NB. Exact photoionization cross-section for hydrogen. NB. Takes array of photon energies in eV. H0_anu=: monad define yy=. y%13.598 NB. add 1 if yy < 1 to avoid errors for such arguments yp=. yy+ (-. yy>1) a=. %: <:yp b=. ^4* 1- a%~ atan a g=. b% yp* 1- ^ _2p1%a NB. fix exceptions: f=0 for yy<1, f=1 for yy=1 f=. (yy=1)+(yy>1)*g 6.308e_18*f %y^3 )