Ejection of high-energy photoelectrons by intense laser light

The mechanism by which a high-energy photoelectron acquires its large drift momentum is analyzed. Ejection by circularly polarized light is found to be most efficient, in accord with experimental observations.     

The determination of the molecular weight and interactions of a polyelectrolyte in aqueous salt solutions by light scattering

Summary The light scattering results for a polyelectrolyte in a binary solvent are experimentally evaluated in terms of the theory for multicomponent systems developped independently by Kirkwood and Stockmayer. Two proven experimental methods for molecular weight determination are discussed.

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Zusammenfassung Die Ergebnisse der Lichtstreuung für Polyelektrolyte in einem zweiwertigen L?sungsmittel werden experimentell mit der Theorie für Mehrkomponentensysteme ausgewertet, wie sie unabh?ngig voneinander Kirkwood und Stockmayer entwickelt haben. Zwei experimentelle Methoden für Molekulargewichtsbestimmung werden diskutiert.

     

Probing molecular symmetry effects in the ionization of N2 and O2 by intense laser fields

High-resolution electron spectroscopy is used to explore the role played by molecular symmetry in determining the morphology of the energy spectra of electrons ejected when N2 and O2 are irradiated by intense laser fields. In O2, the low-energy part of the electron spectrum is curtailed due to the destructive interference brought about by the antibonding nature of the O2 valence orbital. The high-energy tail of the spectrum is also suppressed by virtue of electron rescattering being of little consequence in O2. In contrast, in N2, which has a bonding valence orbital, the electron dynamics follow the pattern that has been established for atomic ionization in strong optical fields.     

The cluster structure of dilute aqueous-alcoholic solutions and molecular light scattering in them

The structures, equations of state, and character of fluctuations of dilute water-glycerol solutions are discussed. Two or three glycerol and about ten water molecules were found to form a fairly stable molecular complex. We call this complex elementary cluster (pseudoparticle). In a certain region of state parameters, the system could be considered a solution of pseudoparticles (clusters). Its properties were modeled by the van der Waals equation. The character of interactions between clusters was analyzed. An anomalous increase in concentration and molecular light scattering fluctuations was caused by the approach to the solution “pseudospinodal.” The experimental data were found to be in quite satisfactory agreement with theoretical estimates.     

Depolarization of light scattering by atomic and molecular solutions with strongly anisotropic translational-orientational fluctuations

Translational-orientational fluctuations are shown to raise or lower the depolarization of light scattered by solutions according to the structure of the near-order region of anisotropic molecules. In solutions of atoms, translational fluctuations induce anisotropic scattering already described in the bi-molecular radial correlations approximation; in accordance with the latest experiments, this scattering undergoes an increase or decrease depending on ternary radial correlations. A general expression for the effective optical anisotropy of the near-order region in solutions is applied to the dilute case permitting, in a novel manner, determination of both the value and sign of the anisotropy for isolated solute molecules.     

Stray light effects in Zeeman atomic absorption spectrometry

A transverse Zeeman atomic absorption spectrometer has been modeled using a polychromatic, optical calculus simulation software program. The models were found to be realistic and greatly facilitated the study of the effects of various Zeeman spectrometer parameters and their interactions. The behavior of the Zeeman spectrometer calibration curves, in the presence of three different types of stray light, was modeled. From the initial results, it seems most likely that the major stray light source in real Zeeman experiments is due to polychromaticity of the light source, even when the source profile is relatively narrow in spectral bandwidth.     

Photofragmentation of phase-transferred gold nanoparticles by intense pulsed laser light

Gold nanoparticles with an average diameter of approximately 20 nm were prepared in an aqueous solution by a wet chemistry method. The parent gold nanoparticles were then capped with a 4-aminothiophenol protecting layer and transferred into toluene by tuning the surface charge of the modified nanoparticles. Gold nanoparticles before and after phase transfer were subjected to photofragmentation by a pulsed 532 nm laser. The effects of solvent properties and surface chemistry on the photofragmentation of the gold nanoparticles have been investigated. Fast photofragmentation has been observed in the organic solvent in which the dielectric constant, heat capacity, and thermal conductivity are lower. The results suggest new approaches for the preparation of very small gold clusters from gold nanoparticles.     

Tests of models of molecular reorientation in liquids

Computer simulations of tetrahedral molecules have been used to test some common models for molecular reorietation in liquids. At high density, the Fokker—Planck and J-diffusion models and a three-term Mori approximation are all unsatisfactory; the two latter provide reasonable fits at low density.     

