Multiphoton Ca2+ production occurring before the onset of Ca+ saturation: is it a fingerprint of direct double ionization?

Abstract

Singly and doubly charged Ca ions have been produced by multiphoton excitation of Ca vapour with 5 ns, low intensity (⩽3 × 1011 W cm−2) dye-laser pulses in the 675–685 nm wavelength range, including the four-photon 4s2 1S0 → 4p2 1S0 excitation. The intensity and wavelength dependence of the ion yields was recorded as well as the fluorescence emission from excited states of the Ca ion in an effort to identify the excitation pathways leading to single and double ionization. Unambiguous evidence for the absorption of at least two photons above the first ionization threshold was recorded, in agreement with earlier results for Mg and Sr obtained under similar conditions. However, certain characteristics of the process differ significantly from those of the earlier results, despite the apparent similarity in both the atomic structure and the excitation scheme. The most striking and unexpected finding is that for a certain wavelength, the Ca2+ yield is observable well before the saturation intensity of Ca+ and, moreover, it grows with intensity and saturates in parallel with the Ca+ yield. Possible mechanisms behind this outcome are discussed in detail as well as their implications for the multiphoton multiple ionization of complex atoms. Our tentative conclusion is that the occurrence of doubly charged ion production before the singly charged ion saturation should not be considered as ‘synonymous’ with direct (or non-sequential) multiphoton double ionization.