Two-color cross-correlation of fs-laser pulses by two-photon induced photoconductivity for near and far field optical measurements

Two-color two-photon induced photoconductivity in a GaAsP diffusion type photodiode is demonstrated by measuring femtosecond cross-correlation functions for widely separated wavelength pairs of 775 and 1300 nm. Results are obtained for a range of tunable wavelengths and average powers of the incident lasers by measuring the two-photon induced photocurrent as a function of the optical delay between the pulses. The temporal autocorrelation of femtosecond laser pulses in the near-field of a small diameter aluminum coated optical fiber tip is also obtained with the same photodiode method for single colors.     

Synthesis and X-ray structure of tetrakis[μ-(3,4-dimethoxy-α-β-dihydrocinnamato)] dimethanoldicopper(II)

The synthesis, magnetic, and structural properties of the title compound are reported. The crystals are triclinic, space groupP¯1, witha=15.084(2),b=11.820(1),c=7.5253(7) Å, =71.50(1),=77.61(1), =68.68(1)° andZ=1; the structure was solved by Patterson and Fourier methods and refined toR 0.0351 for 4147 observed reflections. The content of the unit cell consists of a centrosymmetrical copper acetate-like molecule in which each copper can be described as square-pyramidally surrounded by four carboxylic oxygens in the basal plane [1.955(2)–1.970(2) Å], one methanolic oxygen in the apical site [2.206(2) Å] and a CuCu interaction of 2.612(5) Å at a sixth position of a distorted octahedron. The dimers are packed by means of hydrogen bonds formed between the methanolic OH group and the carboxylic O(2) atom of one adjacent complex and by normal Van der Waals contacts. The subnormal magnetic moment, characteristic of exchange interactions between the two copper(II) ions, and the energy of the d-d band observed are consistent with the reported dinuclear copper(II) acetate-like structure.     

SOME PRELIMINARY INFORMATION ON SYNDIOTACTIC POLYSTYRENE CATALYSTS

Syndiotactic specific polymerization of styrene has been investigated by ~(13)C NMR analysis and isotopic laelling methods. The value of the activation energy involved in the steric control has been determined. Some information of the number of the active sites and on the life of the catalysts is reported.     

Transitions from coherent to incoherent state induced by anharmonic interaction

A non-linear stochastic oscillator driven by a coherent radiation field is studied. Analog simulation is shown to support the arguments of theory on the specific role of the fluctuation-dissipation process and/or anharmonic interaction in making valid a treatment in terms of a Boltzmann-like distribution of energy. The results of analog simulation show that at certain values of the excitation frequency a superimposition of a coherent and an incoherent state may appear thereby opening the way to a more exhaustive theoretical treatment of this problem.     

Spectral heterogeneity of oxygen-deficient centers in Ge-doped silica

We report an experimental investigation of the emission spectra of a 1000 mol ppm sol–gel Ge-doped silica by fine tuning the excitation energy in the ultraviolet (UV) range, around 5 eV, and in the vacuum-UV range, around 7.3 eV, at room temperature and at 10 K. The sample is characterized by a blue (centered at 3.2 eV) and an UV (centered at 4.3 eV) bands. We have found that the ratio between the area of the blue and the UV bands depends on the temperature and on the excitation energy in both the vacuum-UV and the UV range. At both temperatures the spectral features of the blue and the UV bands are weakly affected when the excitation is varied in the vacuum-UV. At variance, under UV excitation the peaks of the bands are shifted and also their widths are changed. These results are interpreted in terms of distinct excitation channels of the luminescence that are influenced in a different way by the structural inhomogeneity of point defects.     

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.