Raman spectra and conformational relaxation in glassy molecular films

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Structure and structural relaxation in amorphous molecular films were studied by Raman and, partly, infrared spectroscopies. The films were prepared by vacuum evaporation onto cold metal substrates. Three groups of substance, normal hydrocarbons, polyphenyls, and polyacenes, were studied. Major results are summarized below.1. Structural Relaxation in Amorphous n-C_<24>H_<50>The initial films evaporated onto cold substrates were amorphous containing molecules with a large number of gauche conformation. When the temperature of the film was elevated stepwise, the number of trans conformation increased. Then, molecules with the all-trans conformation appeared. The films finally changed into the structure of the crystalline state in the temperature region around 260-280 K.2.Structural Relaxation in Amorphous BiphenylBiphenyl molecules were found to be twisted to the angle of about 30ﾟ in the amorphous state, and were changed at about 140 K into the flattened structure as in the room-temperature crystal. The amorphous film was found to change into a crystalline phase with an unknown structure when it was kept for a long time at a temperature below 140 K.3. Structural Relaxation in Amorphous Films of Naphthalene and AnthraceneMolecules of this group of substance may be considered to be rigid bodies. It has been expected that amorphous states of this kind of substance can exist only at very low temperatures. However, an amorphous film of naphthalene kept a disordered structure to the temperature as high as 120 K. In amorphous films of anthracene, an anomalous interaction between molecules found to exist even at temperatures above 200 K. These are new findings on the structure of amorphous molecular states.