Spectral hole-burning study of boson peaks in molecular glasses

Pseudophonon wings have been investigated by means of persistent spectral h ole burning in the optical absorption of tetra-tert-butyl-tetraazaporphine (t-Bu-TAP) embedded in frozen solvent glasses at 6 K. The maxima of pseudop honon sidebands (nu(ph)) are shifted to the lower frequencies by 7-31 cm(-1 ) from the purely electronic hole. The nu(ph) depends on the molecular mass (M) of the glass former, being linear with M-1/2 and extrapolating to zero frequency at infinite mass. The intermolecular force constant is close to the value estimated from the London formula for dispersive interaction. As a result of disorder the "phonon" is most probably confined to a short chai n (or just a pair) of molecules, whereas the moving entity is the molecule as a whole. [S0163-1829(98)01946-8].