SPECTRAL HOLE-BURNING - SPONTANEOUS AND PHOTOINDUCED TUNNELING REACTIONS IN LOW-TEMPERATURE SOLIDS

Spectral hole burning is frequently observed in molecular solids when a solute dye molecule in a solvent matrix at very low temperature is i rradiated with light having a narrow frequency bandwidth. This phenome non relies on photo induced changes of the absorption spectrum of the solute. All photochemical reactions lead in principle to such changes, but the extreme sensitivity of the method makes it particularly suita ble for the characterization of reactions that occur at cryogenic temp eratures with very small yield and that would be difficult to observe by conventional methods. Here we will discuss in more detail the contr ibution of proton tunneling reactions to spectral hole burning and in particular new results obtained in pentacene doped benzoic acid crysta ls, which is an excellent model system.