Valence changes absorption

It is more difficult to study equilibria between transition metal allyl compounds and bases, olefins, etc. In the case of Zr (allyl) 4 and pyridine, a valency change occurs as shown by Eq. (8), and the process is irreversible. The polymerization is considered to be preceded by displacement of one allyl group by the monomer (12) as shown in Eq. (1). In the methyl methacrylate/Cr(allyl)3 system it was not possible to detect any interaction between the olefin and catalyst with infrared radiation, even with equimolar concentrations because of the strong absorption by the allyl groups not involved in the displacement processes. Due to the latter, evidence for equilibrium between monomer and catalyst is less likely to be found with these compounds than with the transition metal benzyl compounds. 

Hydrogen absorption in Ce2Fe14B was studied in detail by D. Fruchart et al. (1987). These authors reported that the hydrogen induced increase in Curie temperature is approximately three times stronger than in the other R2Fe14B compounds and discussed this property in terms of a valence change of Ce. 

A change of the absorption edge energy position is called the chemical shift when the element valence changes. 

Optical properties of rare-earth doped YAG ceramics are important requirements for practical applications, which has been a subject of numerous studied in the literature [170-176]. For example, thermal annealing had a strong effect on optical properties of Cr-doped YAG ceramics [170]. After annealing at high temperatures, the absorption intensity was increased and the two peak wavelengths shifted from 430 to 600 nm to approximately 465 and 612 nm, respectively, which could be attributed to valence changing from Cr " to Cr". The presence of Cr" ion was confirmed by emission measurement. 

The most important and characteristic feature of 3d-4f transitions in Sm-group elements and compounds is the absence of any observable change in the widths of the absorption lines when the valence change occurs. Quantitative estimates of the fractional valence in mixed-valence compoimds (Kaindl et al. 1985) for this group of elements can be performed by using the weighted contributions firom corresponding pure valent, divalent or trivalent compounds. 

The L i spectrum of Eu in EuPd2Si2 exhibits two clearly resolved lines whose individual intensities vary drastically with temperature or pressure induced valence changes. This spectrum may serve as an example for a rather reliable determination of the individual lineshapes and linewidths. Here the atomic-like absorption lines for reasons of simplicity are assumed to exhibit the same shape. The spectral densities dyj/d n and dfj/dsi, however, are clearly different > ejt- In other words, the trivalent (4f M ) absorption line turns out to be broader by e,/ jt = 1.22 than the divalent (4f 5d ) line (the asterisk denotes the excited state). The corresponding... 

Finally we note that the cellular model also provides a hint as to the direction of possible valence changes in the intermetallics RM upon hydrogenation, when R is one of the elements Ce, Eu or Yb. Hydrogen absorption results in the formation of an appreciable area of contact between R atoms and H atoms at the expense of R-M contacts. The R atoms therefore will tend to adopt the same valences they have in the binary hydrides, i.e., Ce tends to be trivalent, Eu and Yb tend to be divalent. This agrees with experimentally observed valence changes (Buschow et al., 1977 Buschow, 1980b). 

The two-dimensional carrier confinement in the wells formed by the conduction and valence band discontinuities changes many basic semiconductor parameters. The parameter important in the laser is the density of states in the conduction and valence bands. The density of states is gready reduced in quantum well lasers (11,12). This makes it easier to achieve population inversion and thus results in a corresponding reduction in the threshold carrier density. Indeed, quantum well lasers are characterized by threshold current densities as low as 100-150 A/cm, dramatically lower than for conventional lasers. In the quantum well lasers, carriers are confined to the wells which occupy only a small fraction of the active layer volume. The internal loss owing to absorption induced by the high carrier density is very low, as Httie as 2 cm . The output efficiency of such lasers shows almost no dependence on the cavity length, a feature usehil in the preparation of high power lasers. 

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