Radiative recombination at low energies

An exact calculation of the direct radiative recombination (RR) cross section is carried out for the hydrogenic system with one electron, and the result is used to improve Kramers' semiclassical formula, with emphasis on low energies and high Rydberg states. The formula is then extended to include explicitly the angular momentum dependence. The corresponding RR rate coefficient is evaluated at scaled electron temperatures Theta ranging 0 less than or equal to Theta less than or equal to 1, where Theta = k(B)T/(Z(2)Ry), but again emphasizing the low temperature region. The interesting feature of the RR rates is that at low T and high n the capture to high l is important; the l-distribution for given n is approximately Gaussian in shape, with the maximum near l similar or equal to n/3. This can affect the field ionization of the recombined ions. (C) 1998 Elsevier Science Ltd. All rights reserved.