;+
; NAME:
;  rt_epsln
; PURPOSE: (one line)
;  return ratio of collision timescale to radiaitve timescale
; DESCRIPTION:
;
; CATEGORY:
;  RT
; CALLING SEQUENCE:
;  ep = rt_epsln(t,ntot,a10,p10,d,m)
; INPUTS:
;  t - temperature (K). May be an array.
;  ntot - total number density (cm^-3). May be an array.
;  a10 - einstein A coefficient. s^-1
;  p10 - V-T transition probability. Unitless.
;  d - collision diameter. cm
;  m - reduced mass. g
; OPTIONAL INPUT PARAMETERS:
;  none
; KEYWORD INPUT PARAMETERS:
;  none
; KEYWORD OUTPUT PARAMETERS:
;  none
; OUTPUTS:
;  c10/a10
; COMMON BLOCKS:
; SIDE EFFECTS:
; RESTRICTIONS:
;  None
; PROCEDURE:
; 
; 
; MODIFICATION HISTORY:
;  Written 2004 May 8, Leslie Young SwRI
;  Modified from epsln.f, Roger Yelle
;-

function rt_epsln, t,ntot,a10,p10,d,m 

    physconstants
    z10 = d^2*sqrt(8. * !pi * !phys.k * t/m)
    c10 = p10 * z10 * ntot
    eps = c10/a10
    
    return, eps

end