A probe method for studying dibromocarbene by time resolved infrared spectroscopy

Dibromocarbene reacts with tertiary-butylisocyanide to form a ketenimine. The absolute rate constant of the reaction (k_TBI = 2.3 × 10⁹ M⁻¹ s⁻¹) was determined by laser flash photolysis techniques with UV-vis detection of the dibromocarbene-pyridine ylide. The ketenimine was detected by TRIR spectroscopy at 2040 cm⁻¹. Isocyanide trapping of carbenes to form ketenimines is proposed as a general method of studying IR silent carbenes by TRIR spectroscopy.     

Influencing parameters in time resolved picosecond spectroscopy and a new light gate method

We show that the open period of a CS₂ light gate, triggered by a picosecond optical pulse, depends on the optical pathlength through the CS₂. An alternative to the light gate is suggested for time resolved spectroscopy. The working principle is based on scattering induced in CS₂. We have found that the broadband emission from H₂O or D₂O excited by a 5300 Å picosecond pulse is also of picosecond duration, but the blue part is delayed with respect to the red. From this delay time the lifetime of the ground state vibrational levels may be measured directly.     

Voxel-based Monte Carlo simulation of X-ray imaging and spectroscopy experiments

A Monte Carlo code for the simulation of X-ray imaging and spectroscopy experiments in heterogeneous samples is presented. The energy spectrum, polarization and profile of the incident beam can be defined so that X-ray tube systems as well as synchrotron sources can be simulated. The sample is modeled as a 3D regular grid. The chemical composition and density is given at each point of the grid. Photoelectric absorption, fluorescent emission, elastic and inelastic scattering are included in the simulation. The core of the simulation is a fast routine for the calculation of the path lengths of the photon trajectory intersections with the grid voxels. The voxel representation is particularly useful for samples that cannot be well described by a small set of polyhedra. This is the case of most naturally occurring samples. In such cases, voxel-based simulations are much less expensive in terms of computational cost than simulations on a polygonal representation. The efficient scheme used for calculating the path lengths in the voxels and the use of variance reduction techniques make the code suitable for the detailed simulation of complex experiments on generic samples in a relatively short time. Examples of applications to X-ray imaging and spectroscopy experiments are discussed.     

Simulating systematic errors in X-ray absorption spectroscopy experiments: Sample and beam effects

The article presents an analytical model to simulate experimental imperfections in the realization of an X-ray absorption spectroscopy experiment, performed in transmission or fluorescence mode. Distinction is made between sources of systematic errors on a time-scale basis, to select the more appropriate model for their handling. For short time-scale, statistical models are the most suited. For large time-scale, the model is developed for sample and beam imperfections: mainly sample inhomogeneity, sample self-absorption, beam achromaticity. The ability of this model to reproduce the effects of these imperfections is exemplified, and the model is validated on real samples. Various potential application fields of the model are then presented.     

Design, simulation and application of a new micromixing device for time resolved infrared spectroscopy of chemical reactions in solution

We present a novel micromachined fast diffusion based mixing unit for the study of rapid chemical reactions in solution with stopped-flow time resolved Fourier transform infrared spectroscopy (TR-FTIR). The presented approach is based on a chip for achieving lamination of two liquid sheets of 10 microm thickness and approximately 1 mm width on top of each other and operation in the stopped-flow mode. The microstructure is made on infrared transmitting calcium fluoride discs and built up with two epoxy negative photoresist layers and one silver layer in between. Due to the highly laminar flow conditions and the short residence time in the mixer there is hardly any mixing when the two liquid streamlines pass through the mixing unit, which allows one to record a mid-IR transmission spectrum of the analytes prior to reaction. When the flow is stopped, the reactant streams are arrested in the flow-cell and rapidly mixed by diffusion due to the reduced interstream distances and the reaction can be directly followed with hardly any dead time. On the basis of two model reactions-neutralisation of acetic acid with sodium hydroxide as well as saponification of methyl monochloroacetate-the performance of the mixing device was tested revealing proper functioning of the device with a time for complete mixing of less than 100 ms. The experimental results were supported by numerical simulations using computational fluid dynamics (CFD), which allowed a reliable, quantitative analysis of concentration, pressure and flow profiles in the course of the mixing process.     

