Yield spectroscopy

Because of their high intensity. X-ray tubes were commonly used as laboratory radiation sources for radiation chemistry experiments until they were superceded by particle accelerators during the middle part ofthe 20th Century. They still retain specialized uses in research applications such as being used as the radiation source for MARY (MAgnetic field effect on Reaction Yield) spectroscopy studies of radical cation lifetimes and reactivity in alkane solvents [14,18]. MARY spectroscopy uses fluorescence to detect variations in singlet-triplet dynamics in radical ion pairs as a function of magnetic field. It is particularly useful for short-lived transients that are difficult to study by ESR. 

The alternative approach involves the so-called photoemission yield spectroscopies, which enable empty surface states to be probed. In these, the incident photon energy is varied while the electron energies are or are not resolved. The technique can firstly be used to investigate transitions between core levels and empty states, usually by using synchrotron radiation so that sufficient photon energy is available for the excitation. Core levels have negligible dispersion in fc-space, so the measurement reveals the unoccupied conduction band and surface state transition state densities, since electrons are generated by optical transitions from a core level to either empty surface states or conduction band states. 

The partial yield of electrons in an energy window AE at a fixed final state energy E, as a function of photon energy, where E is fixed at < 5eV so that only secondary electrons are measured, is referred to as partial yield spectroscopy. When E > 5eV, although the technique is experimentally identical, it is used to study initial state and excitonic effects and is known as constant final state spectroscopy. 

Given adequately prepared surfaces, angle-resolved photoemission and the various yield spectroscopies have been used to investigate filled and empty surface states, respectively. Results of angle-resolved photoemission measurements have been published by Knapp and Lapeyre [181], Williams et al. [182], Knapp et al. [183] and Huijser et al. [184], A typical set of angle-resolved photoelectron energy distributions (AREDCs) due to Huijser et al. [184] is shown in Fig. 16, in which four structures labelled B , SM S2 and B2 are observed. They are ascribed to emission from filled intrinsic states since they disappear on exposure to 10s L of H2. As we shall see below, B , S and S2 are primarily As-derived, while B2 is mainly a Gas-like state bonded to Asp-states. 

Hempelmann A, Piancastelli MN, Heiser F, Gessner O, Rudel A, Becker U (1999) Resonant photofragmentation of methanol at the carbon and oxygen -edge by high-resolution ion-yield spectroscopy. J Phys 32 2677-2689... 

GERISCHER The vacant surface states can be detected in the partial yield spectroscopy because the matrix element for an electronic transition from the core states to these surfaces states is large since they are concentrated at the same atom. What is the chance to observe an optical transition between occupied and vacant surface states ... 

SXE can also be excited by photons (fig. 4a), in which case the thickness of the investigated surface depth is increased by a few orders of magnitude. Conventional SXA of thin samples has in practice no surface sensitivity when compared to the other methods. The detection of the photon absorption can also be performed by measuring the yield of the emitted electrons so that the surface sensitivity may even be very high if the electrons are selected in a narrow energy window at low kinetic energy (partial yield spectroscopy). 

We call this method photoemission yield spectroscopy (PYS) if photons near to threshold are used and all photoelectrons are collected [77S]. If VUV photons are used and photoelecfron spectra are measured, we call the method ARUPS from angle-resolved UV-light photoelecfron spectroscopy. Energy conservation... 

In order to appreciate these and other results of yield spectroscopy on NEA diamond surfaces, it is best to recall briefly Spicer s three-step model of photoelectron emission, which is likely to be nowhere better suited than in the case at hand [109]. This model divides the photoelectron emission process up into three conceptually separate processes, (i) The bulk absorption of light generates photoexcited electrons and holes, and (ii) electrons travel to the surface with the possibility to suffer inelastic losses on their way before they (iii) escape into vacuum where they are being detected. In normal photoelectron spectroscopy interest lies in the so-called primary current, that is, in those electrons that leave the sample without energy loss on their way to the surface. In this case, the photoexcitation, transport, and escape processes are not entirely independent. For crystalline samples with well-ordered surfaces, the wave vector component parallel to the surface, k, is, for example, conserved from the initial electron state to the free electron in vacuum. In this case, a better description of the photoelectron emission is by a one-step excitation from an initial band structure state to a final state constructed as an inverse LEED state (Chapter 3.2.2). The inelastic mean free path of photoexcited electrons, is energy dependent and lies in the nanometer range (Chapter 3.2.3). 

Rdsslein, M., E. Kades, K. Bergmann, and J.R. Huber (1995), Alignment detected photofragment yield spectroscopy demonstratred by photodissociation of f-butyl nitrite in a supersonic jet, Chem. Phys. Lett., 235, 242-246. 

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