We present a nonperturbative, fully correlated calculation of the structure of ${\mathrm{H}}^{\mathrm{-}}$ in an intense laser field, based on the high-frequency Floquet theory. We show that, as the intensity increases at sufficiently high frequencies, the electron affinity manifests a peculiar behavior and can become substantially higher than its field-free value. Besides, ${\mathrm{H}}^{\mathrm{-}}$ acquires excited states, some of which are doubly excited and nonautodetaching. At high intensities ${\mathrm{H}}^{\mathrm{-}}$ undergoes stabilization, similarly to the H atom.

• Received 8 June 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.1693