Two-photon transitions between discretenS states in atomic He

Summary The two-photon transition probabilities from the ground state 1¹ S and from the metastable states 2¹ S and 2³ S to the lowest excited states are computed in He. A configuration interaction approach is used, and a detailed comparison between the ?length? and the ?velocity? gauges leads to an estimate of 1% accuracy. Correlation effects are found to account for about 20% of the transition probabilities. The life-time of the 2¹ S state is ≅19.6 ms. The two-photon transition probabilities display a dramatic dependence on frequency, characterized by resonances and transparencies. Based on work supported in part by the Gruppo Nazionale di Struttura della Materia of the C.N.R. and by the Frascati Synchrotron Radiation Contract (P.U.L.S.).     

On the Green's function technique for the study of multiphoton transitions in atoms

Summary The Green's function method for implicitly evaluating the sums over intermediate states occurring in the study of multiphoton processes is revisited. Partieular emphasis is given to bound-bound transitions and to the numerical techniques used to implement the method. This is also extended to the important case in which the sum over intermediate states must be performed with respect to an incomplete set. The accuracy of the method is tested against very precise results on atomic hydrogen obtained by direct summation over intermediate states.

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Riassunto Si riesamina il metodo della funzione di Green per calcolare implicitamente le somme di stati intermedi che compaiono nello studio dei processi a molti fotoni. L'enfasi é posta su transizioni discreto-discreto e sulle tecniche numeriche utilizzate. Il metodo é anche esteso al caso in cui si debba sommare su un insieme incompleto di stati intermedi. La precisione del metodo è verificata nel caso dell'idrogeno confrontandone i risultati con quelli otenuti mediante una somma accurata sugli stati intermedi.

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Резюме Заново исследуется метод функций Грина для явного вычисления сумм по промежуточным состояниям, которые появляются при рассмотрении многофотонных процессов. Особое внимание уделяется переходам из связаннного в связанное состояние и использованной вычислительной технике. Предложенный метод также обобщается на важный случай, в котором суммирование по промежуточным состояниям должно быть проведено по неполной системе состояний. Точность предложенного метода проверяется посредством сравнения с точными результатами для атома водорода, полученными прямым суммированием по промежуточным состояниям.

     

The opposite beams in He−Ne ring laser at 632.8 nm

Summary In the paper the resonant interaction of the opposite beams in a He−Ne ring laser at 632.8 nm is discussed on the basis of experimental results. These results show that the stable laser generation is attained when no simultaneous interaction of the opposite beams exists in the laser system. In the experiment, the mode-locking generation was obtained by applying a combining mirror, reflecting some part of the output beam into the laser cavity.

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Riassunto Nel lavoro si discute sulla base dei risultati sperimentali l’interazione risonante degli sciami opposti in un laser ad anello He−Ne a 632.8 nm. Questi risultati mostrano che la generazione laser stabile è raggiunta quando nessuna interazione simultanea di raggi opposti esiste nel sistema laser. Nell’esperimento, la generazione di modi vincolati in fase è stata ottenuta applicando uno specchio combinante, che riflette qualche parte del raggio di output nella cavità laser.

     

Avoiding the opposite beams interaction in the He−Ne linear laser at 632.8 nm

Summary In the paper the role of the opposite beams in laser generations is discussed on the basis of experimental investigations performed on the He−Ne long linear laser. It is stated that standing waves do not interact with atoms in one-photon transitions. Laser oscillations always avoid generating standing waves inside the laser medium. The best way to avoid such standing waves is laser generation in the form of one short pulse travelling to and fro inside the resonator—the self-locking generation. Also discussed is the laser generation of two or more pulses inside the resonator. The work was supported by CPBR 8.14.     

Raman processes in free-free transitions in the presence of intense laser beam

The differential cross section for the electron scattering accompanied by simultaneous absorption or emisson of one Raman photon and the inverse bremsstrahlung process in an intense laser beam is calculated.     

Intensity correlation as a means of manifesting coherent atomic excitation

Summary The excitation of atoms into a coherent linear superposition of states may not always be revealed by measurement of the spontaneous-emission intensity at a single detector. Correlation of the intensity at two detectors, however, can under specified circumstances reveal this coherent excitation. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

Effects of a spatially inhomogeneous laser field in non-linear absorption in solids

We investigate the influence of a spatially inhomogeneous laser field on the multiphoton absorption processes occurring in direct-gap semiconductors. We adopt a “non-perturbative” approach to treat the single-mode spatially homogeneous case using a parabolic band model to describe the solid. We show that corrections due to inhomogeneity may change the results in a significant way and thus have to be considered when comparing the calculated multiphoton absorption coefficients with the experimental data.     

