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Semiconductor quantum dots (QDs) have been gaining much attention because of their outstanding properties for multiple-photon microscopy applications. By solving nonperturbatively the time-dependen ...

Temporal oscillations of amplified spontaneous emission of molecules are studied theoretically. From the proposed theory and numerical simulations, it is found that the self-pulsations originate in an interplay between stimulated emission and saturable absorption. A stability analysis demonstrates the crucial role of the photoabsorption in this process, which can be regulated by a proper choice of buffer molecules. Variations in the saturable absorption mediate a transition from damped oscillations to self-sustained pulsations. The role of propagation effects as well as of the interaction of co- and counter-propagating pulses is also investigated. Numerical simulations, demonstrating the theoretical findings, are performed for a model 3-level system and for an organic chromophore; 4-[N-(2-hydroxyethyl)-N-(methyl)amino phenyl]-4'-(6-hydroxyhexyl sulphonyl) stilbene.

We investigate the transmission of short pulses through one-dimensional impurity band based photonic crystals. We found a strong dependence of the delay time on the angle of incidence. The delay time is larger for larger incident angles for the transverse electrical mode, while the delay time of the transverse magnetic mode has a qualitatively different angular dependence, especially in the region below the Brewster angle. The strong anisotropy of the delay time is traced to the anisotropy of the group velocity which is directly related to the angular dependence of the impurity band structure.

One-dimensional photonic crystals with continuous distribution of the dielectric constant were fabricated by the use of photopolymer materials and laser holography. The angular dependence of light propagation through the system was studied experimentally and theoretically. It is shown that the Brewster angles for different bands are different, in contrast to the conventional two-layer Bragg reflector with a step-like distribution of the dielectric constant. Comparison of the theory with experimental data allowed us to define the parameters of the hologram—the dielectric contrast and the shrinkage of the structure.

We develop a strict theory of nonlinear propagation of few interacting strong light beams. The key idea of our approach is a self-consistent solution of the nonlinear wave equation and the density matrix equations of the material beyond the rotatory wave approximation. We assume a Fourier expansion of the density matrix which goes beyond the conventional Taylor expansions of the polarization over the field amplitudes which is inadequate for the field strengths that we are interested in. Two qualitatively different situations are considered, with and without phase matching. Unlike in our previous paper (Baev et al 2003 J. Opt. Soc. Am. B at press) devoted to the three-photon (TP) absorption induced upconverted lasing, we obtain here a strict solution for the nonlinear interaction between different light beams. The general theory is applied to a numerical study of the role of saturation in TP photoabsorption by an organic chromophore in solution.

Near-edge X-ray absorption fine structure spectra have been calculated for different carbon-nitrogen molecules, clusters and solids. The compounds investigated are used to model the chemical bondin ...

We report the observation of the unusually weak decay of the N 1s --> pi* core-excited N-2 molecule to the (B) over tilde (2)Sigma(u)(+) final state of N-2(+), which is only detectable in an exp ...

The de-excitation of higher core-excited Rydberg states of carbon monoxide has been studied. It is found that already with a main quantum number of 4 or higher, Rydberg states of p-symmetry can be successfully treated within the single-particle hydrogenic model for both the core-excited and the final states. From a study of the binding energy of the Rydberg electron in the final state versus the core-excited state, the importance of relaxation in the core-excited intermediate state is shown. Also, for core-excited Rydberg states with main quantum number of 5 or higher, an almost total quenching of strict spectator lines in the de-excitation spectra is observed.

The possibility of observing natural circular dichroism in non-resonant x-ray emission spectroscopy is investigated by means of simulations of the chiral molecules twisted ethylene, propylene oxide and trans-1, 2-dimethylcyclopropane, in a two-step model and at the SCF level, with or without relaxation of the core-hole states. We observe both a chemical and an element dependence of the phenomenon and also an effect of electron relaxation. However, the latter is much less crucial than for circular dichroism in x-ray absorption. The calculations indicate that, at least for the decay of the carbon core-hole states, the effect could be detectable with the present or soon to be available experimental equipment.

We have performed quantum chemical calculations of carbon and oxygen x-ray absorption spectra for a series of carbonyl containing molecules. Systematic variations in energy positions and intensities of the spectra have been investigated. Based on the good reproduction of experimental spectra, quantum defect and population analyses were utilized for detailed assignments. In some cases new assignments are proposed.

