  We have observed the characteristic temperature dependence of the
intermolecular phonon spectrum in the organic dimer Mott insulator
kappa-(ET)2Cu2(CN)3 exhibiting a dielectric anomaly at 30 K. The anomalous
spectral narrowing of the 55 cm-1 phonon peak at 30 K was analyzed in terms of
motional narrowing within the framework of a stationary Gaussian process, i.
e., the phonon frequency is modulated by the ultrafast charge fluctuation. The
spectral narrowing occurs because the time constant of the correlation time
tau_c and the amplitude of the frequency modulation delta satisfy the relation
tau_c<delta at 30 K. At temperatures below 30 K, the motional narrowing is
disturbed by the increasing of tau_c, near the charge-glass or the short-range
order at 6 K. On the other hand, for temperatures above 30 K, the motional
narrowing is disturbed by the increase of delta with increasing temperature.