Generalized implicit-summation technique for multiquantum processes in systems containing continua

Summary Implicit-summation technique has been so far limited to the lowest-order term of the amplitude for multiphoton ionization. In this paper, a previosly derived result in which the higher-order terms are taken into account is considered. The implicit-summation technique is adapted for the treatment of the complete amplitude, resulting in a new set of differential equations. The solution procedure for this set of differential equations is discussed.

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Riassunto La tecnica di somma implicita è stata fino ad ora limitata al termine d'ordine piú basso dell'ampiezza per la ionizzazione multifotonica. In questo lavoro si considera un risultato derivato in precedenza nel quale termini di ordine piú alto sono presi in considerazione. La tecnica di somma implicita è adattata al trattamento dell'ampiezza completa, che risulta in una nuova serie di equazioni differenziali. Si discute la procedura di soluzione per questo gruppo di equazioni differenziali.

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Резюме Техника неявного суммирования до сих пор применялась к члену низшего порядка для амплитуды многофотонной В этой статье рассматривается ранее полученный результат, в котором учитываются члены более высоких порядков. Техника неявного сммирования применяется для рассмотрения полной амплитуды, получается новая система дифференцальных уравнений. Обсуждается процедура решемия этой системы дифференциальных уравнений.

     

Asymmetric angular distributions in two-frequency multiphoton ionization of hydrogen atoms

Summary We report on calculated angular distributions of electrons produced in the ionization of hydrogen atoms by a linearly polarized soft X-ray and a circularly polarized laser light, with both the electric fields vibrating on the same plane. Their shape exhibits asymmetries which are similar to those observed in multiphoton ionization of noble gases by singlefrequency elliptically polarized light. An explanation of these asymmetries is given in terms of lowering of symmetry caused by the simultaneous action of the two fields during the ionization of the atom. The case at hand is another example which allows a useful insight into a new class of physical situations which deserve to be further explored both experimentally and theoretically.

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Riassunto In questo lavoro si calcolano le distribuzioni angolari di elettroni prodotti nella ionizzazione di atomi di idrogeno da raggi X molli e da luce laser polarizzata circolarmente, i cui campi elettrici vibrano nello stesso punto piano. La loro forma mostra asimmetrie che sono simili a quelle osservate nella ionizzazione multifotonica di gas nobili prodotta da un laser polarizzato ellitticamente. La spiegazione di questa asimmetria è data in termini di riduzione di simmetria causata dalla simultanea azione dei due campi durante la ionizzazione dell'atomo. Il presente caso è un esempio di una nuova calsse di situazioni fisiche che meritano di essere esplorate sia sperimentalmente che teoricamente.

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Резюме Приводятся вычисленные угловые распределения электронов, образованных при ионизации водорода линейно поляризованным мягким рентгеновским излучением и диркулярно поляризованным лазерным излучением, причем оба электрических поля колеблются в той же плоскости. Форма распределения обнаруживает асимметрии, которые аналогичны асимметриям, обнаруженным при многофотонной ионизации инертных газов под действием монохроматического эллиптически поляризованного света. Предлагается объяяснение этих асимметрий в терминах понижения симметрии вследствие одновременного действия двух полей во время ионизации атома. Рассматривается другой пример, который позволяет правильно понять новый класс физических ситуаций, которые заслуживают дальнейшего теоретического и эксрериментального исследований.

     

Interaction of auto-decayed states in molecule photodissociation processes

Summary The process of molecule photodissociation (PD) in the vicinity of the ionization threshold is studied with direct transitions into the dissociative continuum being accompanied by occupation of pre-dissociative Rydberg and quasi-stationary valence (non-Rydberg) configuration states interacting with each other. Interaction with valence configuration of ≈XY is shown to result in major rearrangement of series of Rydberg resonances (whose width and profile indices turn out to depend greatly on the position of valence configuration levels, their own widths and configuration interaction constant). Possibilities of interference stabilization of individual resonances are shown and conditions for formation of bound states, with continuum serving as the background, are discussed. Calculation of the NO molecule photodissociation spectrum (in the (68600⋎71200) cm⁻¹ region) is given.

