Experimental evidence of the local character of vibrations constituting the Boson peak in amorphous solids

We report the first measurement of the density of states of low-energy vibrational excitations in a disordered solid via single-molecule (SM) spectroscopy. Optical spectra of many single tetra-tert-butylterrylene (TBT) molecules embedded to amorphous polyisobutylene (PIB) as spectral probes were recorded at low temperatures. The T dependences of SM spectral linewidths showed the broad distribution of local frequencies of vibrations under study. The obtained distribution was compared with the "Boson peak" in pure PIB measured in [R. Inoue, J. Chem. Phys. 95, 5332 (1991)] by neutron scattering. We found that embedding of a small amount of TBT into PIB does not influence markedly on the observed vibrational dynamics. These results prove the local character of low-energy vibrational excitations in glasses and the existence of relationship between these excitations and the Boson peak.