  Molecules that trace the high-density regions of the interstellar medium have
been observed in (ultra-)luminous (far-)infrared galaxies, in order to initiate
multiple-molecule multiple-transition studies to evaluate the physical and
chemical environment of the nuclear medium and its response to the ongoing
nuclear activity.
  The HCN(1-0), HNC(1-0), HCO+(1-0), CN(1-0) and CN(2-1), CO(2-1), and CS(3-2)
transitions were observed in sources covering three decades of infrared
luminosity including sources with known OH megamaser activity. The data for the
molecules that trace the high-density regions have been augmented with data
available in the literature.
  The integrated emissions of high-density tracer molecules show a strong
relation to the far-infrared luminosity. Ratios of integrated line luminosities
have been used for a first order diagnosis of the integrated molecular
environment of the evolving nuclear starbursts. Diagnostic diagrams display
significant differentiation among the sources that relate to initial conditions
and the radiative excitation environment. Initial differentiation has been
introduced between the FUV radiation field in photon-dominated-regions and the
X-ray field in X-ray-dominated-regions. The galaxies displaying OH megamaser
activity have line ratios typical of photon-dominated regions.
