March 2004 NSLS-II Workshop: Spectroscopy Breakout Session

The spectroscopy session featured three invited scientific presentations covering a broad range of x-ray spectroscopy focusing on recent experiments that require extraordinary bright sources. These talks were a platform to initiate a discussion on the role of NSLS-II in the field of spectroscopy.

In the first talk - X-ray Magnetic Circular Dichroism of Dilute Transition Metal Systems, Nanoparticles, and Nanowires - Dr. P. Gambardella (Institut de Physique des Nanostructures, Losanne, Ch.) discussed a series of impressive experiments which he recently performed at ESRF, aimed to the study the magnetic properties of two-, one-, and zero-dimensional systems. Dramatic effects were illustrated both in the magnetic moments and in the magnetic anisotropy of these nanostructured systems. The amount of material present in some of these experiments is extremely small (down to something of the order of 0.002 atomic layers); correspondingly extremely bright sources of X-rays are required to achieve enough sensitivity.

In the second talk - High-Resolution High-Energy Photoemission - Prof. G. Stefani (University of Rome) considered the challenge of extending photoemission to the case of excitation by hard x-rays (about 10 keV). The use of hard x-rays would extend the applicability of photoemission by: (1) Increasing the probing depth to a few hundreds of angstroms, thus allowing studies of bulk systems and buried interfaces, and (2) Allowing a very convenient independence of the final states from the solid state band structure. This should render the spectral interpretation more direct. These obvious advantages come only at a cost: the cross-section of the photoemission process is extremely low at high photon energies. Furthermore, the performance of conventional electron spectrometers is quite poor at high energies. For this reason, various groups in Europe are dedicating considerable effort to the improvement of electron spectrometer performance at high energy. Prof. Stefani described the first successful results of one of these projects (the VOLPE (VOLume PhotoEmission ) collaboration) which recently demonstrated a probing depth of about 150 Angstroms at 8 keV, with an energy resolution of about 100 meV.

In the third talk - Some Ideas about the Future of X-ray Spectroscopy - Dr. F. deGroot (Utrecht University, The Netherlands) considered several important topics that should become more and more important in the new machines. In particular, he emphasized the new possibilities enabled by ultra-short light pulses (of the order of few tens of ps) to perform time-resolved absorption spectroscopy to study dynamic phenomena. Another area to which Dr. deGroot devoted considerable attention is in-situ experiments, to better understand the all-important phenomena of catalysis in the chemical sciences.

Following the formal presentations, there were several informal short presentations to initiate the discussion.

Dr. Steve Hulbert (NSLS) presented an overview of the current status of the NSLS-II machine, with particular emphasis on the possible types of insertion devices currently being considered. This presentation made quite clear that users need to be better informed about the characteristics of these devices.

Dr. Bruce Ravel (NRL) emphasized the necessity of not losing sight of the potential of bending magnet beamlines in the excitement of the new machine. Useful techniques with considerable number of users (like EXAFS) mostly require bending magnet sources rather than insertion devices. Also, the potential for wigglers for EXAFS should be examined carefully. According to Bruce, there are many applications where kinetics occur on the several second timescale. Such experiments would most benefit from the development of quick EXAFS (QEXAFS) techniques, which would greatly increase their throughput by allowing spectra to be collected on the timescale of seconds.

Dr. Anatoly Frenkel (Yeshiva University, NY) emphasized the even faster timescale (milliseconds) where energy dispersive techniques (DEXAFS) would also be useful. In addition to these time-resolved techniques, spatial resolution at the level of microns is also an important capability for µ-XANES and µ-EXAFS experiments. Finally, complete coverage of the 0.1-100 keV energy range is important, although there were differences in opinion expressed as to whether this would best done using a number of optimized beamlines, or by single beamlines that cover a very wide energy range.

Dr. Dan Fischer (NIST) and Dr. Peter Johnson (BNL) emphasized the fact that a new source can also mean a new way to perform spectroscopic work. As an example, Dr. Fischer showed results of a Japanese group working on a project dedicated to a synchrotron-based STM. A micron size light spot would allow the combination of the STM technique with photoemission spectroscopy. In this way, it should be possible to record atomic-scale images with combined spectroscopic information. Another provocative idea came from Dr. Johnson, who is currently studying the problem of recording angular-resolved photoemission spectra with a time-resolved technique based on pulsed light sources. Some of the challenges that these novel techniques are focusing to a small spot size, and limiting the coherence of the beam, as the energy resolution will be degraded if the coherence of the beam is too great.

The discussion session was kept informal and numerous questions and scattered comments came from the workshop participants. As a general impression, we perceived a strong interest and enthusiasm of the users for the NSLS-II project. We believe that the spectroscopy user community will be even more responsive once they are provided with specific detailed information on the characteristics of the new machine. Indeed, if one of our purposes was to gather information on what could be most appropriate for them, most of the discussion actually went in giving them more information about NSLS-II. The spectroscopy users of NSLS-II clearly need access to this detailed information as soon as it can be provided.

Last Modified: January 31, 2008
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