Photoelectron collective excitation

Even though photoelectron spectroscopy has not yet shown evidence for core level shifts corresponding to differences between atoms of a clean surface and atoms in the interior of a solid8), both single particle and collective excitations specific to valence electrons of surface atoms have been observed. 

The intensity of an XPS peak (Ia) is a strong function of (i) the incoming photon flux, (ii) the concentration of the given element, (hi) its photoionization cross-section (which is excitation-energy dependent), (iv) the mean free path of the emitted photoelectron, and (v) further instrumental parameters (such as photoelectron collection and detection efficiency). By defining atomic sensitivity factors (S, as an overall factor summing up the effects of iii-v), the atom fraction of any element in a sample can be calculated as ... 

In this chapter we describe advances in the femtosecond time-resolved multiphoton photoemission spectroscopy (TR-MPP) as a method for probing electronic structure and ultrafast interfacial charge transfer dynamics of adsorbate-covered solid surfaces. The focus is on surface science-based approaches that combine ultrafast optical pump probe excitation to induce nonlinear multi-photon photoemission (MPP) from clean or adsorbate covered single crystal surfaces. The photoemitted electrons transmit spectroscopic and dynamical information, which is captured by their energy analysis in real or reciprocal space. We examine how photoelectron spectroscopy and microscopy yield information on the unoccupied molecular structure, electron transfer and relaxation processes, light induced chemical and physical transformations and the evolution of coherent single particle and collective excitations at solid surfaces. 

In SXAPS the X-ray photons emitted by the sample are detected, normally by letting them strike a photosensitive surface from which photoelectrons are collected, but also - with the advent of X-ray detectors of increased sensitivity - by direct detection. Above the X-ray emission threshold from a particular core level the excitation probability is a function of the densities of unoccupied electronic states. Because two electrons are involved, incident and the excited, the shape of the spectral structure is proportional to the self convolution of the unoccupied state densities. 

The 5950A ESCA spectrometer is interfaced to a desktop computer for data collection and analysis. Six hundred watt monochromatic A1 Ka X-rays are used to excite the photoelectrons and an electron gun set at 2 eV and 0.3 mAmp is used to reduce sample charging. Peak areas are numerically integrated and then divided by the theoretical photoionization cross-sections (11) to obtain relative atomic compositions. For the supported catalyst samples, all binding energies (BE) are referenced to the A1 2p peak at 75.0 eV, the Si 2p peak at 103.0 eV, or the Ti 2p3/2 peak at 458.5 eV. 

Birch et al. have used the Philips XP2254B, an S20 version of the XP2020Q, to study the fluorescence lifetimes of a series of aminotetraphenylporphyrins in a multiplexed fluorometer. 83 The extended red response (S20R) version of this device, the XP2257B, has been used with IR spark source excitation to study the fluorescence lifetimes of carbocyanine dyes up to 930 nm emission in isotropic and anisotropic media. 55,561 % 84) An improved voltage divider network has been developed for linear focused photomultipliers which reduces thermionic noise from the photocathode by an order of magnitude by restricting the collection of photoelectrons to the center of the photocathode. 84 ... 

By varying the excitation energy in direct and inverse photoemission, further important information is collected on hybridized states. Thus, it can be said that by photoelectron methods, the itineracy of open shells (in actinides 5f) is well characterized. 

TRPES has been recently reviewed and details of the experimental method and its interpretation can be found elsewhere [5], Trans-azobenzene was introduced via a helium supersonic molecular beam into the interaction region of a magnetic bottle photoelectron spectrometer. The molecules were photoexcited by a tunable femtosecond laser pulse (pump pulse) with a wavelength of 280-350nm. After a variable time delay, the excited molecules were ionized by a second femtosecond laser pulse (probe pulse) with a wavelength of 200 or 207nm. The emitted photoelectrons were collected as a function of pump-probe time delay and electron kinetic energy. 

ZEKE-PES was pioneered by Miiller-Dethlefs, Sander and Schlag18,25. They and others26 recorded the zero kinetic energy photoelectrons produced by absorption of one or more photons to a resonant ion state as a function of pulsed laser excitation wavelength. ZEKE electrons were extracted by an electric field pulse which permitted a delay between the creation and collection of electrons. During this delay, non-ZEKE electrons departed the focal volume, leaving only ZEKE electrons for collection by the extraction pulse. 

Chlorophylls chlorophylls have a variety of functions in photosynthetic systems, including collection of photons, transfer of excitation energy, operation of the primary photoinduced charge separation, and transfer of the resulting photoelectrons. Resonance Raman spectroscopy offers the possibility of selectively observing chlorophylls in their native structures (Lutz, 1984 Koyama et al., 1986 Tasumi and Fujiwara, 1987 Lutz and Robert, 1988 Nozawa et al., 1990 Lutz and Mantele, 1991). Transient Raman spectroscopy is a unique method of revealing the excited state structures of chlorophylls. The T1 and SI states were revealed by nanosecond Raman spectroscopy (Nishizawa et al., 1989 Nishizawa et al., 1991 Nishizawa and Koyama, 1991). 

Plasmon loss generates another type of satellite peaks these peaks do not provide useful information but they complicate a spectrum. Plasmon loss refers to the energy loss of a photoelectron because it excites collective vibrations in conduction electrons in a metal. The vibrations require a characteristic amount of energy. Such characteristic amounts of photoelectron energy loss in photoelectrons will generate satellite peaks as shown in Figure 7.10c. Plasmon loss peaks are shown in XPS spectra of clean metal surfaces and also in Auger spectra. 

The catalysts were characterized by X-ray photoelectron spectroscopy using an XSAM-800 spectrometer (Kratos) with Al Kai,2 radiation for spectra excitation. Spectra in the O Is, Ti 2p, and Al 2p regions were collected. The C Is binding energy (BE) at 285.0 eV was taken as a reference. The spectra of the Ti 2/73/2 line was analyzed by a conventional peak synthesis procedure using Gaussian functions. 

Photoemission spectroscopy (PES) is by far the most widely used and powerful spectroscopic technique for interface research. XPS and UPS are complementary techniques that utilize different light sources, e.g., x-ray and ultraviolet, to excite electrons in solids via photoelectric effect and then collect the escaped photoelectrons with an energy analyzer. In general, photoemission experiments for interface formation studies are performed in the following way. The study begins with the photoemission analysis of a clean surface of the material that will eventually form one side of the... 

When the laser wavelength A,l is tuned across the spectral range of the absorption lines, the total fluorescence intensity 7fi(/ l) oc ni oikNi monitored as a function of laser wavelength Xl represents an image of the absorption spectrum called the excitation spectrum. According to (1.36) the photoelectron rate nps is directly proportional to the absorption coefficient Niait, where the proportionality factor depends on the quantum efficiency ) ph of the photomultiplier cathode and on the collection efficiency 5 of the fluorescence photons. 

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