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Neutron scattering study of the picosecond dynamics of polybutadiene and polyisoprene.

Neutron time-of-flight data of polybutadiene and polyisoprene in a wide temperature range around the glass transition are reported. The anharmonic part of the spectra is evaluated using a recently developed time domain Fourier transform technique. The results corroborate the recent finding of two different time domains in the dynamics, one above a crossover time of about 2 ps and the other below it. Two different interpretations of the short time behavior are considered, namely a fast Debye regime of the \ensuremath{\alpha} relaxation and a vibrational softening of the boson peak.