Photoemission threshold

We consider a general dissipative environment, using a three-manifold model, consisting of an initial ( ), a resonant ( r ), and a final ( / ) manifold to describe the system. One specific example of interest is an interface system, where the initial states are the occupied states of a metal or a semiconductor, the intermediate (resonance) states are unoccupied surface states, and the final (product) states are free electron states above the photoemission threshold. Another example is gas cell atomic or molecular problems, where the initial, resonant, and final manifolds represent vibronic manifolds of the ground, an excited, and an ionic electronic state, respectively. 

Figure 7. Normalized photoresponses yield spectra for WOs in 1N HsSO The data are taken in order of decreasing negative potential at the potentials indicated. All of the currents are cathodic. The insert shows Fowler plots used to determine the photoemission threshold at each potential.

A large number of studies have been devoted to measuring the ionization potential in the liquid and the solid phases (see Refs. 179-181, 189, 190). Some of these results are presented in Table VII, from which one can see that for most substances V0 is negative, and so the ionization potential in the condensed phase is smaller than the photoemission threshold Eph. However, for some substances (for instance, for n-pentane, /i-decane, and neon), VQ is positive, meaning that in this case it is more advantageous, from the energy point of view, for an electron to make a transition into vacuum than to remain in a quasi-free state. 

Fig. 12. Scheme of energy levels in a dielectric. /c is the ionization potential in the condensed state, E is the photoemission threshold, Va is the energy of the bottom of the conductivity band, and Vs is the solvation energy. (Vu and Vs are negative since they are measured from the energy of the vacuum level.)... 

Figure 12.10 Plots of the anodic and cathodic photocurrent spectra for anodised bismuth, showing how the bandgap and photoemission threshold are obtained. Adapted from Castillo and Peter (1983).

Structure and the built-in potential caused by the use of asymmetric metal contacts. The electric field dependence of the photoemission threshold, including the effects of the built-in potential (see below), agrees with this expression as shown by comparing it (taking = 3) with bias-dependent data in the upper left inset of Figure... 

Here, wJ is the work function at E = 0 relative to the chosen reference electrode. Extrapolating the measured I-E dependence (for the given hv) in F -E coordinates to I -> 0 gives a photoemission threshold potential E,. At E = E, the electrode-to-... 

Two material properties needed in the discussion are depicted in Figure 17. The photoemission threshold is the photon energy required to raise an electron from the top of the valence band to the vacuum level, and the electron affinity x is the energy difference between the bottom of the conduction band and the vacuum level. The quantity - x) is the band gap. 

Figure 17. Energy level diagrams showing the metal work function, I the photoemission threshold, and the electron affinity, x of a wide-band-gap material. Epjs the Fermi level of the metal, and and Ec are the valence and conduction band edges.)...

The photoemissive threshold energy for electron emission in a metal is usually of value in a semiconductor, -n, would be roughly X if the density of electrons in the conduction band (above E. ) were sufficiently high to allow photon absorption. This is seldom the case. More often the high density of electrons available to efficiently absorb incident light is in the valence band, near Eq, so that the threshold for photoemission from a... 

Fig. S.3a-c. Band structure and photoemissive yield for (a) silicon activated to negative electron affinity (b) a thin layer of CSjO (values are approximate) and (c) metallic Cs. For Si, is the energy of intentionally added p-type dopants, while for CsjO, Eq is the energy level of native defects. The relationship between photoemissive threshold energy and the value of

Vq values are of theoretical and practical importance. In the language of physics, Vq is identified as the bottom of the conduction band, while in chemical terms, it is called the electron affinity of the liquid. As discussed in Section 7.6, Vq reflects the subtle balance of repulsive and attractive forces acting on the electron. The more negative the value, the greater the influence of attraction, while positive Vq values point toward the decisive influence of repulsion. Vq data are compiled in Table 3. The measured data were obtained either by the photoelectric method (see Section 6.2), by electron emission from liquids, or by comparison of photoconductivity and photoemission data (see Section 6.5). Data measured by the photoelectric effect may suffer from the effect of a charged double layer at the metal liquid interface. In principle, the values obtained by the electron emission method or from the comparison of photoconductivity and photoemission thresholds should be free from this ambiguity. 

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