Nonphotochemical Hole Burning and the Nature of Amorphous Solids

摘要

Nonphotochemical hole burning in the absorption spectra of impurity molecules dissolved in amorphous solids has been studied for several impurity-host systems. These systems are divided into four classes according to holeburning occurrence and hole shape. The hole formation mechanism and possible expanations for each class of holes are discussed. Hole burning of tetracene in 3:2:2 glycerol/dimethylsulfoxide/N,N-dimethylformamide was studied as a function of burn temperature (T/sub B/), and annealing (hole disappearance) was studied for T > T/sub B/. Hole widths show that the pure dephasing of the impurity electronic state is orders of magnitude faster than found in crystalline media. This anomalously fast dephasing is explained by a theory considering the excited impurity to interact with two level systems (TLSs) is the host solid which undergo rapid phonon assisted tunneling transitions involving very low frequency phonons. The high temperature limit, where the dephasing frequency is linear in T, occurs in this theory for T less than or equal to 1K, proving an estimate of the width of the distribution function f(epsilon), where epsilon denotes the TLS asymmetry. The relationship between this theory and similar theories for TLS dephasing of fluorescence transitions of rare earth ions in inorganic glasses is discussed. A new theory is developed to explain the annealing process. The TLS barrier height is allowed to vary with the order parameter of an order-disorder transition assumed to be characteristic of amorphous solids. A constant distribution of critical temperatures based on the degree of local ordering of the host solid gives a linear intensity decrease with annealing temperature as observed for thermally activated barrier crossings. This theory is shown to be consistent with the hole formation and dephasing theories and all existing data. (ERA citation 08:012737)