root/branches/newmole/source/atom_pop2.cpp

/* This file is part of Cloudy and is copyright (C)1978-2008 by Gary J. Ferland and
 * others.  For conditions of distribution and use see copyright notice in license.txt */
/*atom_pop2 do level population for simple two level atom, no radiative transfer */
#include "cddefines.h"
#include "phycon.h"
#include "dense.h"
#include "atoms.h"

double atom_pop2(double omega, 
  double g1, 
  double g2, 
  double a21, 
  double bltz, 
  double abund)
{
        double boltz, 
          popexc_v, 
          q12, 
          q21, 
          r;

        DEBUG_ENTRY( "atom_pop2()" );

        /* result is density (cm-3) of excited state times a21
         * result normalized to n1+n2=abund
         * cdsqte is eden / sqrte * 8.629e-6
         * */
        boltz = bltz*phycon.teinv;
        if( abund == 0. || boltz > 15. )
        {
                popexc_v = 0.;
                return( popexc_v );
        }

        /*begin sanity check */
        ASSERT( omega > 0. );

        q21 = dense.cdsqte*omega;
        q12 = q21/g1*exp(-boltz);
        q21 /= g2;
        r = (a21 + q21)/q12;
        popexc_v = abund*a21/(r + 1.);
        return( popexc_v );
}