Radiative with band overlap

Fig. Ila-d. Radiative transitions in solids. Conductor a with incompletely filled conduction band b with band overlap c insulator or semiconductor depending on the width of the gap and d insulator with lattice defects. From Alonso M, Finn EJ. Fundamental University Physcs, vol Vlll. Copyright Addison-Wesley Publishing Company. Reprinted by permission...

Radiative transitions may be considered as vertical transitions and may therefore be explained in terms of the Franck-Condon principle. The intensity of any vibrational fine structure associated with such transitions will, therefore, be related to the overlap between the square of the wavefunctions of the vibronic levels of the excited state and ground state. This overlap is maximised for the most probable electronic transition (the most intense band in the fluorescence spectrum). Figure... 

Radiative transfer plays a role essentially when the absorption band of the acceptor ion is allowed. A photon emitted by an ion is absorbed by an other ion before escaping from the material. This requires overlap of the emission spectrum of the donor with the absorption spectrum of the acceptor. Radiative transfer between identical ions causes a modification of the spectral distribution. This is the case for the Ce + emission when the Stokes shift is small. The cerium emission originates from the lowest 5d state and consists of two bands because the ground state 4f is split by spin-orbit coupling into the states Fs/2 and F7/2 (Figme 5), the shorter-wavelength component 5d 4f( Fs/2) having the... 

Surface plays an important role in excited state relaxation processes. In the ideal case of a three-dimensionally confined exciton, one expects to see strong exciton luminescence due to enhanced overlap of the electron and hole wavefunction. The radiative rate of the exciton should increase with increasing cluster size. In reality, this is generally not observed. Most of the luminescence spectra of semiconductor nanoclusters consist of a stokes-shifted broad luminescence band, usually attributed to emission from surface defects. Sometimes near the band edge, an exciton-like luminescence band can be observed. Various passivation procedures have been used to enhance the exciton luminescence. These are discussed in Section III. 

The photophysical processes of semiconductor nanoclusters are discussed in this section. The absorption of a photon by a semiconductor cluster creates an electron-hole pair bounded by Coulomb interaction, generally referred to as an exciton. The peak of the exciton emission band should overlap with the peak of the absorption band, that is, the Franck-Condon shift should be small or absent. The exciton can decay either nonradiatively or radiative-ly. The excitation can also be trapped by various impurities states (Figure 10). If the impurity atom replaces one of the constituent atoms of the crystal and provides the crystal with additional electrons, then the impurity is a donor. If the impurity atom provides less electrons than the atom it replaces, it is an acceptor. When the impurity is lodged in an interstitial position, it acts as a donor. A missing atom in the crystal results in a vacancy which deprives the crystal of electrons and makes the vacancy an acceptor. In a nanocluster, there may be intrinsic surface states which can act as either donors or acceptors. Radiative transitions can occur from these impurity states, as shown in Figure 10. The spectral position of the defect-related emission band usually shows significant red-shift from the exciton absorption band. 

The absorption bands of several trace gases overlap those of water drops and ice crystals. Because of the strong absorption properties of clouds, the infrared absorption by such trace gases is diminished in the presence of clouds. For example, calculations by Ramaswamy reported in IPCC (1995) show that the radiative forcing resulting from a 1 ppb increase in CFCI 3 in an overcast, midlatitude summer atmosphere with cloud top at 3 km is 37% less than the corresponding clear sky value (0.35 W m ). This reduction increases to 83% if the cloud top is at 10 km. 

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