We study the strong-field interaction of a negative hydrogen ion in a short infrared laser pulse.
An imaging technique is employed in a fast ion beam to record angular-resolved energy distributions
of the photodetached electrons. The spectrum exhibits four excess photon channels. A detailed analysis
of the energy and angular distribution is presented, including a comparison with recent theories.
From our data we determine elastic-scattering phase shifts of the partial continuum waves in the
lowest-order detachment channel. Quantitative agreement with scattering theory is obtained.
A quantum interference effect predicted in a recent Keldysh-like theory is observed at energies close to threshold.
We propose an analogy to a double-slit model which explains the interference features in a simple and intuitive way.
We also verify the applicability of the Wigner threshold law in the strong-field regime and discuss its
role in the observed quantum interference. ©2003 The American Physical Society