Monte Carlo studies of a novel X-ray tube anode design

When energetic electrons are incident on high atomic number absorbers, a substantial fraction is back-scattered. This phenomenon is responsible for several undesirable effects in X-ray tubes, in particular a reduction in the X-ray output. The extent of this shortfall has been estimated by using Monte Carlo simulation to start electrons at increasing depth inside the anode, the results indicating that an output enhancement of nearly 50% could be achieved in principle if the electrons wasted in back-scatter events could be trapped inside a tungsten anode. To test this idea a further set of simulations were done for a novel anode geometry. Results showed that X-ray tube efficiencies might be substantially enhanced by this approach.     

Ultralow-energy excitations and prospects for spatially resolved spectroscopy.

The key contribution of electron microscopy methods to condensed matter spectroscopy is undoubtedly spatial resolution. So far this has mainly been manifest through electron energy loss spectroscopy in the 1-eV to 10-keV energy range and has not seriously challenged the dominance of optical, X-ray, and neutron spectroscopy methods over most of the vast field at lower energies. At frequencies up to a few megahertz, corresponding to energies of a few nanoelectron volts and below, direct excitation by pulsed electron beams or electric fields has proved effective. Prospects are discussed for extending spatially resolved spectroscopy to the intermediate energy region, mainly by combining the advantages of electrons with those of photons.     

Infrared spectra for a multi discipline spectroscopy system

Summary The infrared spectra quality requirements of the Spektrendatenbanken-Verbundsystem (SDVS) are reported with special reference to Expert System needs. Some of the potential benefits of the system are given. A Call for Spectra is made and the guidelines for submitting spectra to the database outlined.

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Infrarotspektren für ein multidisziplinäres Spektroskopie-System

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Dedicated to Prof. Dr. G. Tölg on the occasion of his 60th birthday     

Confocal microscopic X-ray fluorescence at the HASYLAB microfocus beamline: characteristics and possibilities

The possibilities of performing non-destructive elemental analysis in three dimensions on a variety of heterogeneous materials by means of an innovative variation of the microscopic X-ray fluorescence analysis (μ-XRF) method are described. Next to employing focusing optics for concentration of the primary beam of X-rays, a second optical element between the sample and the energy-dispersive detector is used in confocal μ-XRF. Thus, only X-ray fluorescence signals from a cube-like volume (within certain limits imposed by the absorption of the radiation in the sample) can be arbitrarily positioned within the sample. The distribution of major, minor and trace elements (down to the sub-ppm concentration level in some matrices) along lines and planes within the sample can be visualized with a spatial resolution of the order of 15–40 μm. The lowest detectable amounts in confocal mode using pink-beam excitation are situated at the sub-femtogram level.     

FOCAL: X-ray optics for accurate spectroscopy

A crystal spectrometer has been constructed in the Focusing Compensated Asymmetric Laue geometry covering the energy range between 30 and 120 keV. We summarize the crystal optics and show the usefulness of the instrument for spectroscopy of stationary and fast moving X-ray sources. Results are reported from several tests employing a ¹⁶⁹Yb gamma-ray source and the Lyman radiation of one-electron Au⁷⁸⁺ ions travelling at a velocity corresponding to β=v/c_o≈0.44.     

A new pulsed light source for lifetime studies and time resolved spectroscopy: the synchrotron radiation from an electron storage ring

Synchrotron radiation from the Orsay Storage Ring (ACO) has been used as a repetitive source for lifetime and time resolved fluorescence studies. Our measurements and results concern: (1) Lifetime of standard compounds (quinine sulfate and fluorescein anion); (2) single vibronic level fluorescence (aniline and pyrazaine in the vapor phase; pyrazine lifetime has been measured for the first time to be 0.5 ± 0.2 ns); and (3) reactivity of excited molecules (decay time and time resolved spectra of 2-naphthol and fluorescein both undergoing fast protolytic reactions in the excited state). Storage ring synchrotron radiation characteristics and performances are discussed and compared to those of conventional “nanosecond” flash lamps. Possible new applications are foreseen.     

