Stimulated bremestrahlung in the presence of a high-intensity radiation field

Summary The theoretical treatment is given for the case when the emission or absorption of a weak-fied photon of arbitrary frequency ω₁ during a scattering process takes places in the presence of a strong laser field. The general formulation of the theory which is given in detail corresponds more properly to the case when an electron is scattered by a static potential in the presence of two arbitrary radiation fields. Part of theory is expected to cover also the process of the stimulated bremsstrahlung. A selected set of calculations is performed in the first order in the scattering potential for the case when one of the two fields is weak. The effects of the two fields are always treated exactly. The chosen geometry is the parallel one, in which the two linearly polarized fields are parallel to the initial electron velocity. The presence of a strong assisting field is found to yield several significant modification of the characteristics of emission/absorption of a weak-field photon. In paritcular, i) the final electron energy distribution is drastically altered; ii) angular distributions and total cross-sections of emission and absorption are modified up to become largely similar; iii) when the oscillatory velocityv ₀ is approximately equal to the incoming velocityv _i, the cross-sections experience rapid variations, generally in the sense of the growth; iv) for higher intensities, whenv ₀>v _i the strong field is found instead to dump considerably all the cross-sections, besides making emission and absorption similar. Accordingly, in the high-intensity domain, the question whether or not emissions prevail over absorptions looses part of its importance. In general, calculation shows that the quantities, accounting for the balance between emission and absorption, exhibit strong oscillations above and below zero either as a function of the strong-field intensity or as a function of the weak-field frequency.

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Riassunto Si propone un trattamento teorico per i processi di emissione e assorbimento di un fotone di campo debole a frequenza arbitraria ω₁ durante un processo di scattering in presenza di un forte campo laser. La formulazione generale della teoria è data in dettaglio relativamente al caso di un elettrone diffuso da un potenziale statico in presenza di due campi di radiazione di intensità arbitraria. La teoria sviluppata, è applicabile anche alla descrizione del processo di bremsstrahlung stimolato. Si presenta un gruppo di risultati ottenuti con una teoria al primo ordine nel potenziale statico, per il caso in cui uno dei due campi è debole. Gli effetti dei due campi sono trattati sempre in modo esatto. I risultati mostrano che la presenza di un forte campo laser è responsabile di numerose importanti modifiche delle caratteristiche dei processi di emissione/assorbimento di un fotone di campo debole. In particolare i) viene alterata drasticamente la distribuzione dell'energia finale degli elettroni; ii) le distribuzioni angolari e le sezioni d'urto totali d'emissione e assorbimento sono modificate fino a divenire largamente simili; iii) quando l'ampiezza della velocità di oscillazionev ₀ è approssimativamente uguale alla velocità di incidenzav _i , le sezioni d'urto subiscono rapide variazioni, generalmente nel senso della crescita; iv) per intensità piú elevate, quandov ₀>v _i , si trova che il campo spettatore riduce sensibilmente le sezioni d'urto, oltre a renderle simili. Di conseguenza, nel dominio delle alte intensità, la questione della prevalenza dell'emissione sopra l'assorbimento perde parte della sua importanza. In generale, il calcolo mostra che le quantità fisiche, che rendono conto del bilancio fra emissione e assorbimento, presentano un andamento oscillatorio, sopra e sotto lo zero, sia come funzioni dell'intensità del campo forte sia come funzioni della frequenza del campo debole.

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Резюме Теоретически рассматривается случай, когда излучение или поглощение фотона слабого поля произвольной частоты ω₁ в процессе рассеяния происходит в присутствии сильного лазерного поля. Предлагается общая формулировка теории, которая соответствует случаю, когда электрон рассеивается на статическом потенциале в присутствиии двух произволных радиационных полей. Часть теории описывает процесс стимулированного тормозного излучения. Проводятся вычисления в первом порядке по потенциалу рассеяния, когда одно из двух полей является слабым. Влияния двух полей всегда рассматривается точно. Выбранная геометрия соответствует случаю, когда два линейно поляризованных поля являются параллельными первоначальной скорости электрона. Обнаружено, что наличие дополнительнго поля приводит к существенному изменению характеристик излучения (или поглощения) фотона слабого поля. В частности: 1) существенно изменяется конечное энергетическое распределение электрона; 2) изменяются угловые распределения и полные поперечные сечения излучения и поглощения и становатся похожими; 3) когда осцилляторная скоростьv ₀ приближенно равна скорости налетающего электронаv _i поперечные сечения испытывают сильные изменения; 4) для высоких интенсивостей, когдаv ₀>v _i , отмечают значительное уменьшение поперечных сечений. В области высоких интенсивностей важным становится вопрос о том, превалируют ли процессы излучения над процессами поглощения или нет. В общем случае вычисления показывают, что величина, определяющая баланс между излучением и поглощением, обнаруживает сильные осцилляции выше и ниже нуля в зависимости от интенсивности поля или в зависимости от частоты слабого поля.

