An nl-model with radiative transfer for hydrogen recombination line
  masers
  Atomic hydrogen masers occur in recombination plasmas in sufficiently dense
HII regions. These hydrogen recombination line (HRL) masers have been observed
in a handful of objects to date and the analysis of the atomic physics involved
has been rudimentary. In this work a new model of HRL masers is presented which
uses an nl-model to describe the atomic populations interacting with free-free
radiation from the plasma, and an escape probability framework to deal with
radiative transfer effects. The importance of including the collisions between
angular momentum quantum states and the free-free emission in models of HRL
masers is demonstrated. The model is used to describe the general behaviour of
radiative transfer of HRLs and to investigate the conditions under which HRL
masers form. The model results show good agreement with observations collected
over a broad range of frequencies. Theoretical predictions are made regarding
the ratio of recombination lines from the same upper quantum level for these
objects.
