0 5 10 15 0 0.5 1 1.5 2 Photon energy(eV) Cross section(a 0 2 ) 0 5 10 15 0 0.5 1 1.5 2 Photon energy(eV) Cross section(a 0 2 ) 0 5 10 15 0 0.5 1 1.5 2 Photon energy(eV) Cross section(a 0 2 ) 0 5 10 15 0 0.5 1 1.5 2 Photon energy(eV) Cross section(a 0 2 ) (a) d=50 a$_{0}$ (c) d=150 a$_{0}$ (b) d=100 a$_{0}$ (d) d=200 a$_{0}$