The Fourier power spectrum of a transient spectrum, calculated for a model composed of a three-state electronic system coupled to a set of displaced harmonic oscillators, is shown to depend on the pulse duration. The condition under which the wavepacket motion on the harmonic potential substantially deviates from that of the classical mass point is derived. In such an excited state, the Fourier power spectrum has enhanced components with frequencies of harmonics even in a system composed of ideally harmonic potentials. The utility of a Fourier analysis of the spectrum for clarification of the squeezed molecular vibrational state is discussed. (c) 2005 Elsevier B.V. All rights reserved.