  In a previous work, we showed that the observed FIR/sub-mm/mm line spectra of
a starburst galaxy (M 82) can be successfully modeled in terms of the
evolutionary scheme of an ensemble of giant molecular clouds (GMCs) and shells,
and such studies can usefully constrain the age(s) or star formation history of
a starburst galaxy. In this paper we present a preliminary study of using the
template of an ensemble of evolving GMCs/shells we developed for M 82. we apply
the model to represent various stages of starburst evolution in a well known
sample of nearby luminous infrared galaxies (LIRGs). In this way, we attempt to
interpret the relationship between the degree of molecular excitation and ratio
of far-infrared (FIR) to $^{12}$CO (or simply CO) luminosity to possibly
reflect different stages of the evolution of star-forming activity within their
nuclear regions.
