engleski

Raman Studies of Structural Relaxation in Amorphous Film of Adamantane

We show that small cage-like molecules like adamantane form amorphous films easily. Adamantane films were prepared by vacuum evaporation and deposition onto cold metal substrate at temperature 10 K. The structure and structural relaxation of amorphous films of adamantane have been studied by Raman spectroscopy. The characteristic spectral changes, which reflect the structural change and transition from amorphous to the ordered crystalline state, are observed during the annealing process. The change of integrated Raman intensity and bandwidth of the 760 cm(-1) band with temperature is shown. This band is assigned to the totally symmetric CC stretching mode of the adamantane cage. The effect of Bose factor Error! Switch argument not specified.was accounted for in these spectra. The 760 cm(-1) band observed in amorphous film at 10 K was weak and significantly broader than that of crystalline films at the same temperature. When the temperature was raised from 10 K to 180 K, the increase in the intensity and decrease in the bandwidth were observed for all the measured bands. The intensity and the bandwidth after the crystallisation remained temperature independent. The weak and broad bands are observed in amorphous systems in the low frequency region because the translational symmetry is destroyed, the usual selection rule q = 0 Error! Switch argument not specified. breaks down and all vibrational modes become Raman active. According to Shuker and coworkers, reduced intensity of light scattering in disordered material is given by equation where C(omega)3 describes the coupling between light and vibrational modes, g(omega)4 is the density of states. The reduced Raman intensity I_R(omega) 5for amorphous adamantane films is shown. At low frequencies the reduced intensity I_R(omega) 6exhibits an (omega)^27 dependence. Such a frequency dependence of I_R(omega)8 has been observed in other amorphous systems as well. Assuming that the Debye-like (omega)^2 9behaviour of the density of states is also characteristic of amorphous materials, C(omega) 10parameter is frequency-independent in the low frequency region in amorphous adamantane films. However, accepting the propotionality, the density of states g(omega) becomes also linearly dependent on frequency. The band shape analysis in the whole region from 180 to 10 cm(-1) and at various temperatures from 10 to 120 K strongly indicates that the observed characteristics could be accounted for by the damped-oscillator mechanism.

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