A microscopic theory of photonucleation: Density functional approach to the properties of a fluid of two-level atoms, a part of which is excited

Abstract

The studies of equilibrium properties of a gas of identical atoms, a part of which is in an excited electronic state, have attracted attention for the last forty years [1–10]. Such atoms may appear due to the electromagnetic irradiation with the frequency, which corresponds to the excitation energy of the atom. Since the life-time of the excited state is essentially larger than the time required for establishing the equilibrium over translational degrees of freedom, the system should exhibit equilibrium properties at a given (nonequilibrium) concentration of excited atoms. Moreover, owing to new effective long-range interatomic interactions – the resonance dipole-dipole interactions – one may expect essential changes of various equilibrium characteristics due to the presence of excited atoms. The analysis performed within the framework of the cluster expansion method confirmed these expectations (see [7,8] and references therein). On the other hand, there are only a few papers which may be related to experimental observations of theoretically investigated features of the gas with excited atoms. We should mention here the papers on the effect of irradiation on the condensation of iodine and anthracene vapor [11,12] and on the photonucleation [13–21], in particular, in vapors of mercury and cesium. The latter studies reported the quantitative results of resonance irradiation effect on the nucleation rate. The main conclusion of these studies is as follows: the resonance irradiation of the nucleation zone leads to a sharp increase of the nucleation rate. The experimental results, to our best knowledge, have not been explained so far. One may