Photoelectron detection

The spectral resolution of photoelectron spectroscopy is limited for a number of technical reasons, not least because of its moderate energy resolution of 10meV(S 80cm ). This is much less than generally encountered with laser spectroscopy, by about three orders of magnitude. 

The resolution of photoelectron spectroscopy was dramatically improved when a threshold technique called ZEKE spectroscopy was introduced. With this technique, a tuneable laser is scanned over the thresholds for the formation of specific ionic eigenstates. The resulting photoelectrons possess very low ( zero ) kinetic energies and can be discriminated from higher kinetic energy electrons, which are also produced as the photo-excitation laser is scanned over the ionic eigenstates. 

There are basically two ways of achieving high-resolution photoelectron spectra, differing in the applied technique and the achievable resolution  

Only the second and most commonly used approach will be addressed here (see Chapter 18 for more details). 

A ZEKE spectrum is thus acquired by recording the yield of electrons, produced by PFI, when the photo-excitation laser is scanned across successive ionization thresholds. The resolution obtainable by ZEKE spectroscopy is of the order 10 —10 cm , which is governed by the line width of pulsed, tuneable lasers with CW lasers, even sub-Doppler resolution is now achievable. This is sufficient to resolve rotational structure, even with many polyatomic species, and thus enable the determination of molecular ion structures. 

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Using photoelectron detection in a femtochemistry arrangement, we studied size-selected clusters of ionic systems, covering the transition from gas phase to condensed phase dynamics [6]. We investigated the solvent effect on Oj dissociation dynamics, and observed... 

Using photoelectron detection in a femtochemistry arrangement, we studied size-selected clusters of ionic systems, covering the transition from gas phase to condensed phase dynamics [6]. We investigated the solvent effect on Oj dissociation dynamics, and observed that the addition of one solvent (O2, N2, Xe or N2O) gives very different effects on the dynamics of the nuclear motion, whose wave packet bifurcates in two channels. These real time studies of the one-solvent dynamics provide the time scale of the distinctive processes of electron recombination and bond rupture, and vibrational predissociation— with both... 

Quanlitative Applications. Once. XPS was not considered to be a very useful quantitative technique. However, there has been increasing use of XPS for determining the chemical composition of the surface region of solids. If the solid is homogeneous to a depth of several electron mean free paths, we can express the number of photoelectrons detected each second / as... 

The intensity/energy response function (lERF) of a spectrometer is the product of the area from which photoelectrons are collected, of the transmission function (T), and of the detector efficiency D). With a microfocused X-ray beam, such as in SSX 100/206 spectrometer, the area of collection is defined by the X-ray spot size and is smaller than the acceptance area of the analyzer. The relative intensity of the peaks is sensitive to the position of the sample on the vertical axis. When a broad X-ray source is used, the lERF includes also the variation of the area of collection as the latter may depend on the energy of the photoelectrons detected (cf. the section on Basic Equations). 

Quantification is in many cases the most important feature of XPS [52]. The X-ray photoelectron current produced by an incident X-ray photon of energy hv, which ionises core level Z in an atom of type A in a solid matrix (M), is a function of both energy- and matrix-dependent terms. Quantification of XPS spectra is thus far from simple. In fact, already the exact definition of quantification is complicated. Even for an ideal sample the actual escape depth may vary substantially. The depth of photoelectron detection, for a given XPS system, may vary dramatically from measurement to measurement, and even within the same measurement (in relation to the energy scale). After some experimental... 

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