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Riassunto Si studia il processo di fotodissociazione delle molecole in vicinanza della soglia di ionizzazione con transizioni dirette nel continuo dissociativo accompagnato dall'occupazione di stati di configurazione di Rydberg pre-dissociativa ed a valenza quasi stazionaria (non di Rydberg) che interagiscono tra di loro. Si mostra che l'interazione con la configurazione di valenza di ≈XY risulta in una maggiore ridisposizione di serie di risonanze di Rydberg (i cui indici di ampiezza e profilo appaiono dipendere in gran parte dalla posizione dei livelli di configurazione di valenza, le loro ampiezze e interazione di configurazione costanti). Si mostrano le possibilità di stabilizzazione d'interferenza di risonanze individuali e si discutono le condizioni per la formazione di stati legati, con il continuo che funge da sfondo. Si dà il calcolo dello spettro di fotodissociazione della molecola di NO (nella regione (68600|71200) cm⁻¹).

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Реэиме Йсследуется процсс фотодиссоциации молекул вблизи порога ионизации в спуае ирямъх переходов в диссциативнъій континуум, которъіе сопобождиіотся заполненим пре-диццоциативнъіх ридберровских и квази-стаіциохарнъих валентнъіх (не-Ридберговских) конфигурагуацоннъіх состояний, которъіе взаимодействуіот друт с друтом. Показъівается, что взаимодействие с валентной конфигурацей ≈XY приводит к перегруппировке ряда Ридбергобских резнансов (у которъх џірина и индексъи профилх сущестбенно завият ояпложения уровней валентной конфигурации, их собственнъіх ширин и конфигурационной постоянной взаимодействия. Ппказывается возможность интерференционной стабилизации индивидуальнных резонансов. Обуждаются условия для образования связанных состояний. Проводится вычисление для спектра фотодиссоциации молекулы NO (в области (68600|71200) см⁻¹).

     

Mode interaction in a spatially inhomogeneous laser

In this paper the influence of spatial matter inhomogeneities on laser action is investigated theoretically. The calculations are based on nonlinear laser theory for the amplitudes of the empty-cavity modes. The nonlinear mode interactions like mode suppression or phase locking are modified by spatial inhomogeneities. There also arises a linear coupling among the cavity modes in general leading to monochromatically oscillating states which are superpositions of the empty-cavity eigenfunctions.     

Strong-field multiphoton ionization of hydrogen. TheS-matrix treatment of the elementary process

Summary A theoretical model is presented to treat the elementary act of strong-field multiphoton ionization of atoms when the electron continuum final-state distribution is important. It is based on theS-matrix formalism and treats the final-state electron-radiation interaction in an essentially nonperturbative way. Selected numerical calculations concern the ionization of hydrogen atoms and include differential and total cross-sections of several multiphoton channels as a function of the laser intensity. Good, qualitative agreement with the experimental observations is found for values of the field intensity which are not critical with respect to the simplifications adopted in constructing the theoretical model. It applies particularly to the use of an ideal model for the laser field. Significant departure from observations is instead found when the implications of the ideal laser model play a critical role, as occurs at channel inversion and suppression. It is concluded that the theoretical treatment to be completely satisfactory needs essentially to incorporate a more realistic laser model such as a multimode one.

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Riassunto Si presenta un modello teorico per il processo elementare della ionizzazione multifotonica di atomi da parte di un forte campo laser quando diventa importante la distribuzione degli stati finali del continuo dell'elettrone. Il modello si fonda sul formalismo della matriceS e tratta l'interazione elettrone-radiazione nello stato finale in modo non perturbativo. I calcoli numerici qui riportati si riferiscono alla ionizzazione di atomi d'idrogeno e includono sezioni d'urto totali e differenziali per diversi canali multifotonici in funzione dell'intensità del laser. Si trova un accordo qualitativo con i dati sperimentali per valori dell'intensità del campo che non sono critici rispetto alle semplificazioni adottate nella costruzione del modello teorico. Un significativo distacco dai dati sperimentali si ha quando le implicazioni del modello di laser ideale giocano un ruolo critico, come si ha per l'inversione e la scomparsa dei canali. Si conclude che il trattamento teorico per essere completamente soddisfacente necessita dell'introduzione di un modello di laser piú realistico, quale, ad esempio, quello a molti modi.

