  Molecules that trace the high-density regions of the interstellar medium may
be used to evaluate the changing physical and chemical environment during the
ongoing nuclear activity in (Ultra-)Luminous Infrared Galaxies. The changing
ratios of the HCN(1-0), HNC(1-0), HCO+(1-0), CN(1-0) and CN(2-1), and CS(3-2)
transitions were compared with the HCN(1-0)/CO(1-0) ratio, which is proposed to
represent the progression time scale of the starburst. These diagnostic
diagrams were interpreted using the results of theoretical modeling using a
large physical and chemical network to describe the state of the nuclear ISM in
the evolving galaxies. Systematic changes are seen in the line ratios as the
sources evolve from early stage for the nuclear starburst (ULIRGs) to later
stages. These changes result from changing environmental conditions and
particularly from the lowering of the average density of the medium. A
temperature rise due to mechanical heating of the medium by feedback explains
the lowering of the ratios at later evolutionary stages. Infrared pumping may
affect the CN and HNC line ratios during early evolutionary stages. Molecular
transitions display a behavior that relates to changes of the environment
during an evolving nuclear starburst. Molecular properties may be used to
designate the evolutionary stage of the nuclear starburst. The HCN(1-0)/CO(1-0)
and HCO+(1-0)/HCN(1-0) ratios serve as indicators of the time evolution of the
outburst.
