Vibrational properties of tetrahedral amorphous carbon from first principles

We study the vibrational properties of hydrogenated tetrahedral amorphous carbon from first principles. Our results reproduce the C density of states (DOS) measured by electron energy loss. We decompose the theoretical DOS in terms of vibrational modes of microscopic units. These partial DOS have large overlaps in frequency. Only the region between 1350 and 1600 cm(-1) is covered by a single type of stretching (sp(2)-sp(2)), whose DOS reproduces the experimental visible Raman spectra. We find a linear correlation between CH stretch frequency and CH bond length, and we show that the spreading of CH distances due to the constraints imposed by the C network is an important source of broadening of the CH stretching spectrum. (C) 1999 American Institute of Physics. [S0003-6951(99)02331-1].