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Recently, there have been a number of beautiful experimentsl-4 on nonphotochemical hole burning (NPHB) in organic glasses. A low concentration of some molecule (eg, tetracene) is put into a glass system (eg, ethanOl-methanol) and the sample is irradiated with a narrow laser line for some length of time (minutes). Because of the amorphous nature of the system, the optical line shape has a large. inhomogeneous width. After the laser irradiation, the line shape hal;> a hole in it which relaxes very slowly (on the scale of hours or more). An explanation for this has been suggested by Small and co-workers1: there are many sites in the glass at which guest molecules sit, with a slightly different optical excitation. In the ground state, the rate of conversion of one configuration to another is very slow; in the excited state, however, this conversion (tunneling) takes place with ease. Thus, the environment of an excited molecule can convert into another environment (with a different absorption frequency for the molecule). The molecule then optically relaxes to the ground state of the new environment. The net effect is thus to deplete the population of molecules at the original site, burning a hole in the inhomogeneous line.

Other pertinent experimental details are (a) the burn times are-20 min-l h for photon fluxes of-l0 mW/mm and (b) the hole can be filled by irradiating with a laser at-1-2 cm-1 from the hole. Finally, this phenomenon occurs mainly in amorphous materials. 1.2