DEPOLARIZED DYNAMIC LIGHT-SCATTERING FROM 3 LOW-MOLECULAR-WEIGHT GLASS-FORMING LIQUIDS - A TEST OF THE SCATTERING MECHANISM

Depolarized dynamic light scattering (DDLS) experiments are performed on the glass forming materials ortho terphenyl (OTP), bis-methyl-metho xy-phenyl-cyclohexane (BMMPC), and bis-methyl-phenyl-cyclohexane (BMPC ). Depolarized spectra are obtained for the bulk liquids and for solut ions of varying concentration in CCl4. Effective optical anisotropies of the molecules are obtained from the solution spectra. A detailed an alysis of the DDLS spectra, as well asa comparison with the results of quasielastic neutron scattering (QENS) experiments, indicates that th e DDLS spectra in the bulk liquids result from at least two physical m echanisms: (i) The low frequency part of the spectrum, in the frequenc y range of the ct peak, arises mainly from the molecular reorientation mechanism. The integrated intensity of this contribution is proportio nal to the square of the effective molecular anisotropy. (ii) At highe r frequencies (but still below the Boson peak), the predominant contri bution to the DDLS spectrum arises from interaction-induced scattering . The intensity of this contribution increases with increasing tempera ture (decreasing density) in the way expected for the cancellation eff ect. The implications of these results for the comparison of DDLS and QENS experiments and for the comparison of the data to the predictions of mode coupling theories of glass forming liquids are discussed. (C) 1997 American Institute of Physics.