Photodetachment cross section of H⁻ calculated from the quantum method with a linear harmonic oscillator potential (red lines), compared with the cross section in the absence of fields (green dashed lines). We choose the angular frequency ω of the oscillator to be 0.002. (a) x and y-polarized light. (b) z-polarized light. The arrows point to the conersponding harmonic oscillator energy levels.