A fast beam of H2+ ions, produced from a low energy ion accelerator, has been used for the first time in intense laser field experiments. The technique has enabled neutral dissociation products to be analysed and detected for the first time in such studies. Energy spectra of neutral and ionized fragments, product yields as a function of focused laser intensity and angular distributions of neutral dissociation products have been measured. Significant differences are observed between the present results and those obtained from experiments involving neutral H2 molecules. These differences are indicative of the precursor H2 molecule playing an important and hitherto neglected formative role in the laser-induced fragmentation processes.

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The interaction of an intense laser field with a beam of atomic ions has been investigated experimentally for the first time. The ionization dynamics of Ar 1 ions and Ar neutrals in a 60 fs, 790 nm laser pulse have been compared and contrasted at intensities up to 10 16 W cm 22 . Our results show that nonsequential ionization from an Ar 1 target is strongly suppressed compared with that from the corresponding neutral target. We have also observed for the first time the strong field ionization of high lying target metastable levels in the Ar 1 beam. DOI: 10.1103/PhysRevLett.88.233001 PACS numbers: 32.80.Fb There is considerable current interest in the interaction of high intensity ͑10 13 10 18 W cm 22 ͒ radiation with dilute matter, where the external electric field strength becomes comparable to molecular bond and valance electron binding strengths. Studies have been carried out with targets of atoms, molecules, clusters [1], and most recently molecular ions [2,3], in order to elucidate the new physics observable in this highly nonlinear regime. These include the study of high harmonic generation [4], strong field effects in molecular dissociation, and the enhancement of multiple ionization in atoms [5,6]. Work is currently underway to generate attosecond pulses through harmonic generation [7,8], and coherent x-ray generation could be possible for highly charged ions in the presence of superintense fields [9,10].Both experiment and theory have shown that, in femtosecond laser pulses, multiple ionization of atoms is enhanced by nonsequential processes [5,6,11,12]. The physical model which best describes the observed behavior is known as the "atomic antenna" [13] or "recollision" [14,15] model. In this picture the first electron is removed from the atom by tunneling through the barrier created by interaction of the external field with the atomic potential. This electron is then accelerated in the external field and, depending on the initial phase, can return to the singly charged ion with energies up to 3.17U P [6], where U P is the classical ponderomotive energy in the field. Multiple ionization is thus via electron interaction with the ionic core. Evidence for this mechanism includes large reductions in nonsequential ionization rates for circularly polarized fields and more recently from the momentum distribution of the recoil ions [16,17], and coincidence measurements between ions and ejected electrons [18]. However, it has proved difficult to predict quantitative yields for the product ions without using large-scale calculations through the S-matrix approach [19] or by obtaining a time-dependent solution of the Schrödinger equation [20].By including the effects of excitation from the recollision followed by tunneling [21] and taking into account the Coulomb focusing on the active electron [22], better agreement with experiment has been obtained. The importance of multiple returns to the core has been demonstrated experimentally by a reduction in nonsequential ionization when few cycle pulses are used [23]....

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