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Резюме Предлагается теоретическая модель для рассмотрения элементарного акта многофотонной ионизации атомов в сильном поле, когда распределение электронных непрерывных конечных состояний является существенным. Подход базируется на формализмеS-матрицы и взаимодействие электронов в конечном состоянии с полем излучения рассматривается непертурбационным образом. Численные вычисления касаются ионизации атомов водорода и включают дифференциальные и полные поперечные сечения для некоторых многофотонных каналов, как функции интенсивности лазерного излучения. Получено хорошее качественное согласие с экспериментальными результатами для значений интенсивности поля, которые не являются критическими по отношению к упрощениям, используемым при конструировании теоретической модели. Используется идеальная модель для лазерного поля. Обнаружено существенное отклонение от экспериментальных результатов в тех случаях, когда имеет место инверсия и подавление каналов. Делается вывод, что теоретический подход становится удовлетворительным, если используется более реалистическая модель лазера, как например, многомодовая модель.

     

Laser-assisted particle-atom ionization. Analysis of triple and double differential cross-sections

Summary Based on a theoretical model worked out previously, an analysis is reported of triplet and double differential cross-section of hydrogen atom ionization by electron impact in the presence of a single-mode, homogeneous, linearly polarized, off-resonance laser field. In particular, calculations are carried out to investigatea) the dependence of the cross-sections on the field strength,b) the average number of exchanged photons,c) the limits of validity of a known sum rule. The reported results improve considerably the present knowledge of this kind of elementary stimulated atomic process. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

AbsoluteL-subshell ionization cross-sections ofZ∼50 targets by4He ion impact

Summary The X-radiation induced by⁴He ion impact on Ag, Cd, In, Sn, Sb, Te and I has been investigated for incident energies ranging from 0.6 to 0.5 MeV. Production cross-sections for various X-ray lines have been obtained by an internal normalization method, by means of simultaneous backscattering measurements. From the intensities on theL _α ,L _γ1,5 andL _γ2,3 lines theL-subshell ionization cross-sections have been extracted. Comparison with current theories shows systematic discrepancies for all the targets at incident energy below 2.0 MeV and, for Ag, Cd and In, even in the higher-energy region. These are attributed to deviations in the atomic parameters used in the data analysis as well as to a failure of the ionization theories employed.

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Riassunto Si sono effettuate misure di fluorescenza X indotta da bombardamento di ioni⁴He di energia compresa tra 0.6 e 5.0 MeV sugli elementi Ag, Cd, In, Sn, Sb, Te e I. Utilizzando un metodo di normalizzazione interna fondato su contemporanee misure di retrodiffusione Rutherford, si sono ottenute le sezioni d’urto assolute per un certo numero di righe X. Da quelle relative alle righeL _α ,L _γ1,5 eL _γ2,3 si sono poi ricavate le sezioni d’urto di ionizzazione per i diversi sottostratiL. Il confronto con le teorie disponibili ha evidenziato la presenza di sensibili disaccordi per tutti i bersagli al di sotto dell’energia incidente di 2.0 MeV e, per Ag, Cd ed In, anche per energie piú elevate, che sono stati attribuiti in parte ai parametri atomici disponibili per i calcoli delle sezioni d’urto ed in parte all’inadeguatezza delle teorie che descrivono il processo di ionizzazione.

     

The eigenmodes of a spatially inhomogeneous laser

The theory of the isothermal Hall-effect in intrinsic material is corrected taking into account arbitrary surface and bulk recombination and the departure from local electrical neutrality. The resulting formulas for the Hall voltage, the Hall field, the charge density, and the electron-hole current are valid for a weak magnetic field.     

Multiphoton transitions in a spin system driven by strong bichromatic field

EPR transient nutation spectroscopy is used to measure the effective field (Rabi frequency) for multiphoton transitions in a two-level spin system bichromatically driven by a transverse microwave (MW) field and a longitudinal radio-frequency (RF) field. The behavior of the effective field amplitude is examined in the case of a relatively strong MW field, when the derivation of the effective Hamiltonian cannot be reduced to first-order perturbation theory in ω₁/ω_rf(ω₁ is the microwave Rabi frequency, ω_rf is the RF frequency). Experimental results are consistently interpreted by taking into account the contributions of second and third order in ω₁/ω_rf evaluated by Krylov-Bogolyubov-Mitropolsky averaging. In the case of inhomogeneously broadened EPR line, the third-order correction modifies the nutation frequency, while the second-order correction gives rise to a change in the nutation amplitude due to a Bloch-Siegert shift.     