We derive symmetry selection rules for polarized resonance inelastic X-ray scattering (RIXS) in randomly oriented molecules belonging to any of the 32 crystallographic point groups or to the two groups of linear molecules. Compact formulae are derived for cross sections of RIXS processes in randomly oriented molecules for photons of arbitrary polarization; linear, circular or elliptical polarization. It is shown that a specification of polarization ratios of RIXS cross sections yields sufficient information to permit an unequivocal identification of the symmetry species of the occupied and unoccupied molecular orbitals (MOS). The given tabulation of RIXS symmetry selection rules can be used as a quick guide to assign symmetry properties of molecules of experimental interest.

The authors present the carbon and fluorine X-ray emission spectra of methane and the fluoromethanes (CH4-xFx, x=0, 1, 2, 3 and 4), measured in high resolution. Relative X-ray emission intensities in dipole velocity and length gauges have been derived from the independent particle approximation using Hartree-Fock ground state molecular orbitals. The results of the calculations are compared with the experimental spectra. In the experimental carbon (K-V) emission spectrum of CF4 the authors observe dipole-forbidden electronic transitions. They suggest that these transitions arise due to vibronic coupling, i.e. intensity is borrowed from the dipole-allowed transitions.

A high-resolution X-ray emission spectrum of molecular oxygen is presented. Comparisons with ab initio calculations are made regarding relative intensities, energies, and band profiles for the outer valence, 2 Pi g, 4 Pi u, 2 Pi u, 4 Sigma -g, 2 Sigma -g, levels. The importance of lifetime-vibrational interference is discussed.

The core hole photoelectron spectra of ethene, 1,3-butadiene and 1,3,5-hexatriene are studied as model molecules, i.e. monomer, dimer and trimer subunits of polyacetylene. The C 1s photoelectron spectra show partially resolved lines for both 1,3-butadiene and the 1,3,5-hexatriene. The chemical shifts between the terminal and nonterminal carbons have been experimentally determined via assignments of the gas phase spectra supported by core-hole-relaxed ΔSCF ab initio calculations. By plotting the measured core-hole binding energies against a connectivity number the binding energies relative to the vacuum level of different carbon atoms in the semi-infinite one-dimensional quasi-crystal can be extrapolated. The experimental C 1s chemical shifts determined in this way show an oscillatory behaviour for the carbon atoms near the boundary of the quasi-infinite chain.

The shake-up Auger satellite spectrum associated with the N 1s Auger spectrum of N2 is recorded using monochromatized X-ray excitation. As many as 11 structures are observed in the spectrum. Assignments in terms of participator and spectator processes are discussed based on data from normal Auger and shake-up spectra and from a simple one-particle model. The interpretation is assisted also by a new N 1s photoelectron shake-up spectrum showing several hitherto unobserved details. A new X-ray excited normal Auger electron spectrum of N2 is also presented.

Model calculations are carried out to explore the origin of shifts of Auger spectra of liquid water and ice. The models employed are the dielectric, the reaction field, the supermolecular, the point charge field and the statistical models. The dependence of the Auger Shift on the model parameters and on the choice of different liquid water and ice structures is explored.

We report the mechanical action of infrared light on atoms and molecules based on the rectification of the electric force. This mechanism is qualitatively different from the conventional ways of controlling photochemistry. The rectification of the electric force originates from the synchronous charge transfer induced by the laser field. This brings about an opportunity to produce a site selective light-induced action, controlled by the tailored intense laser field, on atoms in molecules and clusters. The concept is illustrated by ab initio molecular dynamics simulations of the water hexamer.

Colloidal quantum dots (QDs) have been widely studied for nanophotonics and bioimaging applications for which the lifetime of their fluorescence is of critical importance. We report experimental and theoretical characterizations of dynamic optical properties of multi-shell–coated CdSe/CdS/ZnS QDs. Quantum-mechanical studies of fundamental optical excitations and Monte Carlo simulations of energy relaxation mechanisms indicate that the excitonic states are densely compacted in the QDs and are easily photoexcited by the laser pulse in the presence of nonradiative electron-phonon interactions. For spherical QDs, the decay time of spontaneous radiative emission of individual photoexcited excitonic states with zero angular momenta is found to be only tens of picoseconds. In our multi-shell QDs, high-energy excitonic states of nonzero angular momenta have to go through a number of nonradiative electron-phonon interaction steps in order to relax to zero–angular-momentum excitonic states for radiative emission, resulting in an effective fluorescence peak at about 2 ns in the photoncount-time relationship. This explains the measured long average fluorescence lifetime of 3.6 ns. Such a long lifetime facilitates the applications of colloidal QDs in areas such as QD-based solar cells, bioimaging and metamaterials.