Dissociative ionization of methane by chirped pulses of intense laser light

Measurements have been made of optical field-induced ionization and fragmentation of methane molecules at laser intensities in the 10(16) W cm(-2) range using near transform limited pulses of 100 fs duration as well as with chirped pulses whose temporal profiles extend up to 1500 fs. Data is taken both in constant-intensity and constant-energy modes. The temporal profile of the chirped laser pulse is found to affect the morphology of the fragmentation pattern that is measured. Besides, the sign of the chirp also affects the yield of fragments like C2+, H+, and H2+ that originate from methane dications that are formed by optical field-induced double ionization.     

Abnormally high light absorption by pigments adsorbed on metals

An abnormally high extinction coefficient (almost 100 times as high as in solution) has been found for chlorophyll and pheophytin adsorbed on a smooth metal. These and some literature data suggest this phenomenon may be rather general, particularly for substances having a system of conjugated bonds.     

Tuning the light absorption of a molecular vanadium oxide system for enhanced photooxidation performance

A new mechanism for enhancing the visible light absorption of a homogeneous polyoxovanadate system is described. The photoactive pentanuclear {V(5)} isopolyoxovanadate cluster is formed in situ by a thermally-induced condensation reaction starting from a tetranuclear {V(4)} precursor. Upon irradiation with visible light, {V(5)} undergoes a light induced reduction reaction resulting in the formation of a 2-electron reduced {V(10)} cluster. Simultaneously, the oxidant methanol is selectively oxidized to formaldehyde. The {V(10)} cluster can subsequently be re-oxidized using H(2)O(2) or O(2).     

Dynamic light scattering by polyelectrolyte solutions

A generalized expression for the apparent diffusion coefficient (D_app) for macroions as a function of scattering vector (q) is developed. Mathematica®, a system of doing mathematics on a computer, was used to obtain the eigenvalues for a select set of polyion-electrolyte systems. It is shown that under the conditions of low electrolyte concentration D_app exhibits a marked q-dependence. The second part of this communication focuses on the so-called “ordinary-extraordinary” transition observed in some polyelectrolyte systems. The characteristic D_app versus electrolyte profile for this transition is compared with the “splitting” of relaxation times reported for many other polyelectrolyte systems. General problems associated with dynamic light scattering studies on macroion systems are discussed.     

双光子吸收材料的分子设计研究

介绍了双光子吸收材料分子设计原理.为了设计有大的双光子吸收响应的材料,对多种分子进行了系统的理论研究.用量子化学密度泛函理论和AM1方法进行分子几何构型优化.在优化结构的基础上,用ZINDO和自编程序求得分子的单、双光子吸收性质.设计了一些未知化合物,以期为合成新的具有大的双光子吸收截面的材料提供理论根据.以双层二聚二甲苯邻甲酸衍生物、铂乙炔化物、卟啉衍生物、C60、C70、八极矩分子为例,报道了我们在这方面的研究结果.     

Many-body effects in molecular photoionization in intense laser fields; time-dependent Hartree-Fock simulations

The time evolution of the reduced single electron density matrix for eight electrons in a one-dimensional finite box potential driven by an intense laser field is calculated by numerically integrating the time-dependent Hartree-Fock equations. We study the effects of the Coulomb interaction, field intensity, and frequency on the time profile of the ionization process. Our computed saturation ionization intensity (Isat) is in good agreement with experimental results for decatetraene [Ivanov et al. J. Chem. Phys. 117, 1575 (2002)].     

A cage effect in collision-induced light absorption from a molecular dynamics calculation

A computer experiment on a two-dimensional Lorentz gas is described for which the autocorrelation function of the induced dipole moment exhibits an extremely long positive tail. It is argued that this tail can be ascribed to the trapping of particles in irregular cages.     

Far infrared study of molecular reorientation in some alkoxyazoxybenzenes

Far infrared spectra in the frequency range 20 to 200 cm^-1 for the 4,4δ-di-n-alkoxyazoxybenzene homologous series are presented for the isotropic and nematic phases and for 0.1 M solution in benzene. In addition some partially deuteriated samples were studied. The spectra consist generally of two broad bands. By comparison of the spectra for different deuteriated compounds it is possible to interpret the higher frequency band as being connected with the intramolecular torsional vibration of the end groups. The band at lower frequencies we associate with the Poley absorption of librating molecules. The confined rotator model calculations performed for this band, for all compounds studied, give the rotational correlation time which is in good agreement with the correlation times as derived from quasi-elastic neutron scattering experiments.