  By applying closed-orbit theory to an existing model, a simple formula is
derived for the modulation function of s-wave photo-detachment in the presence
of a static electric field. We then compare the s-wave modulation function with
the p-wave modulation function. We show the maximums (minimums) in the s-wave
modulation function correspond to the minimums (maximums) in the p-wave
modulation function because of a phase difference of $\pi$ in their
oscillations. The oscillation amplitude in the p-wave modulation function can
be larger than, smaller than or equal to the oscillation amplitude of s-wave
modulation function when the angle between the laser polarization direction and
the static electric field direction is tuned to be smaller than, larger than or
equal to a special angle which is approximately $54.74^\circ$.
