Donor transition energy

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). 

Dipole-dipole coupling, in which the transition moments of donor and acceptor are strongly coupled.142 Such interactions can occur over large distances if both of the moments are large and the transition energies are matched so that the overall process is isoenergetic. 

Silicon substituted silylenes attract the chemists interest for a special reason. According to ab initio calculations 85 137 176], substituents acting as cr-donors should induce a relatively large red-shift of the UV maximum or, in other words, the n(Si) —> 3p(Si) transition energy should be relatively small. Therefore these species are potential candidates for the long-sought triplet ground state silylenes, especially if this electronic effect is supported by a steric one. Nevertheless, reports on the matrix isolation of silicon substituted silylenes are comparatively scarce. 

CT transition energy of the high-energy (hvH) and with sterically hindered donors) on the arene donor low-energy (hvi.) bands with the oxidation potential strength. Data from ref. [28]. 

Another feature clearly observed in FIGURE 1 is the sharp transition on the low energy side of the I2 line and labelled I2 (n = 2). It is interpreted as a two electron satellite of the donor bound exciton line, i.e. due to a recombination where the donor is left in its n = 2 excited state [6,19], From the 22 0.5 meV separation from the I2 line, a donor binding energy of 29 1 meV can be deduced [6,19], The weak transitions in FIGURE 1 at 3.27 and 3.18 eV, attributed to donor-acceptor pair recombinations, are discussed below. 

Skromme et al [24] claim to have identified a weak transition in the low temperature spectra of undoped MOVPE and MOMBE-grown GaN films as a two-electron replica of the D°X transition (in which the donor is left in its 2s excited state) which yields a value of 22 meV for the ls-2s separation and, therefore, a donor ionisation energy ED = 29 meV. 

Electron transfer reactions and spectroscopic charge-transfer transitions have been extensively studied, and it has been shown that both processes can be described with a similar theoretical formalism. The activation energy of the thermal process and the transition energy of the optical process are each determined by two factors one due to the difference in electron affinity of the donor and acceptor sites, and the other arising from the fact that the electronically excited state is a nonequilibrium state with respect to atomic motion (P ranck Condon principle). Theories of electron transfer have been concerned with predicting the magnitude of the Franck-Condon barrier but, in the field of thermal electron transfer kinetics, direct comparisons between theory and experimental data have been possible only to a limited extent. One difficulty is that in kinetic studies it is generally difficult to separate the electron transfer process from the complex formation... 

Figure 27 Molecular (D ), exciplex [(DA) — (A D )( T], charge-transfer (A -D+) excited species as generated by photo-excitation and electron-hole recombination processes in electron acceptor (A)-electron donor (D) molecular systems, hvD, hvEx, and hvEc are corresponding transition energies to the ground state, to be observed as different emission bands.

The unsubstituted thiopyrylium ion (2) has been found to form CT complexes in CHjCN with both olefins and aromatic hydrocarbons (72CL17 75BCJ1519). Two CT absorption bands have been observed in the former case, and one in the latter. The slope obtained by the plot of the CT transition energies vs the ionization potentials of donors is 0.27 for the olefin complexes and 1.04 for the aromatic hydrocarbon complexes. These slopes suggest that 2 interacts with olefins more strongly than with aromatic hydrocarbons. Strong interactions in the olefin complexes would manifest themselves also in the appearance of two CT bands. These have been ascribed to electronic transitions from the HOMO of the olefin donor to the lowest and the second lowest vacant orbital of 2. The CT absorption frequencies of the complexes of 2 with olefins and aromatic hydrocarbons have been used to calculate their heat of formation by an empirical relation (81MI4). 

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