No abstract

Interaction of molecular SF6(-) anions with intense near-IR laser pulses is found to produce cationic fragments by nonsequential multiple detachment. Dissociative ionization channels that lie more than 20 eV above the threshold energy for double detachment are observed. Mass-resolved product yields are presented and analyzed as a function of the femtosecond laser pulse energy, pulse shape, and polarization ellipticity. The observed strong suppression of multiple detachment by pre-pulses, induced with negative third-order dispersion of the transform-limited fs laser pulse, is interpreted as suppression of a nonsequential process by early single detachment. However, in contrast to the relatively simple picture of a rescattering mechanism characterized by acute sensitivity to polarization ellipticity that dominates double ionization of neutral species and was reported for the atomic F(-) anion, multiple detachment of the molecular anion is found to exhibit only mild ellipticity dependence.

The efficient intense field double detachment of molecular anions observed in SF6(–) is studied by 3D coincidence imaging of the dissociation products. The dissociation anisotropy and kinetic energy release distributions are determined for the energetically lowest double detachment channel by virtue of disentangling the SF5(+) + F fragmentation products. The observed nearly isotropic dissociation with respect to the linear laser polarization and surprisingly high kinetic energy release events suggest that the dissociation occurs on a highly excited state. Rydberg (SF6(+))* states composed of a highly repulsive dication core and a Rydberg electron are proposed to explain the observed kinetic energy release, accounting also for the efficient production of all possible cationic fragments at equivalent laser intensities.

We describe a method for determining saturation peak intensities of nonlinear intense field processes. The Z-scan method takes advantage of the balance between nonlinear response and interaction volume change as an intense laser pulse is focused onto a sample. We derive a robust geometric factor, directly relating the peak intensity at optimal target displacement from the focal plane and the corresponding saturation intensity. The Z-scan method allows obtaining saturation intensities with no need of a priori assumptions about the nonlinear process; surprisingly even for unfavorable finite depth samples. The method is demonstrated experimentally for an intense laser pulse interaction with molecular anions.

The competition of intense-field multiple-detachment with efficient photodissociation of F¯ is studied as a function of laser peak intensity. The main product channels are disentangled and characterized by 3D coincidence fragment imaging. The presented kinetic energy release spectra, angular distributions, as well as two-color pump-probe measurements allow identification of competing sequential and nonsequential mechanisms. Dissociative detachment, producing two neutral atoms (F + F), is found to be dominated by a sequential mechanism of photodissociation (F¯ + F), followed by detachment of the atomic anion fragment. In contrast, dissociative ionization (F + F) shows competing contributions of both a sequential two-step mechanism as well as a nonsequential double-detachment of the molecular anion, which are distinguished by the kinetic energy released in the dissociation. Triple-detachment is found to be nonsequential in nature and results in Coulomb explosion (F + F). Furthermore, the measured kinetic energy release for dissociation on the Σ state provides a direct measurement of the F¯ dissociation energy, D = 1.26 ± 0.03 eV.