Beam time

The price per tonne, p, of materials is the first of the properties that we talked about in this book. The total price of the beam, crudely, is the weight of the beam times p... 

A XAS experiment involves the irradiation of a sample with a tuneable source of monochromatic X-rays, usually from a synchrotron facility (high brilliance). Third-generation synchrotrons have sufficient intensity to observe XAFS spectra up to 100 keV. Nevertheless, laboratory-scale XAFS spectroscopy is of importance, despite the vast availability of synchrotron beam time [305]. 

It is possible to measure nearly any type of sample for almost any element with little or no preparation. Only a few mg of sample is required, and the measurements are non-destructive in that the sample is generally undamaged. Measurements take only 1-20 min of beam time. Elemental mapping showing the variations in elemental concentrations can be measured over the surface of a sample using the ion microprobe for an area as large as 5 x 5 mm. 

The ordinary user who carries out scattering experiments at a synchrotron beamline will rarely adjust the optics without help. Nevertheless, during the beam time one should be able to assess the quality of the adjustment. Thus the user will most probably have to readjust some slits or to adjust the flux according to the requirements of the experiment. 

Check the output in the monitor file and the functionality of the counters17 several times during the beam time. 

At the beginning of a beam time the users should write down which monitor channel is connected to which signal. 

We are grateful to HASYLAB for offering beam time on the synchrotron radiation facility of DESY and for the access to the EXAFS spectrometer. We also thank J.W. 0rnbo for help in the preparation of the samples, N.-Y. Topsjzfe for helpful discussions, and 0. Sorensen for carrying out the electron microscopy investigations. 

EDXRD is a very powerful technique, although limitations include the requirement for synchrotron radiation. This Hmits the number of experiments that can be performed, due to the high cost and low availabihty of synchrotron beam time. Because of the large voliune of the reaction vessel and the geometry of the instrument, the peak resolution of the energy dispersive data is also rather poor (AE/E). This means that although it is possible to accurately monitor the course of a reaction, using the data for ab initio structure solution or structure refinement is precluded. 

Synchrotron beam time remains a scarce resource world-wide, however wiA over 30 synchrotron raAation centers eiAer operational or under construction, most prospective users can obtain access to Ae facilities (11). In most cases, beam time is available free of charge for non-proprietary research. Access to a facAty is often by means of a peer reviewed proposal. Adequate equipment for sample handling and data collection is available at most synchrotron raAation laboratories. Many laboratories also provide software for data analysis. 

For a number of years now we have been involved in crystallization of bacterial ribosomal particles. From the very beginning of our studies, the crucial need for a stable, very intense, and perfectly focussed synchrotron beam was evident, even for preliminary and basic information (e.g. whether crystals diffract at all). Thus our studies have always been dependent on the availability of synchrotron beam time and hampered by only partial (and very occasional) feedback to assess our experimental procedures for growing bacteria, preparing the ribosomes and obtaining crystals. 

Hundreds of crystals were exposed in several synchrotron beam time periods, bearing in mind that there was little chance to solve the inherent problem of how to produce derivative crystals. It was a relief though, that 508 particles from a mutant of B. stearothermophilus lacking protein LI 1 could be crystallized isomorphously which might serve as a low resolution deficiency derivative. 

B. Single-Beam Time-Resolved CARS Microspectroscopy.184... 

Finely divided 2.0- to 5.0-nanometer Au particles were 69 prepared in an ionized-cluster-beam time-of-flight mass spectrometer system... 

T. B. Zunic is gratefully acknowledged for helpful discussions during preparation of the manuscript. The authors are also grateful to HASYLAB, Deutsches Elektronen-Synchrotron, Hamburg, Germany, for making available the beam time needed to develop the time-resolved techniques. 

Another difference between conventional high pressure Cl and the FTMS Cl experiment is the duration of the electron beam event used to form primary ions and secondary electrons. In the conventional Cl source, the electron beam is on continuously during the experiment. The FTMS, in contrast, uses a pulsed electron beam, and the duration of the electron beam event may be varied from less than a millisecond to over a second. In the NICI studies using the FTMS, the electron beam was typically left on for ten milliseconds. However, it was found that in some cases, which will be discussed later, it was necessary to use a longer beam time, up to 1 sec, in order to observe the product ions normally produced... 

When the electron beam time was increased to longer times (up to 500 msec), additional anions were observed. This is illustrated in Figure 3, which is the NICI spectrum obtained from a reaction of... 

Beam Time The length of time that the electron beam is on... 

From a practical standpoint, the use of the glancing angle X-ray method, while powerful, requires a synchrotron source and therefore, due to the constraints of beam time, is necessarily limited in the number of systems that can be studied in a given time period. Of the optical methods listed, the fluorescence and resonance Raman techniques directly measure spectra of an embedded... 

The authors thank Samsung SDI CAE team for providing Cerius2 program. We are also grateful for beam time allotment by PAL. The experiments at PLS were supported in part by MOST and... 

XRD in dedicated laboratory environments provides and will provide a wealth of useful information, because they allow operation with few time constraints and unsurpassed experimental flexibility. Synchrotron sources, on the other hand, provide unique opportunities to combine XRD with EXAFS spectroscopy, which together provide an enormous advantage as long as the conduct of the experiments is not constrained by beam time limitations. Bulk transformations under reaction conditions are typically slow, being characterized by time scales of hundreds of hours, and are therefore prohibitive for standard user operations at synchrotrons. It is also difficult to handle many catalytically relevant reactants safely at synchrotrons, so that XRD investigations are limited to a few reactants, in contrast to the situation in most catalyst characterization laboratories. 

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