The ρ–θ curves for the photoionization of the Rydberg hydrogen atom in different gradient electric fields. The detector is located at z=-0.5 plane. The scaled energy is ε = −0.1. The scaled electric field gradients are (a) α = 0.0, (b) α = 1.0, (c) α = 6.0, (d) α = 8.0, (e) α = 10.0, and (f) α = 13.0.