Positive hole migration

These bacteria cannot in general oxidize water and must live on more readily oxidizable substrates such as hydrogen sulfide. The reaction centre for photosynthesis is a vesicle of some 600 A diameter, called the chromato-phore . This vesicle contains a protein of molecular weight around 70 kDa, four molecules of bacteriochlorophyll and two molecules of bacteriopheophy-tin (replacing the central Mg2+ atom by two H+ atoms), an atom Fe2+ in the form of ferrocytochrome, plus two quinones as electron acceptors, one of which may also be associated with an Fe2+. Two of the bacteriochlorophylls form a dimer which acts as the energy trap (this is similar to excimer formation). A molecule of bacteriopheophytin acts as the primary electron acceptor, then the electron is handed over in turn to the two quinones while the positive hole migrates to the ferrocytochrome, as shown in Figure 5.7. The detailed description of this simple photosynthetic system by means of X-ray diffraction has been a landmark in this field in recent years. 

Other indirect evidence for fast hole transport in 3-methyloctane at low temperatures (127 K) comes from pulse radiolysis studies (Gillis et al., 1977). Electrons and parent cations are characterized by their optical absorption spectra. While the cations absorb at 625 nm, the trapped electrons absorb at 2100 nm. They disappear after the radiation pulse by recombination. The temporal decay of the absorption spectrum was faster in the pure liquid as compared to a solution of 3-methyloctane and triethylamine, a hole scavenger. In the latter case, the resonant positive hole migration is terminated by formation of the triethylamine positive ion which has a much lower mobility. 

The positive hole (C204) migrated rapidly to a surface where subsequent decomposition may proceed either by the thermal or the catalytic path. The adsorbed species C02 has been identified [1057] by ESR in the decomposition of MgC204. 

Hole transfer (HT) is the movement of a positive chaige, say in cation radicals. In reality, it is still an ET process but it is often more convenient to focus on the migration of the positive hole (vacated by an electron) rather than the electron (which moves in the opposite direction to the hole). 

An alternative picture of the conductivity of the electrons and holes in intrinsic semiconductors is to consider the electrons in the conduction band as migrating, as expected, toward the positive potential, and to consider the holes as discrete, positive charges migrating in the opposite direction. Although electrons are responsible for conduction in both cases, the hole formalism represents a convenient physical picture. 

Both the styrene monomer and the neutral dimer can trap a migrating positive hole or positive charge from solvent radical-cations (solventt) or related cationic species, which leads to the formation of radical cations, dimer cations, and bonded dimer cations. 

---