We are studying the electronic structures of new and advanced materials. We are using synchrotron radiation to perform soft x-ray emission and absorption spectroscopy of systems like biomaterials, superconductors and transition metal compounds.
X-ray Absorption Spectroscopy (or XAS) involves observing the fraction of photons which are absorbed when monochromatic synchrotron radiation is incident upon a sample. The mechanism by which photons are absorbed is the promotion of core-level electrons into higher-energy unoccupied states. Therefore, XAS is a direct probe of the partial density of unoccupied electronic states within the sample, allowing direct probing of the conduction band.
To provide a picture of the density of unoccupied states within a certain energy range, the excitation energy is increased by fixed amounts, and the amount of energy absorbed is plotted as a function of the excitation energy. These plots have large, distinct features at energies corresponding to resonant excitation from one bound state to another. In this way, the energy levels of the unoccupied states (relative to the occupied states) can be determined.
There are several methods for measuring XAS. One method is total electron yield (TEY) measurements. TEY involves measuring the amount of electrons needed to neutralize the sample as electrons are ejected. The amount of photons absorbed through excitation to bound states can be measured, even though these excitations do not directly cause electrons to be ejected. In this case, measurements are made possible by the fact that the excited electron quickly relaxes into a lower-energy state, releasing energy. This energy is often transferred to a bound electron, which is then ejected from the sample, leaving a hole which must be refilled. This produces a net positive current out of the sample.
Although the ejection of electrons is the most probable decay mechanism, it is also possible for the energy to be released in the form of a photon. Absorption spectra created by detecting these photons are known as fluorescence yield measurements. Fluorescence methods have advantages over TEY measurements, in that they are not sensitive to sample charging, and have greater bulk-sensitivity. This is due to the fact that photons have a greater escape depth than electrons. Radiative transitions are governed by the selection rule describing the change in l = +/-1 (l is the orbital quantum number).
XAS and XES are extremely sensitive to the chemical state each element is in. Not only does each element have its own characteristic binding energies, XAS measurements can distinguish the form in which the element crystallizes (for example one can distinguish diamond and graphite, which both entirely consist of C, see Fig. 2), and can also distinguish between different sites of the same element.
The plot to the right shows Si 2p emission spectra for different forms of Silicon. Fluorescence photons are emitted when valence band electrons of Si fill a 2p Si core hole, directly probing the valence band. Since the spectra of the compounds displayed are different, it is cleat that the technique allows to determine in which form any element is present in any new or complex material.