Photodetachment of $H^{-}$ in a Strong Infrared Laser Field

Negative hydrogen ions are exposed to a short infrared laser pulse of $2.15\mu \mathrm{m}$ wavelength and $250\mathrm{fs}$ duration. An imaging technique is used to record the energy resolved angular distribution of photoelectrons. The energy spectrum reveals at least three excess photon detachment channels. A quantum interference effect, resulting in an unusual dependence of the angular distribution on the electron kinetic energy, is most prominent in the lowest two-photon detachment channel. The appearance of this effect is discussed in terms of the threshold law of photodetachment.