Photodetachment in a strong laser field: An experimental test of Keldysh-like theories

We investigate the photodetachment process in the negative fluorine ion in a strong linearly polarized laser field. Angle-resolved momentum distributions of photoelectrons are measured with the use of an imaging technique for a wide range of laser frequencies and peak intensities. The nonmonotonic structure recorded in photoelectron spectra is interpreted in terms of the quantum interference effect predicted by a Keldysh-like theory. In particular, the dependence of the interference term on the laser parameters is used to explain the origin of the observed spectral features. Our results unambiguously show that the length gauge is the proper one to use in the frame of the strong-field approximation.