Photoelectron yield

In order to estimate the photoelectron yield from a chemisorbed layer we follow the approach of Henke (S), applied by Madey et al. (9), and modified by Carley and Roberts (10). A monoenergetic beam of x rays strike a surface at an angle 0 to the surface normal the photoelectrons, assumed to be generated in a layer of depth X and thickness dx, escape from the sample to a detector positioned at an angle with respect to the surface normal. The differential probability dP of absorption of x rays at depth X within the thickness dx is... 

If we divide Eq. (6) by Eq. (4), then the photoelectron yield from the mono-layer (F ) relative to the total yield from the substrate (Fj) is given by... 

We must also take into account two further factors. First, the fact that the transmission efficiency of the analyzer is a fimction of the kinetic energy (K.E.) of the photoelectrons in the ESCA-3 Vacumn Generators instrument the transmission is inversely proportional to the K.E. of the electrons (3a). Second, photoelectron yields must refer to total yield from a particular ionization process and this need not, for example, be just the area of the relevant peak. Account must be taken of all processes that divert electrons from the primary peak, e.g., shake-up, shake-oflF, and plasmon peaks. In some cases, e.g., emission from the Cu 2P3/2 level, the contribution of additional processes is small but in others, and emission from the Al(2p) shell is an example, the no-loss peak is substantially less than the true Al(2p) emission. 

We will not concern ourselves here with problems associated with line broadening, overlapping peaks, and background subtraction. There are, however, examples discussed later where both deconvolution and curve fitting procedures are shown to be essential in unraveling the contributions of differently bonded species of the same molecule to the total photoelectron yield. Carley and Joyner (14) have discussed recently deconvolution procedures for photoelectron spectra. 

Figure 3 Absorption spectrum of the DNA film around the K-shell absorption edge of phosphorus [18]. Dotted line is the total photoelectron yield of KH2PO4. Three vertical arrows indicate the X-ray energies used for the irradiation experiments shown in Fig. 4.

In all experiments, we start with an in situ compensation of the residual spectral phase of the IR pump pulse, introduced by dispersive elements in the pump beamline. By this means, we make sure to have BWL pulses in the experiment if the zero phase is applied to the SLM. The residual phase is found by adaptive optimization of the total photoelectron yield from multiphoton ionization of ground state Xe atoms, which are led effusively into the interaction region. It turned out to be sufficient to parameterize the trial phase by a fifth-order polynomial. In the... 

The first measurements to deduce the electron affinity of these nitride materials involved measurements of the photoelectron threshold of GaN [7], This study involved measurement of the electron emission for monochromatic light incident on the surface. The wavelength is scanned and the total photoelectron yield is measured. The photoelectron yield, Y, is related to the photothreshold, E, as follows ... 

Two-photon photoemission spectroscopy is known for its capability to reveal not only occupied but also unoccupied electronic density of states [10]. In this scheme, one photon excites an electron below the Fermi level to an intermediate state. A second photon then excites the electron from the intermediate state to a final state above the vacuum le vel. The photoelectron yields are strongly enhanced if the excitation photon energy is tuned to the resonance conditions, and the photoelectron spectrum reflects the electron lifetime in the intermediate states as well as their density of states. It is necessary to keep the employed photon energy below the work function of the sample, otherwise one photon photoemission signal becomes excessive and buries the 2PPE signals. 

Fig. 1.11. Total photoelectron yield from Age clusters deposited on HOPG, integrated over all photoelectron energies, as a function of the substrate temperature [95]...

Fig. 11.2. The total photoelectron yield spectrum of lanthanum in the solid phase, as obtained by synchrotron radiation spectroscopy in the neighbourhood of the 3d threshold. Curve (a) shows the experimental data points with the sum of two Lorentzian profiles fitted to them. Curve (b) shows the same data points for the higher of the two resonances, with a Fano profile fitted to it. The improvement in the fit is clear evidence for a slight asymmetry due to autoionisation (after R.C. Karnatak et al. [599]).

Malinowski and co-workers at Sandia National Eaboratories have established a correlation between the electric field at the surface of the multilayer film, the photoelectron yield, and the carbon growth rate, indicating that the photoelectrons play a critical role in cracking hydrocarbons. Figure 14.13 is a plot of the electric... 

XPS (x-ray photoelectron spectrometer)— the method uses x-ray irradiation to cause core electrons to be ejected from elements near the surface of a sample. Analysis of the kinetic energies of these ejected electrons, also known as photoelectrons, yields information about the elemental composition and chemical state of the surface. 

Fig. 7.2 Ambient pressure photoelectron spectroscopy combination of differential pumping with electron optics to allow for realistic photoelectron yield from in situ cells. Reproduced from [21] with permission from Elsevier...

T, Maniv and H. Metiu, Electrodynamics at a metal surface. III. Reflectance and the photoelectron yield of a... 

The photoelectron yield estimated for the HAPD readout is given by ... 

We have used an HAPD to observe Cerenkov photons produced by electrons from a Ru / source. The spectrum is shown in Figure 4.14 the photoelectron yield is shown in Table 4.4. 

Since currently available HAPD s are of small diameter, we masked a 2 photomultiplier tube to investigate the dependence of the photoelectron yield on detector area. A masked PMT (with sensitive area 18 mm in diameter) gave 5.4 0.2 photoelectrons or 0.68 0.03 pe/cm with the n = 1.032 aerogel sample. The observed spectrum is shown in Figure 4.15. These results are well-reproduced by a detailed Monte Carlo simulation. We are thus confident that it is possible to extrapolate with results taken with HAPD s of the current size to larger area devices. 

Table 5.2, Light yield and noise measurements with small and large CsI(Tl) crystal read out with an APD, yin is the initial photoelectron yield, Yout is the output electron yield from the APD, and ENC is the equivalent noise charge.

Figure 3. UPS work function versus photoelectron yield...

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