Photoexcitation of electrons

A silicon electrode with a small amount of deposited noble metals such as Pt does not behave as a buried junction and the photoelectrode properties are still determined by contact of the electrolyte, not the metal. The photovoltaic response of the Pt-deposited silicon electrode is due to the incomplete coverage of the surface by Pt so that the interface energetics remains dominated by the contact of the semiconductor surface with the electrolyte. The mechanism of the hydrogen reduction processes involves first photoexcitation of electron-hole pairs followed by charging of the small Pt islands with excited electrons. The charged Pt islands react with H2O at a high rate. 

The absorption cross section for x-rays in the range 100-20000 eV is determined by photoexcitation of electrons from atomic core levels. In this energy range pair production is forbidden and the weakness of electromagnetic field is such that only first order processes are important. 

The possibility that thermal or photoexcitation of electrons between these bands could lead to mobile charge carriers in either or both bands, and consequently to anisotropic semiconductivity or photoconductivity, was anticipated. Subsequent observations by Collman and co-workers (22) and by Gomm, Thomas, and Underhill (23) verified these predictions... 

OPTICAL DETECTORS sense and measure electromagnetic radiation in the broad spectral region from the longest infrared wavelength atmospheric window at A, = 10 /u.m to the ultraviolet end of the visible spectrum at A. = 0.3 /u.m. The detection process may depend upon the heat generated by the incident radiation (thermal detection) or by the photoexcitation of electrons or... 

In recent studies, clusters of ions such as copper or iron and their oxide were deposited (grafted) to induce photoexcitation of electrons in VB of base metal oxide such as titania to the electronic level... 

Quantum Well IR Photodetector (QWIP) Intraband photoexcitation of electrons horn confined states in the conduction band of quantum wells into the continuum... 

It must absorb light and thus allow photoexcitation of electrons from the ground state. The vacancy left behind (a hole) and the photo-excited electron form a pair of charge carriers, which can be separated by the action of an electric field. 

Figure 5.10 Determination of the work function from the total width of a photoelectron energy distribution. The left part shows a schematic energy diagram for the photoexcitation of electrons from the Fermi level into final states, which can propagate into the vacuum. The right side shows a typical photoelectron energy distribution.

Fig. 11.7 Important processes in the surface photocatalysis processes photoexcitation of electron-hole pair, charge transfer processes, bulk and surface recombination processes, and electron- and charge-induced chemistry at surfaces (Reproduced with permission from Ref [27]. Copyright 1995 American Chemical Society)...

Dicarbocyanine and trie arbo cyanine laser dyes such as stmcture (1) (n = 2 and n = 3, X = oxygen) and stmcture (34) (n = 3) are photoexcited in ethanol solution to produce relatively long-Hved photoisomers (lO " -10 s), and the absorption spectra are shifted to longer wavelength by several tens of nanometers (41,42). In polar media like ethanol, the excited state relaxation times for trie arbo cyanine (34) (n = 3) are independent of the anion, but in less polar solvent (dichloroethane) significant dependence on the anion occurs (43). The carbocyanine from stmcture (34) (n = 1) exists as a tight ion pair with borate anions, represented RB(CgH5 )g, in benzene solution photoexcitation of this dye—anion pair yields a new, transient species, presumably due to intra-ion pair electron transfer from the borate to yield the neutral dye radical (ie, the reduced state of the dye) (44). 

The copper system appears to behave similarly to the silver system, and it may be used here in order to illustrate the idea of "selective, naked-cluster cryophotochemistry 150,151). A typical series of optical-spectral traces that illustrate these effects for Cu atoms is given in Fig. 15, which shows the absorptions of isolated Cu atoms in the presence of small proportions of Cu2, and traces of Cus molecules. Under these concentration conditions, the outcome of 300-nm, narrow-band photoexcitation of atomic Cu is photoaggregation up to the Cus stage. The growth-decay behavior of the various cluster-absorptions allows unequivocal pinpointing of UV-visible, electronic transitions associated with Cuj and Cus 150). With the distribution of Cui,2,3 shown in Fig. 15, 370-nm, narrow-band excitation of Cu2 can be considered. Immediately apparent from these optical spectra is the growth (—10%) of the Cu atomic-resonance lines. Noticeable also is the concomitant... 

Research on the molecular basis of photoexcitation and electron transfer, including interactions of electron donor and acceptor molecules, could lead to new photochemicals. Development of model photosensitive compounds and methods of incorporating them into membranes containing donor, acceptor, or intermediate excitation transfer molecules, and... 

Recently, we have shown that the combination of barium tetratitanate, BaTi40g and sodium hexatitanate, NagTigOis, with ruthenium oxides leads to active photocatalysts for water decomposition[1,2]. The unique feature of these photocatalysts is that no reduction of the titanates is required to be activated this is intrinsically different from conventional photocatalysts using TIO2 which are often heat-treated in a reducing atmosphere. Such different photocatalytic characteristics suggest that efficiency for the separation of photoexcited charges (a pair of electrons and holes) which is the most important step in photocatalysis is... 

Photoexcitation of the electrode (of the electrons in its surface layer) and subsequent reaction of ordinary, nonactivated reactants at the electrode (Section 29.3)... 

Electron photoemission from an electrode into an electrolyte solution, which yields solvated (hydrated) electrons, can be regarded as a particular case of reactions with photoexcitation of the electrode (Section 29.2). 

Upon illumination, photons having energy higher than the band gap (eg = ec — v) are absorbed in the semiconductor phase and the electron-hole-pairs (e //i+) are generated. This effect can be considered equivalent to the photoexcitation of a molecule (Fig. 5.57) if we formally identify the HOMO with the ec level and LUMO with the v level. The lifetime of excited e //i+ pairs (in the bulk semiconductor) is defined analogously as the lifetime of the excited molecule in terms of a pseudo-first-order relaxation (Eq. 5.10.2). 

Run/Nin heteronuclear complexes such as (653), in which a photosensitizer [Ru(bpy)3]2+ or [Ru(phen)3]2+ is covalently attached to the Ni1 cyclam complex, have been synthesized in order to improve the efficiency of electron transfer from the photoexcited photosensitizer to the catalytic site.1 44-1646 However, these complexes did not perform particularly well, either due to unfavorable configuration of the Nin-cyclam subunit and the resulting steric hindrance or due to short lifetime of the excited states of the Ru photosensitizer moieties. A stable catalytic system has been prepared by immobilizing macrocyclic Ni11 complexes and [Ru(bipy)3]2+ in a nafion membrane.164... 

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