Controlling Two-Electron Threshold Dynamics in Double Photoionization of Lithium by Initial-State Preparation

Abstract

Double photoionization (DPI) and ionization-excitation (IE) of Lið2sÞ and Lið2pÞ, state-prepared and aligned in a magneto-optical trap, were explored in a reaction microscope at the free-electron laser in Hamburg (FLASH). From 6 to 12 eV above threshold (@! ¼ 85, 91 eV), total as well as differential DPI cross sections were observed to critically depend on the initial state and, in particular, on the alignment of the 2p orbital with respect to the VUV-light polarization, whereas no effect is seen for IE. The alignment sensitivity is traced back to dynamical electron correlation at threshold.

Double photoionization of excited lithium and beryllium

“…P o →P e transitions are dipole-forbidden with one-electron targets, but are fully allowed in many-electron atoms. Transitions to the P e continua are not mentioned in the two recent studies of DPI from 2 P excited Li [2,8]. We note that while there is no selection rule that can be used to predict the ratio of integral cross sections (either total or SDCS) for parallel to perpendicular polarization, since more than one total symmetry component is involved, a straightforward application of the Wigner-Eckart theorem shows that the ratio of D 0 to D x , for both total cross sections and SDCS at all energy sharings, must be 4/3.…”

“…Excited atomic targets with L = 0 can be aligned and then offer the possibility of studying the dependence of DPI cross sections on the direction of photon polarization, posing additional challenges to theory. Indeed, the first experimental measurements on the orientation dependence of DPI cross sections have recently been performed on laserexcited and aligned lithium [2]. These initial measurements only examined orientation dependence of the total cross sections, but given the continued rapid developments in trapping, laser-preparation and advanced detection techniques, coupled with the availability of intense VUV optical sources, it should not be long before results from kinematically complete experiments on DPI from excited atomic targets become available.…”

Nonsequential double ionization of the hydride ion by two-photon absorption

“…With these developments, the role of electron correlations in few-photon multiple ionization processes in atoms [5][6][7][8][9] and molecules [10] can be experimentally investigated. A parallel development of ab initio numerical methods, capable of addressing highly correlated electron motion in few-photon [11,12] and multiphoton [13] ionization processes at the most detailed level, has taken place.…”

Electron and recoil ion momentum imaging with a magneto-optically trapped target

“…Generally, this issue is solved by periodically switching between two operation modes, in the first one trapping and cooling the atoms in the MOT, and in the second one acquiring the time and position data of the atomic fragments in the reaction microscope with the quadrupole field being switched off. The switching of the MOT magnetic field was realized already in earlier MOTRIMS experiments in order to allow for a higher recoil ion momentum resolution 20,22,23 . In the present case, the requirements for the field switching are particularly challenging.…”