Donor ionization

Optical absorption measurements give band-gap data for cubic sihcon carbide as 2.2 eV and for the a-form as 2.86 eV at 300 K (55). In the region of low absorption coefficients, optical transitions are indirect whereas direct transitions predominate for quantum energies above 6 eV. The electron affinity is about 4 eV. The electronic bonding in sihcon carbide is considered to be predominantiy covalent in nature, but with some ionic character (55). In a Raman scattering study of vahey-orbit transitions in 6H-sihcon carbide, three electron transitions were observed, one for each of the inequivalent nitrogen donor sites in the sihcon carbide lattice (56). The donor ionization energy for the three sites had values of 0.105, 0.140, and 0.143 eV (57). 

Since the energy of the transfer band is determined by the difference between the donor ionization potential and the acceptor electron affinity, this fact points to the increase of the PCS ionization potential with decreasing conjugation efficiency. Therefore, the location of the transfer band of the molecular complexes of an acceptor and various PCSs can serve as a criterion for the conjugation efficiency in the latter. In Refs.267 - 272) the data for a number of molecular complexes are given, and the comparison with the electrical properties of the complexes is made. 

Rhenium (III) halides appear to retain their cluster structure in donor solvents, although with strong donors ionization takes place 4 ). 

Both t-1 and c-1 form ground state charge-transfer complexes with strong electron acceptors (72-79). Equilibrium constants and absorption data for their complexes with several electron-poor alkenes are given in Table A. The absorption maxima of a family of charge-transfer complexes can be related to the donor ionization potential (IPp) and acceptor electron affinity (EA ) using eq. 16 (79). 

Fluorescence from the excited state complexes of t-1 and electron poor alkenes has been observed only with dimethylfuma-rate and fumaronitrile, both of which form weak ground state complexes with t-1 (76). Fluorescence of the same wavelength and lifetime is observed upon quenching of t or excitation in the charge-transfer absorption band of the complexes of t-1 with these acceptors. Some properties of these excited complexes and other exciplexes of t-1 are summarized in Table 7. Fluorescence maxima, like the absorption maxima, of related charge-transfer complexes, can be correlated with the donor ionization potentials (eq. 16). As shown in Fig. 3, the point for t-1 falls well below the line obtained by Shirota and co-workers (87) for the com-... 

Further indication of the minor contribution of charge-transfer to the stability of t-l-diene or alkene exciplexes is provided by the relative insensitivity of fluorescence quenching rates to donor ionization potential (Table 8). A plot of log kq versus Pip for quenching of t by dienes and vinyl ethers is shown in Fig. 6. Data for the two families of quenchers fall on different lines, both of which have slopes which are less steep... 

Donor ionization potentials were calculated with aid of the equation 0.87/D = hv + 4.86. 

Using Eq. (10) with z — 8.75 and m — 0.28m [10] for ZnO, the donor ionization energy for an electron is obtained to be 0.050 eV. The value of the donor ionization energy for an electron is about one third of the value for the hole, proportional to The experimental donor ionization energies of the... 

We have to make a comparison of the donor ionization energies between the experimental and calculated values, at least, for the monovalent substitutional impurities of group-IIIA elements at the host Zn site. It is, however, difficult due to a lack of detailed information of the experimentally obtained energy levels. 

The donor electrons are thermally excited into the larger density of conduction band states at elevated temperatures, reducing the neutral donor concentration. Donor ionization occurs in crystalUne siUcon near 20 K, but the corresponding effect in a-Si H begins at about 200 K. The ESR spin density of the neutral phosphorus donor... 

For complexes combining different donors with the same acceptor a linear relation between the ionization potential IP of the donor and the wave number of the CT band has been observed. (See Figure 2.39.) This correlation is much better than expected it requires that the Coulomb term C is either constant or proportional to the donor ionization potential IPp. 

The stabilization will Increase in these complexes as the difference between the donor ionization potential and the acceptor electron affinity Increases. 

Note that is the donor ionization potential in the polar solvent that may be different from the vacuum value. 

A the acceptor electron affinity and I donor ionization energy. When z = 0, the molecules are neutral overall, but E and EpQ arises from the charge variations within the molecules. These are both small, and we neglect them for simplicity. Moreover, we suppose that Ey yy and E,.R depend only weakly on z. Consequently, we have that... 

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