Intersystem crossing transition

Intersystem crossing is a spin-forbidden process between states of different multiplicity, so the magnitude of the spin-orbit coupling is important in controlling the rate of intersystem crossing. Transitions between... 

In this simple description, we have not considered possible additional effects such as excitation to a higher excited state, saturation effects, or intersystem crossing (transition to the triplet state). These will be ignored throughout for simplicity. 

Unimolecular reactants are energized by a variety of experimental techniques including collisional and chemical activation, internal conversion and intersystem crossing transitions between electronic states, and different photo-activation techniques, which include excitation of isolated resonance states for reactants with a low density of states (see also Sect. 3). Trajectory simulations usually begin with the preparation of an ensemble of trajectories, whose initial coordinates and momenta resemble — as close as possible — those realized in a particular experiment [20,329]. 

The rate of internal conversion (IC), a radiationless transition between isoenergetic levels of different states of the same multiplicity, may be of the same order of magnitude or even faster than vibrational relaxation. It depends, however, on the energy separation AEqo between the zero-vibrational levels of the electronic states involved (energy gap law, see Section 5.2.1). Similar relations hold for intersystem crossing transitions between states of different multiplicity, which are slower by 4-8 orders of magnitude. 

In the case of intersystem crossing transitions between states of different multiplicity, an additional spin-orbit coupling term Ago has to be considered. From the perturbational expansion it follows that the contribution... 

Fig. 1. Diagramme of radiative (a - absorption, fl - fluorescence, ph - phosphorescence), radiationless (ic - internal conversion, isc - intersystem crossing) transitions and processes of vibrational relaxation (vr) in organic molecules...

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

FIGURE 7.4 Modified Jablonski diagram showing transitions between excited states and the ground state. Radiative processes are shown by straight lines, radiationless processes by wavy lines. IC = internal conversion ISC = intersystem crossing, vc = vibrational cascade hvf = fluorescence hVp = phosphorescence. 

Thus if one starts with one pure isomer of a substance, this isomer can undergo first-order transitions to other forms, and in turn these other forms can undergo transitions among themselves, and eventually an equilibrium mixture of different isomers will be generated. The transitions between atomic and molecular excited states and their ground states are also mostly first-order processes. This holds both for radiative decays, such as fluorescence and phosphorescence, and for nonradiative processes, such as internal conversions and intersystem crossings. We shall look at an example of this later in Chapter 9. 

Figure 9.1. A Jablonski diagram. So and Si are singlet states, Ti is atriplet state. Abs, absorption F, fluorescence P, phosphorescence IC, internal conversion and ISC, intersystem crossing. Radiative transitions are represented by full lines and nonradiative transitions by dashed lines...

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. 

The interconversion between different spin states is closely related to the intersystem crossing process in excited states of transition-metal complexes. Hence, much of the interest in the rates of spin-state transitions arises from their relevance to a better understanding of intersystem crossing phenomena. The spin-state change can alternatively be described as an intramolecular electron transfer reaction [34], Therefore, rates of spin-state transitions may be employed to assess the effect of spin multiplicity changes on electron transfer rates. These aspects have been covered in some detail elsewhere [30]. 

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