Photoelectron emission

When solutions of CdS colloids containing no additional electron and hole acceptor in the solution, are exposed to a high intensity laser flash, a rather large absorption of an intermediate is observed around 700 nm, similarto that described for the laser excitation of Ti02 in the previous section. The absorption spectrum of the intermediate is given in Fig. 9.17 [52]. It is not due to trapped electrons and holes but it is identical with to the well-known spectrum of hydrated electrons as proved by radiolysis experiments [52]. The half-life of the hydrated electrons is a few microseconds. In the presence of typical hydrated electron scavengers, such as oxygen, acetone or cadmium ions, the decay of the intermediate became much faster. 

Photoelectron emission was only found in aqueous solutions. No Cgq formation was observed in acetonitrile or alcohol solutions. It also has been reported that the electron emission occurred only with CdS colloids stabilized by polyphosphates or colloidal Si02. The negative charge of the stabilizer prevents the emitted electron from rapid return to the particle by electrostatic repulsion [3]. 

A quantum yield of about 0.07 electrons emitted into the solution per absorbed photon was found. Interestingly, this value exceeds, by several orders of magnitude, the 

4 X 10 M ZnS/CdS and 2 x 10 M Na2S in H2O. Insert decay of absorption signal after the laser pulse. 

The absorbance (log 7()/i) and therefore the concentration of e q increased with increasing density of absorbed photons as measured immediately after the laser flash (Fig. 9.18). Since the extinction coefficient is well known, the concentration could easily be determined from absorption measurements. In comparison, the absorption of holes trapped at the particle surface is also shown in Fig. 9.18. The latter has a spectrum peaking around 600 nm and has a long lifetime ( 1 ms), and was obtained after the eaq absorption had been quenched by adding acetone to the solution. Details are given in ref. [54]. The absorption of the trapped holes saturates at higher intensities. The holes were probably used in an anodic corrosion reaction. 

When solutions of CdS colloids containing no additional electron and hole acceptor in the solution are exposed to a high intensity laser flash, a rather large absorption of an intermediate is observed around 700 nm, similar to that described for the laser excitation of Ti02 in the previous section. The absorption spectrum 

A quantum yield of about 0.07 electrons emitted into the solution per absorbed photon was found. Interestingly, this value exceeds, by several orders of magnitude, the yields encountered in photoemission experiments with compact semiconductor electrodes [53]. This result indicates that the particle size may be important and indeed it was found that the e absorption occurs only with small particles (nm range) and the absorption coefficient increased with decreasing par- 

Concerning the nature of the excited state, it must be first reahzed that at high doses each colloidal particle absorbs many hundred photons during one flash. From their measurements, Henglein et al. concluded that the following mechanism occurs [3, 54]  

Comparison between Reactions at Semiconductor Particles and at Compact Electrodes 

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Wlien photons of sufiBciently high frequency v are directed onto a metal surface, electrons are emitted in a process known as photoelectron emission [ ]. The threshold frequency Vq is related to the work fimction by the expression... 

In principle, therefore, the surface concentration of an element can be calculated from the intensity of a particular photoelectron emission, according to Eq. (2.6). In practice, the method of relative sensitivity factors is in common use. If spectra were recorded from reference samples of pure elements A and B on the same spectrometer and the corresponding line intensities are and respectively, Eq. (2.6) can be written as... 

In X-ray photoelectron spectroscopy (XPS), a beam of soft X-rays with energy hv s. focused onto the surface of a solid that is held under an ultra-high vacuum, resulting in the ejection of photoelectrons from core levels of the atoms in the solid [20]. Fig. 15 shows an energy level diagram for an atom and illustrates the processes involved in X-ray-induced photoelectron emission from a solid. 

Cause, R.L., A nonconiacting scanning photoelectron emission technique for bonding surfaces cleanliness inspection. NASA Technical Memorandum NASA TM-100361, 1989. Schirato, R.C., Polichar, R.M. and Shreve, D.C., In Proc JANNAF Nondestructive Evaluation Subcommittee Meeting. Chemical Propulsion Information Agency, Columbia, MD, 1992. 

During the photoelectron emission event there are electronic relaxation effects occurring, which are usually divided into intra- and inter-molecular relaxation effects. These effects can be rationalized in a classical picture as follows. An elec-... 

