According to the modern conception, the dynamics of amorphous solids in the intermediate interval of low temperatures (from a few up to dozens of Kelvins) is determined mainly by quasi-localized low-frequency vibrational modes (LFMs). Up to now, it is known very little about a nature and properties of these excitations in disordered solids. High-selective laser spectroscopy of impurity centres, embedded to transparent disordered matrix as a probe, is a very powerful method for deriving information about LFMs. In the two presented papers the results of our photon echo (PE) and single molecule spectroscopy (SMS) studies of LFMs in organic amorphous solids are discussed. In the first part we review the recent results of our PE-studies. Two cases are analyzed: (a) a coupling of chromophores with a continuous broad spectrum of LFMs, which shape was taken from light scattering experiments, and (b) a coupling of chromophores with continuous LFMs spectra, calculated on the base of the soft potentials model. In the second part we consider the results of our studies of LFMs in a glassy polymer on microscopic level using SMS.
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