FUNCTION VERN18(E,IZZ)
C     ======================
C
C     Photoionization cross-sections for ground states
C       of all Argon ions.
C           Verner D.A. et al. 1996, ApJ 465
C           Verner & Yakovlev 1995, A&AS 109, 125
C
      INCLUDE 'IMPLIC.FOR'
      INCLUDE 'BASICS.FOR'
      PARAMETER (T18=1.D-18,MVER=18)
      DIMENSION S0(MVER),E0(MVER),EMX(MVER)
      DIMENSION Y0(MVER),Y1(MVER),YW(MVER),YA(MVER),PV(MVER)
      DIMENSION S95(MVER),E95(MVER),Y95(MVER),YW95(MVER),P95(MVER)
C
      DATA S0/2.106D1,2.503D1,3.58D1,2.035D1,9.946D0,1.080D0,3.693D0,
     +        3.295D1,8.279D-1,8.204D0,1.76D3,7.018D-1,2.459D-2,
     +        4.997D-2,2.571D4,2.135D1,3.108D1,1.69D2/
      DATA E0/17.09,24.94,14.17,6.953,10.31,0.544,0.02966,3.844,
     +        0.1926,10.4,0.1257,5.31,0.3209,1.557,18.88,41.54,
     +        446.8,139.9/
      DATA EMX/249.2,266.2,280.1,298.7,320.,342.6,366.7,392.5,3361.,
     +         3446.,3523.,3613.,3702.,3798.,3898.,3988.,5.D4,5.D4/
      DATA Y0/1.688,0.9299,2.384,7.501,6.406,1.7D2,4.383D-4,0.,38.14,
     +        38.04,3.286D-3,1.099D2,2.068D3,4.552D2,2.445D-2,0.,0.,0./
      DATA Y1/0.8943,0.7195,1.794,0.1806,3.659D-3,15.87,2.513,0.,4.649,
     +        0.639,0.3226,0.2202,21.13,6.459,1.054D-2,0.,0.,0./
      DATA YW/0.4185,0.5108,0.6316,0.8842,0.4885,11.07,1.363D-2,0.,
     +        1.434,9.203D-4,1.975,0.4987,0.6692,0.2938,29.09,0.,0.,0./
      DATA YA/2.645D2,1.272D2,3.776D1,1.4D1,7.444D1,9.419D2,9.951D3,
     +        7.082D2,2.392D2,1.495D1,1.579D3,1.001D2,2.285D3,5.031D2,
     +        1.475D0,4.118D1,3.039D1,3.288D1/
      DATA PV/4.796,4.288,5.742,9.595,6.261,7.582,7.313,4.645,11.21,
     +        11.15,6.714,8.939,8.81,8.966,26.34,4.945,2.092,2.963/
      DATA S95/8.372D1,1.937D2,2.281D2,2.007D2,2.474D2,2.786D2,3.204D2,
     +         4.198D2,2.931D1,1.585D1,2.796D1,1.666D1,2.888D1,2.874D1,
     +         2.883D1,3.003D1,3.108D1,1.69D2/
      DATA E95/164.7,108.5,102.5,107.3,98.35,92.33,85.63,73.68,467.9,
     +         612.6,478.9,602.8,473.1,474.9,475.6,468.,466.8,139.9/
      DATA Y95/54.52,70.,43.8,70.,42.84,42.2,42.3,44.19,17.44,50.,
     +         19.17,50.,20.42,22.35,26.15,28.54,30.39,32.88/
      DATA YW95/0.627,0.1,7.167D-3,0.1,7.283D-3,7.408D-3,7.258D-3,
     +          7.712D-3,10*0./
      DATA P95/3.328,3.7,4.046,3.7,4.125,4.227,4.329,4.492,2.362,
     +         1.65,2.271,1.65,2.234,2.171,2.074,2.037,2.092,2.963/
C
      VERN18=0.
      IVER=IZZ
C
C     1996 Expression
C
      IF(E.LT.EMX(IVER)) THEN
        XX=E/E0(IVER)-Y0(IVER)
        YY=SQRT(XX*XX+Y1(IVER)*Y1(IVER))
        AA=(XX-UN)*(XX-UN)+YW(IVER)*YW(IVER)
        BB=YY**(HALF*PV(IVER)-5.5)
        CC=(UN+SQRT(YY/YA(IVER)))**PV(IVER)
        FY=AA*BB/CC
        VERN18=S0(IVER)*T18*FY
      ELSE
C
C     1995 Expression for high energies
C          (ionization of inner shell electron)
C
        YY=E/E95(IVER)
        XL=0.
        IF(IZZ.LE.8) XL=UN
        Q=HALF*P95(IVER)-5.5-XL
        AA=(YY-UN)*(YY-UN)+YW95(IVER)*YW95(IVER)
        BB=YY**Q
        CC=(UN+SQRT(YY/Y95(IVER)))**P95(IVER)
        FY=AA*BB/CC
        VERN18=S95(IVER)*T18*FY
      END IF
C
      RETURN
      END