Thermal Physics of Clusters in Atomic Gases

The chapter presents a review of the author's discovering in the cluster physics of equilibrium atomic gases. The pure atomic gas system has been deeply investigated experimentally and theoretically by many researchers, but the nature of clusters was poorly understood. The clusters are considered now as a new state of matter. To study them is both challenging and educative. The pure real atomic gas presents an advanced platform for thermodynamics education: It is the first step after the atomic ideal gas simplified model towards more realistic model accounting for the atomic interactions. The author bases his research on a wonderful feature of pure real atomic gases: The universal basic particles' chemical potential for atoms in all clusters. It permits to find the monomer fraction density from an experimental pressure dependence of the total density. The author's analysis is based on the gas potential energy density series expansion by the monomer fraction density. The author applies his computer aided analysis of precise thermophysical data to atomic gases to demonstrate the unknown and no trivial properties of clusters in them. For this analysis precise experimental thermophysical data have been taken from the US National Institute of Standards and Technology (NIST) online Webbook. The found temperature dependences of the atomic interaction bond energies signal about the directional bonding of atoms in clusters near the triple point due to the lack of rotational symmetry of their electronic outer shells. The analysis accounts for the elastic repulsions’ contribution to the potential energy of the gas at elevated temperatures. The discovered anomalous density dependences of the constant volume heat capacity in gaseous Helium and Neon tell about unusual atomic interactions in clusters of these gases.