Abstract

Photochemical hole-burning has been used to measure the temperature dependence (between 0.3 and 20 K) of homogeneous linewidths (Γ_hom) of 0-0 S₁ ← s₀ transitions of a variety of organic molecules in alcoholic glasses and polymers ^[1]. It was found that Γ_hom obeys a T^1.3±0.1 law independent of the system studied. The results suggest that the guest only acts as a sensor to probe low-temperature relaxation processes in the amorphous state. Furthermore, it was observed for the first time that some of the organic glassy systems reach the lifetime-limited value of a few MHz at T ⩾ 0.3 K, which implies a very weak coupling between impurity and amorphous host. In all systems Γ_hom extrapolates to the T₁- limited value for T → 0. In addition, a cross-over from a T^1.3 to a linear power-law occurs in systems where Γ_hom ≲ 60 MHz, at a temperature which depends on the structure of the glass. This structure also determines the magnitude of Γ_hom: stiff alcoholic networks with a high degree of hydrogen bonding or linear polymer chains with no side chain groups (and, presumably, a high degree of short range order) yield very narrow linewidths of the same magnitude as in crystalline solids. This is in contrast to previous results in organic and inorganic glasses where differences of two to three orders of magnitude have been reported between amorphous and crystalline solids. From these results it appears that hole-burning can be used as a sensitive technique to study slow motions of polymer side chain groups at low temperatures.

© 1984 Optical Society of America