c
c###################################################################
c
      subroutine heexcen(n, l, mult, sumen, eexc, kermsg)
c
c     this subroutine passes the excitation energy for excited states
c     of  helium taken from tables a.4 and a.5 given by janev et al.
c     (see doc=h-he-plasma.)
c
c     the input subroutine parameters are
c
c    n = principal quantum number of the excited electronic state
c
c    l = orbital angular momentum quantum number of the exited state
c
c    mult = the spin multiplicity (2s+1) of the state
c
c    sumen =  indicates choice of excitation enegy to be returned.
c             if sumen=0  the excitation energy for the specific
c              state (quantified by n, l amd mult) is returned
c             if sumen=1  the excitation energy taken as an average
c             over angular momentum and toatal spin is returned
c
c     the output subroutine parameters are
c
c    eexc = excitation energy
c
c    kermsg = blank if no errors
c
c     written by j. j. smith , iaea atomic and molecular data unit
c
c------------------------------------------------------------------------
c
      character*(*) kermsg
      integer n, l, mult, sumen, lp, multp
      dimension enl(4,4,2), en(7)
      data (((enl(i,j,k),i=1,4),j=1,4),k=1,2) /32*0.0/
c
c----   n=1 state
c
c     data enl(1,1,1)
c    *    / 24.588 /,
c
c----   n=2 excited states
c
c    1   enl(2,1,1),enl(2,1,2),enl(2,2,1),enl(2,2,2)
c    *    / 20.614, 19.818, 21.217, 20.963 /,
c
c----   n=3 excited states
c
c    2   enl(3,1,1),enl(3,1,2),enl(3,2,1),enl(3,2,2),enl(3,3,1),
c    *   enl(3,3,2)
c    *    / 22.919, 22.717, 23.086, 23.006, 23.073, 23.072 /,
c
c----   n=4 excited states
c
c    3   enl(4,1,1),enl(4,1,2),enl(4,2,1),enl(4,2,2),enl(4,3,1),
c    *   enl(4,3,2),enl(4,4,1),enl(4,4,2)
c    *    / 23.672, 23.529, 23.741, 23.706, 23.735, 23.735, 23.736,
c    *     23.736 /
c
c-----   excitation energies summed over angular momentum and
c-----   total spin
c
      enl(1,1,1)=24.588
c----   n=2 excited states
      enl(2,1,1)=20.614
      enl(2,1,2)=19.818
      enl(2,2,1)=21.217
      enl(2,2,2)=20.963
c----   n=3 excited states
      enl(3,1,1)=22.919
      enl(3,1,2)=22.717
      enl(3,2,1)=23.086
      enl(3,2,2)=23.006
      enl(3,3,1)=23.073
      enl(3,3,2)=23.072
c----   n=4 excited states
      enl(4,1,1)=23.672
      enl(4,1,2)=23.529
      enl(4,2,1)=23.741
      enl(4,2,2)=23.706
      enl(4,3,1)=23.735
      enl(4,3,2)=23.735
      enl(4,4,1)=23.736
      enl(4,4,2)=23.736
      data (en(k),k=1,2) /2*0.0/
      data en(3),en(4),en(5),en(6),en(7)
     *    / 22.9799 , 23.699, 24.07, 24.30, 24.71/
c
      kermsg =' '
      if (sumen .eq. 1 .or. n .gt. 4) then
        if (n .ge. 8) then
          eexc =enl(1,1,1) - 13.58/(n*n)
          return
        else
           eexc =en(n)
           if (eexc .eq. 0.0) kermsg =
     *        'excitation energy for n value not in table in heexcen'
        endif
      else
           lp = l+1
           if (mult .eq. 1) then
             multp = 1
           else if (mult .eq. 3) then
               multp = 2
             else
                 kermsg = 'invalid spin multipclity as input in heexcen'
             endif
           eexc = enl(n,lp,multp)
           if (eexc .eq. 0.0) kermsg =
     *        'excitation energy not in table in heexcen'
      endif
      return
      end
c##################################################################
c
c###################################################################
c
      subroutine jeexc4(pe, pcf, kncf, pxs, kermsg)
c
c     this is a subroutine to calculate cross sections (cm[2])
c     versus energy (ev) for electron impact excitation for dipole
c     forbidden transitions with spin change.
c     the formula is taken from doc=h-he-plasma reaction 2.3.4
c
c     pcf is the coefficient data array, where
c
c     pcf(1) =   coefficient q for the transition
c
c     pcf(2) =   lin, the orbital angular monentum of the inital state
c
c     pcf(3) =   multi, spin multiplicity (2s+1) of the inital state
c
c     pcf(4) =   lfin, the orbital angular monentum of the final state
c
c     pcf(5) =   multf, spin multiplicity (2s+1) of the final state
c
c    - warning- .
c
c        the coefficient array pcf is updated by this routine to
c     include energy independent constants. these coefficients can be
c     used in subsequent calls for the same entry. the coefficeients
c     added are:
c
c     pcf(6) = eth, threshold excitation energy for the transition (ev)
c
c     pe = electron energy (ev)
c
c     kermsg = blank if no errors
c
c     pxs = cross section in cm[2]
c
c------------------------------------------------------------------------
c
      double precision pe, pcf, pxs
c
      dimension pcf(6)
      integer lin, lfin, multi, multf
      character*(*) kermsg
c
      data ry /1.358e+01/
c
      q   = pcf(1)
      lin = pcf(2)
      multi = pcf(3)
      lfin = pcf(4)
      multf = pcf(5)
      kermsg = ' '
      if (kncf .eq. 5) then
c
c        first call to jeexc4 determine energy independent
c        parameters and place in pcf for further use
c
c        determine the threshold energy from the eneries of the initial
c        and final states
c
          call heexcen(2, lin, multi, 0, eexc1, kermsg)
          call heexcen(2, lfin, multf,0, eexc2, kermsg)
          eth = abs (eexc1 - eexc2)
c
c        place energy independent parameters in coefficient array and
c        update kncf
c
          pcf(6) = eth
          kncf = 6
      else if (kncf .eq. 6) then
          eth  = pcf(6)
c
      else
          kermsg = ' incorrect number of coefficients passed to jeexc4'
          return
      endif
c
      if (pe .lt. eth) then
        pxs  = 0.0
        return
      endif
c
      u = pe / eth
      pxs  = 3.52e-16 * q * (u - 1.0) / (u ** 2)
c
      return
c
      end