Multiphoton ionization of magnesium in a Ti-sapphire laser field

In this paper we report the theoretical results obtained for partial ionization yields and the above-threshold ionization (ATI) spectra of magnesium in a Ti: sapphire laser field (804 nm) in the range of short pulse duration (20-120 fs). Ionization yield, with linearly polarized light for a 120 fs laser pulse, is obtained as a function of the peak intensity motivated by recent experimental data [9]. For this, we have solved the time-dependent Schrödinger equation nonperturbatively on a basis of discretized states obtained with two different methods; one with the two-electron wavefunction relaxed at the boundaries, giving a quadratic discretized basis and the other with the two-electron wavefunction expanded in terms of Mg ⁺ -orbitals plus one free electron allowing the handling of multiple continua (open channels). Results, obtained with the two methods, are compared and advantages and disadvantages of the open-channel method are discussed.Received: 17 June 2003, Published online: 26 August 2003PACS: 32.80.Rm Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)G. Buic-Zloh: Permanent address: Institute for Space Sciences, P.O. Box MG-23, 76900 Bucharest-Mgurele, Romania.     

Multiphoton transitions in helium atoms in intense electromagnetic field

The T-matrix for the interaction of intense electromagnetic field with helium-like atoms is obtained using a time-dependent unitarity transformation. Essentially the transformation acts as a space transition operation.     

Binary correlation function of a random field in a spatially inhomogeneous semiconductor

A relationship is obtained expressing the binary correlation function of an intrinsic random field in a spatially inhomogeneous semiconductor in terms of the atomic pseudopotentials and a factor determined by the binary correlation functions of the atoms.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 75–78, November, 1984.In conclusion, the author is grateful to V. L. Bonch-Bruevich and M. A. Krivoglaz for a number of useful discussions.     

Excimer laser photolysis of Zn dialkyls for Zn deposition

Summary Excimer laser-induced photochemistry of dimetylzinc (DMZn) and diethylzinc (DEZn) has been investigated by multiphoton ionization time-of-flight mass spectroscopy. After the results achieved, DEZn was chosen as precursor compound for zinc film photodeposition. The organometallic was photolyzed by a focussed KrF excimer laser beam perpendicularly impinging onto a quartz substrate. Localized zinc deposition with thicknesses in the range (300⋎3000) ? has been obtained and the deposition process has been investigated by monitoring the time evolution of the fraction of a He-Ne laser beam transmitted by the growing zinc film.

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Riassunto Le reazioni fotochimiche indotte nelle molecole di dimetilzinco e di dietilzinco dalla radiazione emessa dal laser ad eccimeri sono state investigate utilizzando la tecnica della ionizzazione multifotonica abbinata a spettrometria di massa a tempo di volo. In base ai risultati ottenuti il dietilzinco è stato scelto come composto precursore per la fotodeposizione di film di zinco. La fotolisi del composto organometallico è stata indotta dal fascio focalizzato del laser a KrF incidente perpendicolarmente su un substrato di quarzo; in questo modo sono stati realizzati depositi localizzati di Zn con spessore compreso tra 300 e 3000 ?. Inoltre il processo di deposizione è stato studiato misurando l'evoluzione temporale della frazione del fascio di un laser ad He-Ne trasmessa durante la crescita del film.

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Резюме Исследуется фотохимия диметилцинка и диэтилцинка, индуцированная эксимерным лазером с помощью времени-пролетной масс-спектроскопии. Используя полученные результаты, диэтилцинк выбирается как предвестник для фотоосаждения пленки цинка. Фотолиз металлорганики осуществляется с помощью сфокусированного пучка KrF эксимерного лазера, падающего перпендикуларно на кварцевую подложку. Получастся локализированное осаждение цинка. С толщинами в области (300|3000) ?. Процесс осаждения исследуется посредством мониторирования временной эволюции доли He-Ne лазерного пучка, проходящего через растушую пленку цинка.