The PEEM technique (photoelectron emission microscopy),58 which additionally allows for spatial resolution of about 1 mm2. 

Imbihl, Kiskinova, Janek and coworkers67 have also used XPS and spatially-resolved photoelectron emission microscopy (SPEM) to investigate oxygen backspillover between YSZ and evaporated microstructured Pt films prepared using microlithographic techniques (Figure 5.38). 

H. Schade, Irradiation-Induced Metastable Effects L. Ley, Photoelectron Emission Studies... 

Figure 7.24 Photoelectron emission microscopy images of two Fe304 surfaces that were used as model catalyst in the dehydrogenation of ethylbenzene to styrene at 870 K, showing carbonaceous deposits (bright). These graphitic deposits grow in dots and streaks on a surface of low defect density, but form dendritic structures on surfaces rich in point and step detects (from Weiss et al. f731).

Figure 12.21. Photocathode spectral sensitivity curves in imlliampcres of cathode current per watt ofradiam power falling on the photocathode, q is the photocathode quantum efficiency, i.e, the probability of photoelectron emission (Adapted from Ref 76.)...

The conduction band electron of liquid water, which cotild be produced by photoelectron emission fi m metal electrodes, is very unstable with its lifetime being 10 seconds it is readily captured by water molecules to form a hydrated electron. The hydrated electron is also very unstable being rapidly absorbed in electron scavenger particles such as H3O, NO 2 and O2. The level of the hydrated electron has been estimated at 0.3 to 0.5 eV below the conduction band edge [Battisi-Trasatti, 1977 Watanabe-Gerischer, 1981]. 

The orbital energies can be experimentally determined e.g., through ionization mediated by light excitation (photoelectron emission). According to Koopmans ... 

Let us now briefly outline the structure of this review. The next section contains information concerning the fundamentals of the electrochemistry of semiconductors. Part III considers the theory of processes based on the effect of photoexcitation of the electron ensemble in a semiconductor, and Parts IV and V deal with the phenomena of photocorrosion and light-sensitive etching caused by those processes. Photoexcitation of reactants in a solution and the related photosensitization of semiconductors are the subjects of Part VI. Finally, Part VII considers in brief some important photoelectrochemical phenomena, such as photoelectron emission, electrogenerated luminescence, and electroreflection. Thus, our main objective is to reveal various photo-electrochemical effects occurring in semiconductors and to establish relationships among them. 

Photoelectron Emission from Semiconductors into Solutions... 

Calculation of the current I of photoelectron emission from nondegenerate semiconductors into electrolyte solutions, performed within the framework of quantum mechanical wave approach, yields the following expression (Gurevich, 1972) ... 

Photoelectrochemistry (PEC) is emerging from the research laboratories with the promise of significant practical applications. One application of PEC systems is the conversion and storage of solar energy. Chapter 4 reviews the main principles of the theory of PEC processes at semiconductor electrodes and discusses the most important experimental results of interactions at an illuminated semiconductor-electrolyte interface. In addition to the fundamentals of electrochemistry and photoexcitation of semiconductors, the phenomena of photocorrosion and photoetching are discussed. Other PEC phenomena treated are photoelectron emission, electrogenerated luminescence, and electroreflection. Relationships among the various PEC effects are established. 

Although measurements of the photoelectron emission from organic solids began about 1910, and the first measurements of the external photoeffect from dye films date from the 1930 s,39 precise measurements... 

The mechanism of the photoelectron emission is evidently the same as in the hydrocarbons. The loss in sharpness of the maxima and a relative abundance of slow electrons may have an explanation, if the sequence of the molecular orbitals in these pigments is more narrowly spaced than are those of the hydrocarbons. Possibly, there is an excitation of molecular vibrations. 

In a previous paper of the authors on metal-free and Zn-phthalocyanines45 an overestimated value for

photon energies below the sharp threshold, attributed to impurities. This was corrected in later work. 

Fig. 12. Energy level diagram representation of (a) photoelectron emission and (b) X-ray absorption.

Fig. 1. Comparison of the four different physical processes which can be observed during the interaction of X-ray photons with matter 2 1. The two phenomena scetched below, namely photoelectron emission and Auger electron emission, can be detected and measured in a photoelectron spectrometer by determining the kinetic energy of the ejected free electrons...

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