Structural evolution during evaporation of a 3-glycidoxypropyltrimethoxysilane film studied in situ by time resolved infrared spectroscopy

Time resolved infrared spectroscopy has been applied to study in situ the evaporation process of a 3-glycidoxypropyltrimethoxysilane hybrid sol by casting a droplet on a ZnSe substrate; the analysis has been performed in the middle-infrared range and in the near-infrared range. The experiment has allowed following the structural changes induced by water evaporation and the formation of ordered structures within the cast film; the CH(2) scissoring bands have been used as a fingerprint for the disorder to order transition of the hybrid. The experiment has been done using both a fresh sol and an aged sol which produce respectively an amorphous material and a crystalline hybrid material. The analysis has shown that the epoxy groups do not react during the evaporation while the silica structure shows only a slight condensation and an increase in open cage-like species. At the end of evaporation the hybrid has a "soft-like" state which allows structural rearrangements to self-order.     

Experimental characterization of metallic titanium-laser induced plasma by time and space resolved optical emission spectroscopy

Time and space resolved optical emission spectroscopy has been successfully employed to investigate the evolution of the plasma produced by the interaction of UV laser beam with a metallic target of titanium at two different pressures (10⁻⁵ and 3.4×10⁻² torr) and at distances up to 3 mm from the target. By time of flight measurements and Boltzmann plots both the dynamic and the kinetic aspects have been discussed. The quasi-equilibrium state of the laser-induced plasma has been established on the basis of the failure of Saha balance equation. The effect of three-body recombination on atomic titanium temporal distribution has been explained. Temporal evolution of electron number density, as determined by Stark effect, has been used for the estimation of the three-body recombination rate constant.     

X-ray absorption and X-ray photoelectron spectroscopy of a rhodium colloid

Transition metal colloids are potential precursors of heterogeneous catalysts with application to selective chemical reactions. Sample preparation techniques are described. Experimental details are given of the characterization of these often air-sensitive particles by X-ray photoelectron and X-ray absorption spectroscopy. First results obtained with both techniques for a Rh-colloid show that the metal is mainly present in the zerovalent chemical state. But the spectra indicate further chemical states of Rh which can be assigned to the outermost metal atoms of the colloid interacting with organic ligands or to the educt Rh-halides.     

X-ray absorption and X-ray photoelectron spectroscopy of a rhodium colloid

Transition metal colloids are potential precursors of heterogeneous catalysts with application to selective chemical reactions. Sample preparation techniques are described. Experimental details are given of the characterization of these often air-sensitive particles by X-ray photoelectron and X-ray absorption spectroscopy. First results obtained with both techniques for a Rh-colloid show that the metal is mainly present in the zerovalent chemical state. But the spectra indicate further chemical states of Rh which can be assigned to the outermost metal atoms of the colloid interacting with organic ligands or to the educt Rh-halides.     

Picosecond time resolved photoluminescence spectroscopy of a tetracene film on highly oriented pyrolytic graphite: dynamical relaxation, trap emission, and superradiance

A detailed time resolved investigation of the photoluminescence of a thin tetracene film deposited on highly oriented pyrolytic graphite is presented. In agreement with Lim et al. [Phys. Rev. Lett. 92, 107402 (2004)], we find strong evidence for superradiance: an increase of the relative intensity of the pure electronic transition with respect to the vibronic sideband and a concomitant decrease of the radiative lifetime from 10 to 1.83 ns upon cooling from 300 to 4 K. For lower temperatures, a redshift (approximately 200 cm(-1)) of the free exciton is observed. Previously, this shift was attributed to a structural phase transition. Our time resolved spectra reveal that the spectral shift is related to a dynamical relaxation process which occurs within the first 50 ps.