1. E /eV
2. sigma /cm²

• B7: R. K. Janev, J. J. Smith, "Cross sections for collision processes of hydrogen atoms with electrons, protons, and multiply charged ions", IAEA- Atomic and Plasma–Material Interaction Data for Fusion, Vienna 4 (1993). [https://www.iaea.org/publications/1839/atomic-and-plasma-material-interaction-data-for-fusion]

c
c##############################################################################
c
      subroutine neexh1(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.
c
c     pe = collision energy in ev
c
c     pcf(1) = threshold energy (ev), eth.
c     pcf(2) = upper value of energy range over which the
c              cross section, sig, is approximated in a linear form
c                     sig = a + b*(e-eth)
c              where a,b and c are parameters and e is the incident
c              energy in ev
c              (lower end of the range is the threshold energy)
c
c     pcf(3) = parameter a
c     pcf(4) = parameter b
c     pcf(5) = upper value of energy range over which the cross section
c              is approximated by a constant value. (lower limit for
c              this range is pcf(2).
c     pcf(6) = constant value for second region.
c     pcf(7-12) = parameters for fit to the cross section in the third
c              region.
c
c     kermsg = blank if no errors
c
c     pxs = cross section in cm[2]
c
c------------------------------------------------------------------------
c
      double precision pe, pcf, pxs
      double precision cstr, ryd, ksq, x, alog
c
      dimension pcf(12)
      character*(*) kermsg
c
      data ryd/1.36d+01/
c
      eth = pcf(1)
      if(pe .ge. eth) then
        kermsg = ' '
      else
        pxs = 0.0d0
        return
      endif
c
      if (pe .le. pcf(2)) then
        pxs =  1.0e-16 * (pcf(3) + (pcf(4) * (pe - eth)))
c
      else if (pe .gt. pcf(2) .and. pe .le. pcf(5)) then
         pxs =  pcf(6)
c
      else
        ksq= pe/ryd
        etrans = eth/ryd
        x=ksq/etrans
        xsq = x*x
        alog = 0.0d0
        if (pcf(12) .ne. zero) alog = pcf(12) * dlog (x)
        cstr = alog + pcf(7) + pcf(8)/x + pcf(9)/xsq + pcf(10)/(x*xsq)
     &        + pcf(11)/(xsq*xsq)
        pxs =   5.984d-16 * cstr / pe
c
      endif
c
      return
      end