Recombination at ultra-low energies

Three-body electron-ion recombination is described at ultralow electron temperatures T-e. At 4K, the initial stage involves extremely rapid collisional capture into high Rydberg states n > 200 with high angular momentum l approximate to n - 1 at a rate similar to T-e(-4.5). This is followed by extremely slow collisional-radiative decay. The key collisional mechanism appears to be collisional I-mixing of the Rydberg atoms A(n) by ions and electrons until sufficiently low l's are attained so as to permit relatively rapid radiative decay to the lowest electronic levels. This sequence is in direct contrast to the sequence of much slower collisional capture at higher T-e followed by the much faster decay of A(n) by electron collisions to lower levels where radiative decay completes the recombination. At ultra-low temperatures, the rate limiting sequence is therefore collisional I-mixing followed by radiative decay in contrast to recombination at much higher energies and electron densities N-e similar to 10(8) cm(-3), where the rate limiting step is he initial collisional or radiative capture at intermediate T-e(similar to I eV) and higher T-e (similar to 10 eV) respectively. An exact classical solution for l-mixing is obtained.