Many molecules studied by ultrashort pulse spectroscopic techniques contain modes which have both short and long periods relative to the time scale of excitation. The influence of the short period (high frequency) modes on the ultrashort pulse absorption process is usually neglected. In this paper we examine the effect of the high frequency modes on the energetics of ultrashort pulse absorption, and construct a method for calculating observables for polyatomic systems containing both high and low frequency modes. We compare our results with experimental data for the iodobenzene system.