Autoionizing states of negative ions in strong resonant laser fields: The negative rubidium ion

We have performed an experimental study of the atomic system ${\mathrm{Rb}}^{\mathrm{-}}$, for which the ground state 5${\mathit{s}}^2$ is coupled to the autoionizing state 5${\mathit{p}}_{1/2}$6s by means of a strong resonant laser field. The investigations are based on irradiation of ${\mathrm{Rb}}^{\mathrm{-}}$ with intense (I≤1.5×${10}^{10}$ W/${\mathrm{cm}}^2$) laser pulses of duration ∼1 ns. Single-photon detachment of ${\mathrm{Rb}}^{\mathrm{-}}$ is suppressed due to the Fano interference between transitions directly to the continuum and transitions via the autoionizing state. In contrast, two-photon absorption is at the same time resonantly enhanced since no such destructive interference is present in this process. These observations constitute experimental evidence of general features predicted from theoretical considerations of autoionizing states in strong laser fields. Furthermore, we discuss the role of autoionizing states in multiphoton double-ionization processes. Electron spectroscopy has been used to show that double ionization of ${\mathrm{Rb}}^{\mathrm{-}}$ primarily is the result of a sequential removal of the two electrons via an excited Rb state. The perspectives for direct two-electron ionization in negative ions are briefly discussed.