Radiative electron rearrangement (RER) and hole-mixing effects in molecular ultra-soft X-ray spectra (USX) are theoretically analyzed at several levels of approximation and compared with X-ray photoelectron (XPS) transitions which result in the same residual ionic states. Numerical applications are carried out for the CO molecule with configuration interaction wavefunctions built on orbitals separately optimized for initial and final states. The USX spectra are more complex than the XPS spectrum in the energy ranges corresponding to RER and break-down of the MO-model due to interference from excited components of the initial state wavefunctions and due to non-orthogonality. This breaks the branching ratio rule that predicts equal satellite to parent ionic intensities for the XPS and the different core-sited USX spectra and does accordingly also limit the applicability of local selection rules for the hole-mixing in the USX spectrum.

An efficient method for calculating intensities of molecular radiative transitions in the vacuum UV and soft X-ray wavelength regions using MCSCF wave functions is presented. The initial and final states are expanded within large configurational spaces and separately optimized with sets of molecular orbitals that are mutually non-orthogonal. An optimal construction of the transition density matrix is achieved through a sorting algorithm of the symbolic configuration elements in conjunction with the biorthogonalization technique of Malmqvist [1] for transforming the molecular orbitals. The performance of the method is demonstrated for the soft X-ray emission spectra of CO and N2.

Energies and intensities of transitions in the discrete and near continuum parts of the K-shell EELS spectra are evaluated by means of ab initio methods. The fine structure superponing the carbon continuum spectrum is interpreted as due to an analogous shake effect associated with the strong 2σ-2π* discrete resonance.

A 10 m grazing incidence spectrometer for studies of soft X-ray emission from gases is described. The spectra are excited by a microprocessor controlled high power electron beam with electron optical arrangements for achieving differential pumping of sample gas. A grating at grazing incidence is used as the dispersive element. Adjustment of the analyzer is reduced to one degree of freedom (radius of curvature of grating) by using precision machined optical elements. The wavelength resolution of the analyzer is better than 10 m? without further adjustments. A recording of the 1st order L2, 3 emission spectrum of argon gas is presented.

The X-ray emission of NH3 and N2O are reported and the main X-ray transitions are identified. A correlation between nitrogen 1s binding energies obtained by electron and X-ray emission spectroscopy is given and differences between measured X-ray energies and X-ray energies calculated from electron spectroscopy are discussed.

Recent developments of molecular X-ray emission spectroscopy and its interrelation with electron spectroscopy are discussed. Deep-lying level energies obtained by X-ray emission and ESCA are compared. Valence electron structure of small molecules is studied as well as multiply excited inner vacancy states of free atoms.

The vibrational envelopes of the core ESCA bands of H2O and NH3 and of the 3a1 X-ray emission band of NH3 have been investigated by means of Franck-Condon analysis. The data for the equilibrium geometry and the force constants of the neutral ground, N1s and 3a1 ionized states of NH3 have been computed from ab initio wave functions, whereas the data for the neutral ground and the O1s state of H2O have been taken from literature [1]. The N1s state of NH3 is found to be planar with a significantly shortened N-H bond length in comparison with the neutral ground state. The ionization of the O1s state of H2O leads to an increase of the HOH angle by ~ 16° with negligible change in the O-H bond distance. The vibrational intensities and energies computed from the ab initio results explain the band profiles of the spectra.

Using multiconfiguration linear response and Stieltjes imaging moment theory, accurate predictions are given for the photodetachment cross sections of Li-.

The carbon K emission spectrum of gaseous CO2 has been recorded in high resolution using the grazing incidence technique. Vibrational fine structure corresponding to vibrational excitations in the 2 Pi u and 2 Sigma u+ final states is clearly resolved in the spectrum. From a Franck-Condon fit of harmonic oscillators the natural lifetime width of the C 1s state has been determined to be 0.07+or-0.02 eV.

A high-resolution K emission spectrum of O2 is presented. Four bands are interpreted in a molecular-orbital description. Five satellite lines are found in the spectrum.

A high resolution Ultra Soft X-ray (USX) emission spectrum of gaseous benzene has been recorded. The spectrum is interpreted by means of ab initio one-center intensities and electron spectroscopic data. A comparison is made with USX spectra of solid benzene showing a uniform shift towards higher photon energies for the gas phase spectrum. The solid and gas phase USX spectra show quite different satellite structure which is interpreted in terms of photon and electron excitation of spectra.

The lifetimes of L and LM vacancy states in argon have been determined from the natural widths of L X-ray emission lines in a high resolution experiment.