def heexcen(n, l, mult, sumen):
   """
    This function passes the excitation energy for excited states of  helium 
    taken from tables a.4 and a.5 given by janev et al. (Elementary processes in H-He plasmsas.
 
    n: principal quantum number of the excited electronic state
    l: orbital angular momentum quantum number of the exited state
    mul: the spin multiplicity (2s+1) of the state
    sumen: indicates choice of excitation enegy to be returned.
    """
    enl = np.zeros((4, 4, 2))
    en = np.zeros(7)
    
    # Define the excitation energy values
    enl[0, 0, 0] = 24.588
    enl[1, 0, 0] = 20.614
    enl[1, 0, 1] = 19.818
    enl[1, 1, 0] = 21.217
    enl[1, 1, 1] = 20.963
    enl[2, 0, 0] = 22.919
    enl[2, 0, 1] = 22.717
    enl[2, 1, 0] = 23.086
    enl[2, 1, 1] = 23.006
    enl[2, 2, 0] = 23.073
    enl[2, 2, 1] = 23.072
    enl[3, 0, 0] = 23.672
    enl[3, 0, 1] = 23.529
    enl[3, 1, 0] = 23.741
    enl[3, 1, 1] = 23.706
    enl[3, 2, 0] = 23.735
    enl[3, 2, 1] = 23.735
    enl[3, 3, 0] = 23.736
    enl[3, 3, 1] = 23.736
    en[2:5] = [22.9799, 23.699, 24.07, 24.30, 24.71]
    
    if sumen == 1 or n > 4:
        if n >= 8:
            eexc = enl[0, 0, 0] - 13.58 / (n * n)
        else:
            try:
                eexc = en[n - 1]
            except IndexError:
                raise ValueError('Excitation energy for n value not in table in heexcen')
    else:
        lp = l + 1
        if mult == 1:
            multp = 1
        elif mult == 3:
            multp = 2
        else:
            raise ValueError('Invalid spin multiplicity as input in heexcen')
        
        try:
            eexc = enl[n - 1, lp - 1, multp - 1]
        except IndexError:
            raise ValueError('Excitation energy not in table in heexcen')
    
    return eexc

###############################################################
def jeexc4(pe, pcf, kncf):
    """
    This is a function to calculate cross sections (cm^2) versus energy (eV)
    for electron impact excitation for dipole forbidden transitions with spin change.
    
    pe: electron energy (eV)
    pcf: coefficient data array
    kncf: number of coefficients
    """
    
    q = pcf[0]
    lin = pcf[1]
    multi = pcf[2]
    lfin = pcf[3]
    multf = pcf[4]

    ry = 13.58

    # Initialize energy-independent parameter
    eth = 0.0

    if kncf < 5:
        # First call to jeexc4 determines energy-independent parameters
        # and places them in pcf for further use

        try:
            eexc1 = heexcen(2, lin, multi, 0)
            eexc2 = heexcen(2, lfin, multf, 0)
            eth = abs(eexc1 - eexc2)
        except ValueError:
            raise ValueError("Error calculating threshold energy")

        # Place energy-independent parameters in the coefficient array and update kncf
        pcf[5] = eth
        kncf = 6

    elif kncf == 6:
        eth = pcf[5]
    else:
        raise ValueError("Incorrect number of coefficients passed to jeexc4")

    if pe < eth:
        pxs = 0.0
    else:
        u = pe / eth
        pxs = 3.52e-16 * q * (u - 1.0) / (u ** 2)

    return pxs