Probing inhomogeneous bands

In Section 9.3 we have seen that there is in principle a close relationship between an absorption lineshape and the underlying dynamics of a molecule excited to the corresponding spectral region. The discussion in the previous section however has taught us that life is less simple In many systems the absorption lineshape is an average over many individual molecules that experience different local environments, 

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Suppose the dye probe molecule is photoreactive. Then, to understand the essential features of hole burning, it suffices to consider three states only the ground state Sq, the first excited singlet state Si, and the photoproduct state Pq (Figure 3). For simplicity we assume that Pq has an infinite lifetime. The absorption spectrum of Pq is shifted as compared to that of Sq because Pq represents a structurally different molecule. Consider Figure 4 narrow-bandwidth laser light is tuned to some position within the inhomogeneous band. We denote the respective frequency by Vl. Excitation leads to photochemistry hence, population is transferred to Pq and, concomitantly, Sq... 

EPR spectral profiles of paramagnetic probes in solid phases are determined by the anisotropic distribution of spin packets due to the nonaveraged magnetic anisotropies. The inhomogeneously broadened bands are scarcely informative, and the so-called hyperfine methods (see below) are used in these cases to extract information on the hyperfine parameters. 

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