08/13/2014

New kind of spectrometer for the analyses of sample surfaces



HZB's own Prof. Dr. Gregor Schiwietz and his team have developed a new kind of spectrometer capable of particularly sensitive analyses of sample surfaces. It has allowed them to be the first to obtain measurements of a copper sample's Auger electron spectrum at the HZB's synchrotron radiation source BESSY II's femtoslicing beamline at extremely high temporal resolutions.

The spectrometer is based on the principle of what's known as the "Bessel-Box," which, in its latest installment, however, has been much improved and expanded upon. Going forward, BESSY II will make yet another high-power tool for analysis of rapid surface processes available to the user community.

The distribution of energy (spectrum) of electrons at a sample surface yields precise information about its atomic composition. Powerful X-ray pulses yield Auger electrons whose energy can be measured. There is a worldwide trend to use synchrotrons to measure the spectra of these types of electrons at temporal resolutions of a mere 100 femtoseconds (1 fs = 10-15 s). For it is within this timeframe that many chemical processes that modify the sample surfaces are actually taking place. On the other hand, the synchrotron light's pulse lengths are considerably greater; in the case of BESSY II, they lie within the range of 15 picoseconds (1ps= 10-12s), 150 times longer than the 100 femtoseconds they had originally aimed for.

However, in the case of femtoslicing, HZB physicists have come up with a method for cutting out these kinds of ultrashort pulses from the synchrotron pulses. The femtoslicing beamline allows scientists to conduct pump-probe experiments: A laser pulse initially stimulates the sample and, after a delay of variable duration, a second pulse, this time an X-ray pulse originating from the synchrotron source, records the system's response. Now, at the HZB Institute for Methods and Instrumentation for Synchrotron Radiation Research, Schiwietz' team has developed a new kind of spectrometer capable of highly efficiently detecting excited electrons' energy distributions. The spectrometer is based upon the principle of a "Bessel-Box," which the experts have expanded upon and turned into a new kind of "Retarding Bessel-Box". Electrons are decelerated, energy-selectively focused, and separated according to their respective flight durations. "This allows us to measure a number of Auger electrons per second that is some 60 times greater than before, representing an enormous gain in terms of both data and precision," says Schiwietz.

By international comparison, the new RBB spectrometer means the HZB is now able to offer an ideal option for time-resolved electron spectroscopy using synchrotron pulses: The robust cylinder-symmetrical RBB spectrometer has already been used for purposes of pump-probe experiments at low temporal resolutions and has now also passed the additional particularly short and therefore relatively weak X-ray pulse test at the femtoslicing beamline. "Looking ahead, this could mean we're able to glean insights into the behavior of 'hot' electrons within a crystal lattice as well as study the coupling of electrons and atomic lattice or the interactions between electrons and the spin system. These types of questions are very topical and will help deepen our understanding of switching mechanisms in metastable systems, like, for instance, for the development of new materials to use in information technology," explains Schiwietz.

Source: Helmholtz-Zentrum Berlin (HZB)




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