  Based on accurate Lennard-Jones type interaction potentials, we derive a
closed set of state equations for the description of warm atomic gases in the
presence of ionization processes. The specific heat is predicted to exhibit
peaks in correspondence to single and multiple ionizations. Such kinetic
analogue in atomic gases of the Schottky anomaly in solids is enhanced at
intermediate and low atomic densities. The case of adiabatic compression of
noble gases is analyzed in detail and the implications on sonoluminescence are
discussed. In particular, the predicted plasma electron density in a
sonoluminescent bubble turns out to be in good agreement with the value
measured in recent experiments.
