Suppression of Multiple Ionization of Atomic Ions in Intense Ultrafast Laser Pulses

Abstract

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]....

Double ionization of atomic negative ions in an intense laser field

“…The ease of the initial ionization is critical in determining the efficiency with which laser energy is absorbed and multiple ionization proceeds. Supporting evidence for this assertion comes from the only experiment to measure multiple ionization from a target which is initially positively charged (Greenwood et al 2002). In this study it was found that non-sequential ionization from Ar + was dramatically suppressed, when compared with neutral Ar.…”

Volumetric intensity dependence on the formation of molecular and atomic ions within a high intensity laser focus

“…For example, studies have revealed that even in the case of atoms, subtle features of the ionization process may be masked by integration over the entire focal region [4,5]. Additionally, Greenwood et al applied the technique of "intensity selective scanning" to an Ar ϩ ion beam target which revealed for the first time strong field ionization of high lying target metastable levels in an Ar ϩ beam [9]. Due to the spatial selectivity achieved using the 0.5 mm pin hole, the various charge states of Xe detected had very different volumetric weightings than those achieved in the conventional mode.…”

Electromagnetically confined plasma target for interaction studies with intense laser fields

“…Many surprises were found and models are revised continuously and range from a sequential ionization [4], i.e. 2+ comes from 1+ and 3+ via 2+, etc., to the tunneling ionization model and above threshold ionization, to a re-scattering model of the ionized electron that causes multiple ionization [5]. In these cases, the strong electrical field modulates the Coulomb potential and subsequent ionization processes are responsible for the degree of ionization [1].…”