Tunable synchrotron sources

The absorption of the Mj y and N,v y transitions are located in the soft X-ray range ( 800-1700 eV) and in the ultraviolet ( —98-205 eV), respectively. In the early period of X-ray spectroscopy, such absorption measurements, in particular those in the ultraviolet, were hampered by the low intensities emitted from conventional X-ray sources, by the difficulty of preparing thin (— 100 A) uncontaminated samples of defined thickness, and by the involved detection systems. Meanwhile most of these experimental problems have been overcome through the availability of intense and tunable synchrotron sources, the concomitant improvements of the detection systems (e.g., total electron yield detectors) and through developments of appropriate ultra high vacuum preparation techniques. Nevertheless, because of possible intrinsic surface effects, the question of whether one measures bulk properties remains a drawb k here. 

Instrumental methods have become more sophisticated to face these challenges. In particular, Westmoreland and Cool have developed a flame-sampling mass spectrometer that has provided several revelations in terms of relevant molecular intermediates in combustion. " Their setup couples a laminar flat-flame burner to a mass spectrometer. This burner can be moved along the axis of the molecular beam to obtain spatial and temporal profiles of common flame intermediates. By using a highly tunable synchrotron radiation source, isomeric information on selected mass peaks can be obtained. This experiment represents a huge step forward in the utility of MS in combustion studies lack of isomer characterization had previously prevented a full accounting of the reaction species and pathways. 

The MAD method is performed on a single crystal, but it requires access to tunable radiation (synchrotron source). Moreover, selenomethionine-labeled protein must be produced, purified and crystallized. This is more easily done for proteins which can be expressed in E. coli. 

Refinements in vuv spectroscopy W, aided by the development of synchrotron radiation (7 ) and equivalent-photon electron-impact ( ) tunable light sources, and closely related advances in photoelectron, fluorescence-yield, and electron-ion coincidence spectroscopy measurements of partial cross sections (9), have provided the complete spectral distributions of dipole intensities in many stable diatomic and polyatomic compounds. Of particular importance is the experimental separation of total absorption and ionization cross sections into underlying individual channel contributions over very broad ranges of incident photon energies. 

The study of ion-molecule reactions using state-selected reagents has become a very exciting area of molecular dynamics. We have developed an experimental apparatus in Orsay which utilizes the properties of our tunable synchrotron radiation source at LURE to prepare ions in selected vibronic levels and then to study their reactions. The ions are state-selected using the TPEPICO (threshold-photoelectron/photoion coincidence) method [1]. 

Anomalous dispersion effects have been used for a considerable time, to some extent, in crystal structure analyses. In combination with a tunable synchrotron X-ray source there is considerable scope for a dramatically increased use and benefit of these small but significant effects. 

The XSW technique is not exclusively a synchrotron-based technique it can be performed with use of a conventional fixed-tube X-ray source or rotating anode. However, several practical considerations make it far more advantageous, and often essential, to perform these experiments at a synchrotron source. Some of these considerations are generally true for any X-ray experiment, while others are specific to the XSW techniques. These considerations are often of great practical importance in carrying out XSW experiments. We briefly discuss some of these factors, which include X-ray source brightness, tunability, and polarization. 

Another fruit of modern technology is the advent of synchrotron radiation from electron storage rings This produces a pulsed, polarized tunable continuum source extending into the X-ray region, having a typical pulse duration 0.4 ns and a repetit ion (1—10) MHz, and is an ideal source for time-resolved spectroscopy in the vacuum ultraviolet. The expected intensity of the system at Daresbury, for example,... 

The tunable photon energies available at a synchrotron source can be used to significantly enhance the elemental contrast available in PEEM by choosing a photon energy that corresponds to an absorption edge of a particular element of interest, thus greatly increasing the number of photoelectrons emitted from those areas that specifically contain that element. 

The nearest that one can get in practice is to use monochromatic ultraviolet light from a synchrotron source. Such sources have the advantage of being tunable—so that the wavelength of the radiation can be chosen to be the optimum for the measurement—but the disadvantage that they are only available at a relatively small number of national or international facilities. 

Useful X-ray photon beams are provided by classical sealed X-ray tubes, rotating anode generators and also high energy electron storage rings, named synchrotron sources. Synchrotron sources are in many cases preferred because they provide powerfiil, continuously tunable and well collimated X-ray beams that are particularly adequate for SAXS experiments. 

---