ADAS Subroutine r8zeta

      FUNCTION R8ZETA ( ZEFF , N , L )
C
C
C-----------------------------------------------------------------------
C
C  **************** FORTRAN77 REAL*8 FUNCTION: R8ZETA ******************
C
C  PURPOSE: HYDROGENIC SPIN-ORBIT INTERACTION ENERGY ZETA
C
C  CALLING PROGRAM: GENERAL USE
C
C  FUNC:   (R*8)  R8ZETA  = ZETA
C
C  INPUT:  (I*4)  ZEFF    = EFFECTIVE ION CHARGE.
C  INPUT:  (I*4)  N       = PRINCIPAL QUANTUM NUMBER.
C  INPUT:  (I*4)  L       = ORBITAL QUANTUM NUMBER.
C
C          (R*8)  XN      = REAL VALUE = N.
C          (R*8)  XL      = REAL VALUE = L.
C
C ROUTINES: NONE
C
C AUTHOR:  JONATHAN NASH (TESSELLA SUPPORT SERVICES PLC)
C          K1/0/81
C          JET EXT. 5183
C
C DATE:    11/10/93
C
C VERSION: 1.1                          DATE: 11-10-93
C MODIFIED: JONATHAN NASH
C               - FIRST RELEASE
C
C VERSION: 1.2                          DATE: 17-04-07
C MODIFIED: HUGH SUMMERS
C               - FIRST FULLY COMMENTED RELEASE
C
C-----------------------------------------------------------------------
      INTEGER             L,           N
      REAL*8              ZEFF