the ZnS crystal is given. It shows that: i) The electron-hole interaction enhances the magnitude of α _ex ⁽²⁾ (ω₁, ω₂, ε) and also the electric-field-induced oscillations after the absorption edge. ii) The order of magnitude of α _ex ⁽²⁾ (ω₁, ω₂, ε) decreases as ε increases. iii) Before the edge and as ε increases, the exciton peak broadens and shifts towards higher energies.

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Riassunto Si presentano gli effetti di un campo elettrico, ε, sull’assorbimento eccitonico a 2 fotoni per semiconduttori e isolatori con gap diretto. Sono state calcolate le espressioni per il coefficiente di elettroassorbimento α _ex ⁽²⁾ (ω₁, ω₂, ε) adottando un modello a tre bande ed elementi di matrice a transizione permessa. Lo stato finale permette uno stato eccitonicos. Si dà una applicazione numerica al cristallo ZnS. Si mostra che: i) L’interazione elettrone-lacuna aumenta la grandezza di α _ex ⁽²⁾ (ω₁, ω₂, ε) e anche le oscillazioni indotte dal campo electtrico dopo il limite di assorbimento. ii) L’ordine di grandezza di α _ex ⁽²⁾ (ω₁, ω₂, ε) diminuisce quando ε aumenta. iii) Prima del limite e quando ε aumenta, il picco eccitonico si allarga e si sposta verso energie piú alte.

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Резюме Исследуется влияние электрического поля, ε на экситонное двухфотонное поглощение в полупроводниках и дизлектриках. Исрользуя трех-зонную модель и матричные элементы разрешенных переходов, вычясляются выражения для коэффициента электрологолощения, α _ex ⁽²⁾ (ω₁, ω₂, ε). Конечное состояние допускаетs-экситонное состояние. Вычисления проводятся для кристалла ZnS. Показывается, что: 1) электрон-дырочное взаимодействие возрастает на величину α _ex ⁽²⁾ (ω₁, ω₂, ε) и за краем поглощения возинкают осцилляции, индуцированные электрическим полем; 2) порядок величины α _ex ⁽²⁾ (ω₁, ω₂, ε) убывает при увеличении ε; 3) экситонный пик уширяется и сдвигается в сторону больших энергий перед краем и при увеличении ε.

     

Evidence of the exciton-plasma transition in the emission spectra of GaSe

Summary We present luminescence spectra of gallium selenide at 2 K in which a slow and continuous evolution from excitonic recombination to an electron-hole plasma emission is observed when the excitation intensity is increased. We find a small red-shift and a broadening of the direct exciton emission line which is followed by its disappearing. We explain these results with a model which takes into account the electron-hole correlation. Part of this work was carried out during a stage at the Institut de Physique Appliquée of the Ecole Polytechnique Fédérale de Lausanne (Switzerland).     

Core excitons in the electronic polaron model

Summary The electronic polaron model of the exciton is used to study the dielectric response of a medium to the excitation of a ?core? level, by adopting the method of direct solution of the Eulerian functional variational equations. The dynamical response of the electronic polarization affects the electron-hole attraction and the exciton binding energy, in a way which depends on the basic parameters of the crystal (dielectric constant, effective masses, lattice parameter) through the core exciton radius and the polaron radius. When the former is much larger than the latter, static dielectric screening results. When the exciton radius is comparable to the polaron radius, the screening is reduced and the binding energy is increased. Core exciton binding energies are computed in a number of substances using the effective-mass approximation. Space dispersion of the dielectric function coupled to intervalley interaction may, however, contribute in some cases to reducing the excitonic radius and bringing about an instability to a deep state that would invalidate the effective-mass approximation. Based on work supported by the Italian Research Council (C.N.R.) through a contract G.N.S.M.     

Optical properties of Wurtzite-type semiconducting crystals near the isotropic point

Summary Using Stahl's coherent wave approach we calculated the linear interband susceptibility near the band gap. The starting point was the ?MGM? formula with a smeared-out dipole densityM(r) and the Green's functionG for the effective-mass Hamiltonian with a screened Coulomb potential. The susceptibility so obtained was used for calculating transmission spectra of CdS platelets for energies near the so-called isotropic point lying far below the nearest excitonic resonance. By fitting experimental spectra we determined components of the residual dielectric tensor for polarization of the incident beam perpendicular and parallel to thec-axis, the mixed-mode dipole moment corresponding toB-exciton, and decay radii characterizing transition dipole densities. The established set of parameters determines the dielectric tensor for a quite wide energetic range.