Internal conversion multiple electronic states

Internal conversion between electronic states of the same spin multiplicity and intersystems crossing between singlet and triplet states take place because of weak interactions between the initial and final states. The perturbing interaction is usually limited in time and space, and the total Hamiltonian can be considered as the sum of two terms... 

Section V.D described the competition of two pathways in the H2 + CO molecular channel. There are also multiple pathways to the radical channel producing H + HCO. In aU cases, highly vibrationally excited CH2O is prepared by laser excitation via the So transition. In the case of the radical channel discussed in this section, multiple pathways arise because of a competition between internal conversion (S o) and intersystem crossing ( i T ), followed by evolution on these electronic states to the ground-state H + HCO product channel. Both electronic states So and Ti correlate adiabatically with H + HCO products, as shown in Fig. 7. 

Internal conversion refers to radiationless transition between states of the same multiplicity, whereas intersystem crossing refers to such transitions between states of different multiplicities. The difference between the electronic energies is vested as the vibrational energy of the lower state. In the liquid phase, the vibrational energy may be quickly degraded into heat by collision, and in any phase, the differential energy is shared in a polyatomic molecule among various modes of vibration. The theory of radiationless transitions developed by Robinson and Frosch (1963) stresses the Franck-Condon factor. Jortner et al. (1969) have extensively reviewed the situation from the photochemical viewpoint. 

The possible fate of excitation energy residing in molecules is also shown in Figure 2. The relaxation of the electron to the initial ground state and accompanying emission of radiation results in the fluorescence spectrum - S0) or phosphorescence spectrum (Tx - S0). In addition to the radiative processes, non-radiative photophysical and photochemical processes can also occur. Internal conversion and intersystem crossing are the non-radiative photophysical processes between electronic states of the same spin multiplicity and different spin multiplicities respectively. 

Internal conversion is a non-radiative transition between two electronic states of the same spin multiplicity. In solution, this process is followed by a vibrational relaxation towards the lowest vibrational level of the final electronic state. The excess vibrational energy can be indeed transferred to the solvent during collisions of the excited molecule with the surrounding solvent molecules. 

Radiationless transitions among electronic states of molecules represent a class of relaxation processes that are electronic in nature. The general term electronic relaxation appears to be appropriate for these processes,23 but it is convenient to divide those transitions involving a change in the bound electronic states of a molecule into two categories Transitions between states of the same multiplicity, referred to as internal conversion, and transitions between states of different multiplicity, referred to as intersystem crossing. Although there are several early experimental... 

Internal Conversion.—Nonradiative transition between states of like multiplicity. The process is conceived of as involving iso-energetic transitions from a higher electronic state to an upper vibrational level of a lower state (cf. Fig. 1). To consummate the change, transfer of vibrational energy to the environment (external conversion) must occur rapidly. Since some authors feel that the transition between states in solution is directly coupled with solvent phonon states, the distinction... 

An internal conversion (IC) is observed when a molecule lying in the excited state relaxes to a lower excited state. This is a radiationless transition between two different electronic states of the same multiplicity and is possible when there is a good overlap of the vibrational wave functions (or probabilities) that are involved between the two states (beginning and final). 

Non-radiative processes can also be distinguished on the basis of the spin multiplicity of the initial and final electronic states. Internal conversion (IC) is the non-radiative crossover between two states of identical multiplicity, while intersystem crossing (ISC) is a process in which spin is not conserved. In both instances, crossover between the states is isoenergetic, regardless of the multiplicity. Subsequent vibrational relaxation (VR) occurs to release excess vibrational energy (see Figure 2.12). 

Internal conversion A photophysical process. Isoenergetic radiationless transition between two electronic states of the same multiplicity. When the transition results in a vibrationally excited molecular entity in the lower electronic state, this usually undergoes deactivation to its lowest vibrational level, provided the final state is not unstable to dissociation. 

FIGURE 1. A schematic photochemical mechanism, showing some of the possible elementary transformations. For the purpose of illustration, it is assumed that the states A and A2 have the same multiplicity, and correspond to the ground and lowest excited singlet states of most organic molecules. The state A] would then represent the lowest triplet state. Thus 21 and 11 are radiative transitions, fluorescence and phosphorescence, respectively, and 23 and 13 (intersystem crossing) and 22 (internal conversion) are nonradiative. All of 8, C, D, and F are chemical species distinct from A. Only vibrationally equilibrated electronic states are included in this mechanism (see discussion in Section III.A.l). 

In the same sense, in condensed media, it is often assumed that upper excited electronic states relax rapidly by internal conversion and vibrational equilibration to the lowest excited electronic state of the same multiplicity. Thus, referring to Figure 3, which extends the mechanism of Figure 1, if two excited states, A2 and A, have the same multiplicity, it is likely that the upper state, A, would undergo rapid internal conversion to A2 in solution. The sequence 03-32 could then be considered as the equivalent of step 02, provided that neither of steps 34 or 35 could compete, under these conditions, with step 32. A common diagnostic test (but not proof) of this situation is independence of the quantum yield on wavelength. 

Various primary processes induced upon photon absorption by this molecule are also shown in Fig. 2-1. The photon absorption processes associated with the vibrational-electronic transitions from So to Si and S2 are represented by So ->Si Abs. and So —>82 Abs., respectively. By internal conversion (IC) we mean a radiationless process between two different electronic states of the same spin multiplicity. In Fig. 2-1, IC from S2 to S and IC from Si to So are shown. Usually, the rate constants of S2 Si IC and Si —>So IC are more than lO s and 10 -10 s , respectively. By intersystem crossing (ISC) we mean a radiationless process between two different electronic states of two different spin multiplicities. In Fig. 2-1, Si Ti ISC and Ti—>So ISC are shown. The rate constants of the... 

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