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

An atomic system, Rb super(-), where the Rb super(-)(5s super(2)) ground state was coupled to the Rb super(-)(5p sub( is equivalent )6s) autoionizing state (AIS) by an intense visible laser pulse was experimentally investigated. It was demonstrated how the single-photon ionization Fano profile, typical of weak field probing of an AIS, was still apparent at high field strengths of the laser. Two-photon absorption by way of AIS occurred proficiently and could be monitored by detecting the Rb super(+) ions. The observation of the resonant enhancement of two-photon absorption in the present case of Rb super(-) confirmed the fact that two-photon absorption via an intermediate AIS always resulted in a Lorentzian peak independent of the shape of the single-photon ionization cross section. Moreover, the detection of the energies of the emitted photoelectrons strongly implied that double ionization mainly was due to two-photon ionization by Rb super(-)(5s super(2)) via the Rb super(-)(5p sub( is equivalent )6s) AIS to Rb(8s) or Rb(6d) followed by single-photon ionization of these excited Rb states.