We have measured absolute triple differential cross sections for photo-double ionization of helium at 20 eV excess. The measurement covers the full ranges of energy sharing and emission angles of the two photoelectrons. We compare our data for selected geometries with the convergent close-coupling (CCC) calculations as well as 2SC calculations by Pont and Shakeshaft and 3C calculations by Maulbetsch and Briggs. In terms of the absolute magnitude and the trend in the shapes of the triple differential cross section for different geometries we find good agreement of the CCC and published 2SC calculations with our measurement, though differences with respect to the observed shape of individual patterns still exist.

We have measured and explained a new mechanism of molecular ionization near the appearance intensity that produces a sequence of peaks in the nuclear kinetic energy spectrum separated by the photon energy. Our interpretation is based on an internally consistent model for the nuclear motion during an intense laser pulse. Within this model, the same concepts and language can be used for both dissociation and ionization, leading to a more unified understanding of the dynamics.

A method for determining the laser-induced dissociation pathways of multielectron diatomic molecules is developed. Despite the abundance of possible dissociation pathways inherent to such molecules, this technique allows one to resolve the dissociation pathways that contribute to the measured intensity-dependent threedimensional momentum distribution. To illustrate this method, the unique dissociation mechanisms and pathways producing a few predominant features in the laser-induced dissociation momentum distribution of O 2 + are determined.

Shaping ultrafast laser pulses using adaptive feedback can manipulate dynamics in molecular systems, but extracting information from the optimized pulse remains difficult. Experimental time constraints often limit feedback to a single observable, complicating efforts to decipher the underlying mechanisms and parameterize the search process. Here we show, using two strong-field examples, that by rapidly inverting velocity map images of ions to recover the three-dimensional photofragment momentum distribution and incorporating that feedback into the control loop, the specificity of the control objective is markedly increased. First, the complex angular distribution of fragment ions from the no þis controlled via a barrier-suppression mechanism, a result that is validated by model calculations. Collectively, these experiments comprise a significant advance towards the fundamental goal of actively guiding population to a specified quantum state of a molecule.

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