  Limiting structure of thermodynamic functions of gaseous plasmas is under
consideration in the limit of low temperature and density. The remarkable
tendency, that was claimed previously [High Temp. 19, 799 (1981)], is carried
to extreme. In the limit discussed, when temperature and density tend to zero
at constant value of electron chemical potential, both equations of state
(thermal and caloric) obtain almost identical stepped structure ("ionization
stairs") when their special forms being exposed i.e. PV/RT for thermal EOS and
(U - 3/2PV) for caloric EOS. At the same time all thermodynamic differential
parameters (heat capacity, compressibility, etc.) obtain their remarkable
delta-like structures ("thermodynamic spectrum"). This limiting structure
appears within a fixed (negative) range of electron chemical potential
("intrinsic energy scale") that is bounded below by value of major ionization
potential and above by the value connected with sublimation energy of
substance. Binding energies of all possible complexes (atomic, molecular and
ionic) in its ground state are the only quantities that manifest itself in
meaningful details of this limiting picture as location and value of every
step. The limiting stepped structure of gaseous zero-Kelvin isotherm is generic
prototype of well-known "shell oscillations" in EOS of gaseous plasmas at low,
but finite temperatures and non-idealities. At the same time this limiting form
of plasma thermodynamics could be used as a natural basis for rigorous
deduction of well-known quasi-chemical approach ("chemical picture") in frames
of asymptotic expansion (on temperature, but not density) around this reference
system.