     

Ionic strength dependence of formation constants-I Protonation constants of organic and inorganic acids

The protonation constants of formic, acetic, benzoic, oxalic, phthalic, maleic, malonic, succinic, dl-malic, dl-tartaric, aminoacetic, citric, nitrilotriacetic, ethylenediaminetetra-acetic, sulphuric and orthophosphoric acids have been determined from pH measurements, in tetraethylammonium iodide solution, at various ionic strengths in the range 0.01-1.0M (for phosphoric and sulphuric acids 0.01-0.5M). For each acid the dependence of the protonation constants on ionic strength was determined and an equation, valid for all the acids studied, to describe this was derived. The use of tetraethyl-ammonium salts as background to avoid ion-pair formation is discussed.     

Quantum dots-based reverse phase protein microarray

CdSe nanocrystals, also called quantum dots (Qdots) are a novel class of fluorophores, which have a diameter of a few nanometers and possess high quantum yield, tunable emission wavelength and photostability. They are an attractive alternative to conventional fluorescent dyes. Quantum dots can be silanized to be soluble in aqueous solution under biological conditions, and thus be used in bio-detection. In this study, we established a novel Qdot-based technology platform that can perform accurate and reproducible quantification of protein concentration in a crude cell lysate background. Protein lysates have been spiked with a target protein, and a dilution series of the cell lysate with a dynamic range of three orders of magnitude has been used for this proof-of-concept study. The dilution series has been spotted in microarray format, and protein detection has been achieved with a sensitivity that is at least comparable to standard commercial assays, which are based on horseradish peroxidase (HRP)-catalyzed diaminobenzidine (DAB) chromogenesis. The data obtained through the Qdot method has shown a close linear correlation between relative fluorescence unit and relative protein concentration. The Qdot results are in almost complete agreement with data we obtained with the well-established HRP-DAB colorimetric array (R² = 0.986). This suggests that Qdots can be used for protein quantification in microarray format, using the platform presented here.     

Audio-biofeedback for balance improvement: an accelerometry-based system

This paper introduces a prototype audio-biofeedback system for balance improvement through the sonification using trunk kinematic information. In tests of this system, normal healthy subjects performed several trials in which they stood quietly in three sensory conditions while wearing an accelerometric sensory unit and headphones. The audio-biofeedback system converted in real-time the two-dimensional horizontal trunk accelerations into a stereo sound by modulating its frequency, level, and left/right balance. Preliminary results showed that subjects improved balance using this audio-biofeedback system and that this improvement was greater the more that balance was challenged by absent or unreliable sensory cues. In addition, high correlations were found between the center of pressure displacement and trunk acceleration, suggesting accelerometers may be useful for quantifying standing balance.     

Cycle Action on Treelike Structures

The purpose of this paper is to study the action on cycles of several known classes of oligomorphic groups, that is, infinite permutation groups of countable degree having only finitely many orbits on k-sets for each k. The groups studied here are all related to trees and treelike relational structures. The sequence whose k-th term is the number of orbits in the action on k-cycles is called Parker sequence. It turns out that, if we are dealing with the automorphism group of a suitable relational structure, this sequence counts also the finite substructures admitting a cyclic automorphism; in calculating these sequences for various groups, we shall thus describe and enumerate such substructures.Di più dirò: ch’a gli alberi dà vita [I shall say more: the trees are given life spirito uman che sente e che ragiona. by a human spirit that perceives and reasons.Di più dirò: ch’a gli alberi dà vita spirito uman che sente e che ragiona. Per prova sollo; io n’ho la voce udita che nel cor flebilmente anco mi suona.[I shall say more: the trees are given life by a human spirit that perceives and reasons. I know it by experience: I heard their voice and it still resounds faintly in my heart.]Torquato Tasso, Gerusalemme liberata, XIII, 49     

Preconditioned HSS methods for the solution of non-Hermitian positive definite linear systems and applications to the discrete convection-diffusion equation

Summary. We study the role of preconditioning strategies recently developed for coercive problems in connection with a two-step iterative method, based on the Hermitian skew-Hermitian splitting (HSS) of the coefficient matrix, proposed by Bai, Golub and Ng for the solution of nonsymmetric linear systems whose real part is coercive. As a model problem we consider Finite Differences (FD) matrix sequences {A_n(a,p)}_n discretizing the elliptic (convection-diffusion) problem with being a plurirectangle of R^d with a(x) being a uniformly positive function and p(x) denoting the Reynolds function: here for plurirectangle we mean a connected union of rectangles in d dimensions with edges parallel to the axes. More precisely, in connection with preconditioned HSS/GMRES like methods, we consider the preconditioning sequence {P_n(a)}_n, P_n(a):= D_n^1/2(a)A_n(1,0) D_n^1/2(a) where D_n(a) is the suitably scaled main diagonal of A_n(a,0). If a(x) is positive and regular enough, then the preconditioned sequence shows a strong clustering at unity so that the sequence {P_n(a)}_n turns out to be a superlinear preconditioning sequence for {A_n(a,0)}_n where A_n(a,0) represents a good approximation of Re(A_n(a,p)) namely the real part of A_n(a,p). The computational interest is due to the fact that the preconditioned HSS method has a convergence behavior depending on the spectral properties of {P_n^-1(a)Re(A_n(a,p))}_n {P_n^-1(a)A_n(a,0)}_n: therefore the solution of a linear system with coefficient matrix A_n(a,p) is reduced to computations involving diagonals and to the use of fast Poisson solvers for {A_n(1,0)}_n.Some numerical experimentations confirm the optimality of the discussed proposal and its superiority with respect to existing techniques.Mathematics Subject Classification (1991): 65F10, 65N22, 15A18, 15A12, 47B65     

