An instantaneous normal mode (INM) theory of condensed phase absorption spectra is presented. The present theory approximately includes contributions to the spectra from the imaginary frequency modes. The line-shapes of the absorptions are analyzed by way a motional narrowing parameter, Gamma = Delta tau(c). It is found that the entire intramolecular vibrational spectrum is motionally narrowed, and line-widths determined in this manner are in good agreement with intramolecular spectra obtained from time correlation function results. The low frequency, intermolecular vibrational spectrum is found to be in the motionally narrowed limit only below 50 cm(-1). The frequency region from 50-300 cm(-1) is intermediate between the slow and fast modulation limit. The far infrared (IR), collision-induced spectrum of dense liquid CO2 is calculated using the present theory and found to be in near quantitative agreement with experimental results. (C) 1997 American Institute of Physics.