Conditions for forming Rydberg matter: condensation of Rydberg states in the gas phase versus at surfaces

The recent experiments on processes in cold Rydberg gases produced by laser excitation have so far only a weak relation to Rydberg matter (RM) and its formation, despite some attempts to make such a connection. The problems of forming RM from cold Rydberg gases are severe and due to at least five obvious factors: the condensation energy, the non-circular states formed, the spatial and temporal incoherence of the electronic motion, the low density of Rydberg states formed and the too large interionic distance. Evidence on why these factors are important is given. It is furthermore not obvious that formation of RM would have been detected in the laser excitation experiments since no suitable diagnostics methods were employed. The methods of forming RM using condensation in the boundary layer at surfaces are discussed for comparison. Better designed experiments seem necessary to achieve true condensation to RM in cold Rydberg gases, since large differences exist between Bose-Einstein and Rydberg condensation.