Multiphoton free-free transitions in a spatially inhomogeneous laser field

Summary The results of an investigation are presented in which it is shown how the spatial inhomogeneity of a laser field modifies the multiphoton free-free transition cross-sections compared to the case of a homogeneous field. This kind of investigation is required to make more close contact with experiments in intense fields, as in these cases the inhomogeneity is produced by the focusing of the laser beam. Furthermore, taking into account the intensity spatial distribution allows us to achieve in an effective way the asymptotic decoupling of the scattered particles from the field, which is very important for theoretical models using asymptotic initial and final states embedded in the field. Differential and total cross-sections are calculated over a wide range of parameters as functions of the scattering angle, of the incoming-particle energy and of the laser intensity and frequency. The laser spatial inhomogeneity is found to wash out most of the oscillating behaviour of multiphoton differential cross-sections, derived within the model of a homogeneous laser. Little modifications are, instead, found in the total cross-sections which are simply scaled to slightly lower values. Crosssections with zero photon exchange are increased, while those with photon exchanges are lowered. This yields the result that the sum of all the ≪inhomogeneous≫ cross-sections is equal to the sum of all the ≪homogeneous≫ ones (sum rule). The multiphoton free-free transition differential cross-sections are found to be very sensitive quantities which may be used to get information on the laser properties and on their nonlinear behaviour, when these are not precisely known. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Polymeric photoinitiators based on side-chain benzoin methyl ether and tertiary amine moieties for fast UV curable coatings

The synthesis is reported of a new series of polymeric photoinitiators obtained by copolymerization of a-methylolbenzoin methyl ether acrylate (MBA) with different N,N-dialkylamino alkyl acrylates. The copolymers have been fully characterized and employed in the photoinitiated polymerization and crosslinking reactions of a standard acrylic formulation for clear curable coatings. The photoinitiation activity of the above systems has been measured by using microwave dielectrometry and differential photocalorimetry, and compared with that found for the corresponding low-molecular-weight models. The results clearly indicate that the copolymeric systems display a remarkable decrease of the induction period and an improved overall activity in the UV curing of the acrylic coatings. An interpretation of the experimental findings is also reported.     

Aggregation tendency and reactivity toward AgX of cationic half-sandwich ruthenium(II) complexes bearing neutral N,O-ligands

The aggregation tendency of complexes [Ru(eta6-cymene)(N,O)Cl]X [N,O = 2-benzoylpyridine (2-bzpy), 1, and 2-acetylpyridine (2-acpy), 2, X- = BPh4- or PF6-] has been studied by means of PGSE NMR experiments. It was found that complexes with PF6- as counterion are mainly present in CD2Cl2 as ion pairs at low concentration, as a mixture of ion triples and free anions at medium concentration and as ion quadruples at elevated concentration. 19F, 1H-HOESY NMR experiments revealed that in ion triples and ion quadruples two cationic Ru-units pair up. Consistently, in the solid-state structure of 1PF6, determined through X-ray single-crystal investigation, two cationic Ru-units are held together by an intermolecular pi-pi stacking interaction between the pyridyl rings. Complexes having BPh4- as counterion are only present in solution as even aggregates, namely ion pairs at low concentration and ion quadruples at elevated concentration. In such a case a counteranion bridges two cationic Ru-units as observed in the solid-state structure of 1BPh4. The reactivity of complexes 1-2 toward AgX salts has been investigated in different solvents. Bicationic [Ru(eta6-cymene)(N,O)(MeCN)]X2 (N,O = 2-bzpy, 3, and 2-acpy, 4) and [Ru(MeCN)4(N,O)]X2 (N,O = 2-bzpy, 5, and 2-acpy, 6) complexes were obtained by the reaction of 1 and 2 with AgX in the presence of three equivalents of acetonitrile or in acetonitrile, respectively. The reaction of 1 with AgPF6 in acetone afforded complex [Ru(eta6-cymene)(N,O,O)]PF6 (7, where N,O,O = 4-alcoxide-4-phenyl-4-(pyridin-2-yl)butan-2-one) from the C-C coupling of a deprotonated methyl group of the coordinated acetone and the C=O moiety of 2-bzpy ligand.