Multiple strong field ionization of metallocenes: Applicability of ADK rates to the production of multiply charged transition metal (Cr, Fe, Ni, Ru, Os) cations

Abstract

The reactions of a neutral molecule with a singly charged ion in the gas phase have been studied extensively in connection with catalytic reactions, interstellar and planetary atmospheric chemistry, and the MALDI process. In contrast, the reactions of a neutral molecule with a multiply charged ion have rarely been reported, in part because it is difficult to produce such ions in abundance. In this study, we report the production of multiply charged transition metal cations from metallocenes in intense femtosecond laser fields. The most highly charged metals observed at 3 × 1015 W cm−2 are Cr6+, Fe6+, Ni6+, Ru7+, and Os8+. The production of carbon-containing cations originating in cyclopentadienyl ligands is small relative to that of metal cations because the ligands are dominantly liberated as neutral fragments. Metal cations having higher charges are formed by the sequential (stepwise) tunnel ionization of the singly (doubly) charged metal cations that are liberated from the excited metallocene cation (dication). The experimentally measured and theoretically calculated saturation intensities of cation generation are compared. The quasiclassical tunneling theory under a single active electron approximation has been known to underestimate the saturation intensity of singly charged metal cations. However, we investigate that this theory overestimates the saturation intensity of multiply charged metal cations referred to as an enhanced ionization. Moreover, the degree of overestimation becomes significant as the charge number of the metal cation increases. The characteristic deviation of the theoretical predictions from the experimental results is a key issue for the future clarification of the multiple-tunneling ionization processes. We suggest that the polarizabilities depending on the charge number, and presumably the excitation of cations, might be important in the multiple ionization of transition metals. In addition, the production of multiply charged metal cations in abundance, and the reduced production of carbon and hydrocarbon fragment cations achieved by femtosecond laser ionization, will be valuable as sources of multiply charged metal cations for future ion–molecule reaction studies.