Nonphotochemical hole burning and relaxation dynamics of amorphous solids at low temperature

Nonphotochemical hole burning (NPHB) is applied to cresyl violet perchlorate (CV) in polyvinyl alcohol (PVOH) films. The results of temperature-dependent NPHB, polarization NPHB, spontaneous hole filling (SPHF) and laser induced hole filling (LIHF) are presented. A new mechanism of NPHB is proposed. The relaxation dynamics of amorphous solids at low temperature is extensively studied and a high degree of positive correlation between hole burning and SPHF has been found;New temperature-dependent NPHB results indicate that the simple model for spectral hole production based on a static distribution of two-level systems (TLS) provides an inadequate description. The temperature-dependent hole profile are ascribed to thermally assisted photo-induced relaxation processes. A new mechanism for NPHB, based on an outside-in hierarchy of tunnelling events is proposed in which the time evolution of free volumes plays an important role. The model is consistent with the available data and lends itself to testing by further experiments. The results of polarized hole burning experiments suggest that there is a rotation of the probe molecule at higher temperatures and support the new model;Both NPHB and SPHF can be described with dispersive kinetics theory. A more physically reasonable interpretation of SPHF kinetics is described by a new methodology in which absolute hole filling is considered. The data on absolute hole filling together with the observation of a high degree of positive correlation between the rates of burning and filling indicate that the dominant mechanism for filling is not global spectral diffusion but rather anti-hole reversion. The results of LIHF on cresyl violet in PVOH films established that the primary mechanism of laser-induced hole filling (LIHF) is electronic excitation of the anti-hole sites produced by the primary burn at [omega][subscript] B for [omega][subscript] S \u3e [omega][subscript] B, which can account for the principal features of LIHF. The observation that anti-hole sites undergo a red-shift upon [omega][subscript] S-excitation shows that the anti-hole sites retain at least a partial memory of their pre-burn configurations and suggests that the notion of a more or less well-defined TLS[subscript] ext coordinate(s) at a given burn temperature can be retained in mechanistic models which go beyond the standard two-level systems model