Transitions from neutral donor states

Evaluation of the Work Term from Charge Transfer Spectral Data. The intermolecular interaction leading to the precursor complex in Scheme IV is reminiscent of the electron donor-acceptor or EDA complexes formed between electron donors and acceptors (21). The latter is characterized by the presence of a new absorption band in the electronic spectrum. According to the Mulliken charge transfer (CT) theory for weak EDA complexes, the absorption maximum hv rp corresponds to the vertical (Franck-Condon) transition from the neutral ground state to the polar excited state (22). 

The removal of an electron from an acceptor level or a hole from a donor level denotes, as we have seen, not the desorption of the chemisorbed particle but merely its transition from a state of strong to a state of weak bonding with the surface. The neglect of this weak form of chemisorption (i.e., electrically neutral form) which is characteristic of all papers on the boundary-layer theory of adsorption makes it quite impossible to depict the chemisorbed particle in terms of an energy level, i.e., to apply the energy band scheme depicted in Fig. 10 and used in these papers. ... 

The values of gh derived for the U and I9 lines are very close to the gh = -1.24 obtained for the hole involved in the exciton bound to ionized donor and to the g factor of the hole in li free exciton state. " On the other hand, the expected g/ values of the holes involved in the acceptor bound exciton transitions differ significantly from the g values of the holes involved into excitons bound to ionized or neutral donors. This is similar to the situation found in CdS. Therefore we conclude that both U and I9 transitions should be assigned to the (Do,2Ci(r7)) complex rather than to the (y4o(T7) A(T7)). 

Identification of residual and doped donors have been identified in expitaxial GaAs using the photolumines-cence technique in the presence of applied magnetic fields. Transitions occur between excited initial and final states of the neutral-donor-bound-exciton complexes. The magnetic field compresses the wave function which sharpens the optical transitions. The magnetic field also separates the different donors when viewed from the neutral-donor-bound-exciton transitions. These two effects make possible the identification of donors when the donor concentration is in the mid lOlScm" range. 

The donors were Identified from the collapse of excltons bound to neutral donors. The optical transitions involve the collapse of exclton complexes whose initial state consists of an exciton bound to the first nonrlgld rotational state of the neutral donor and whose terminal state consists of the 2p state of the neutral donor. The most intense transitions in this process are the spin conserving transitions. The measurements were made in an applied magnetic field of 36kG. The transitions to the 2p, state are shown in Figure 6. The dashed curve for sample A shows the spin conserving transitions for the Si donor in this Si doped... 

In crystals which are composed of two different partner molecules, CT excitations and with them CT excitons are frequently the predominant lowest excitation states and are thus responsible for the lowest-energy transitions in the singlet system. We will illustrate this using the example of the weak donor-acceptor complex anthracene/pyromellitic acid dianhydride, (A/PMDA) (Fig. 6.14). The ground state is neutral and nonpolar, with only a small charge-transfer fraction. The lowest optical excitation starts from the ground state of the donor D, anthracene, (from its highest occupied orbital or HOMO) and leads to the lowest unoccupied orbital (LUMO) of the acceptors A, PMDA, within the mixed stack DADADA. A polar ex-... 

The main question concerning the primary step in photosynthesis is as follows how the excited state of the primary electron donor P in reaction centers (Res) is converted into a charge transfer state (see / / for review) Is the latter state formed within P directly from its excited state P or does a neutral P donate an electron to neighboring bacteriochlorophyl1 (BL) and bacteriopheophytin (HL) monomers located in L protein subunit (revealed by x-ray studies /2/) Femtosecond measurements yielded contradictory conclusions /3, / The first measurements /5"7/ of holes burned at absorption band of P have not shown any narrow bands for P+Q.A"formation, yet narrow-band features have been observed for different reactions in Res /8,9/. Here we show that in open R.vi ridi s Res the holes burned in the region of 0-0 transition of P are broad (>50 cm ) while in closed Res a narrow hole is observed with weak electron-phonon coupling /10,11/ in agreement with the Stokes shift between absorption and fluorescence spectra at 1.7 K. 

---