Relaxation of cold plasmas and threshold lowering effect

Low temperature plasmas are produced by photoionization of cold trapped atoms. Due to the threshold energy shift caused by overlapping ion fields, the effective kinetic energy of the free electrons is increased by ${\Delta }_P{=2C}_P/a,$ where a is the Wigner-Seitz radius and $C_P\simeq 11$ is a universal constant. Detailed discussion is given on the self-consistent determination of $C_P,$ using a Debye shielded, fluctuating lattice model. The attainable minimum electron temperature is given by the plasma density alone, as $T_e\simeq {\Delta }_P/3\mathrm{}\simeq 7/a.$ No Wigner crystallization is possible in such plasmas, unless a strong confining external field is present. The shift imposes a stringent cutoff on the high Rydberg state contributions to the three-body recombination probabilities, and a new estimate of the rates is presented. For a freely expanding cold plasma, an additional mode of adiabatic motional recombination is found to dominate plasma relaxation.