Target electron ionization in Li2+-Li collisions: A multi-electron perspective

Target electron removal in Li2+-Li collisions at 2290 keV/amu is studied experimentally and theoretically for ground and excited lithium target configurations. It is shown that in outer-shell ionization a single-electron process plays the dominant part. However, the K-shell ionization results are more difficult to interpret. According to our calculations, the process is shown to be strongly single-particle like. On one hand, a high resemblance between theoretical single-particle ionization and exclusive inner-shell ionization is demonstrated, and contributions from multi-electron processes are found to be weak. On the other hand, it is indicated by the discrepancy between experimental and single-particle theoretical results that multi-electron processes involving ionization from the outer-shell may play a crucial role

Two-Photon Double Ionization of Lithium at FLASH

Two- and three-photon double ionization of atomic lithium by intense (I ≈ 1014 W/cm2) VUV light was investigated at the free electron laser in Hamburg (FLASH). Using a reaction microscope the recoil ion momentum distributions were recorded for two different photon energies of 50 and 59 eV. The former covers the case of non-resonant double ionization, whereas the latter gives rise to resonant excitation followed by double ionization. These reactions are of special interest as theoretical calculations for few-photon - few-electron reactions just become feasible for simple atomic targets

Coherence in multistate resonance-enhanced four-photon ionization of lithium atoms

A joint experimental and theoretical study of four-photon ionization of Li is performed using a pump-probe setup with the laser frequency tuned in between the excitation energies of the 2s–4p and 2s–4f three-photon transitions. The three-dimensional photoelectron momentum pattern is recorded as a function of the delay time between two otherwise identical laser pulses. The process is modeled by numerically solving the time-dependent Schrödinger equation. While complicated interference patterns arise in the electron spectrum when the two pulses partially overlap in time, the expected quantum-beat phenomenon between Li atoms in either the 4p or the 4f state are seen when the fast oscillations involving the 2s initial state are averaged over. The qualitative agreement between the predicted and observed beating patterns is very good

Multi-photon ionization of lithium

A systematic study of multi-photon ionization (MPI) of atomic lithium in ultra-short and intense near-infrared laser pulses has been conducted both experimentally and theoretically. The cross section was measured for intensities spanning across the transition from MPI to over-the-barrier ionization (OBI). It turns out that this transition manifests itself in a substantial change in the photo-electron angular distributions. In a second study pump-probe experiments exciting bound wave packets and probing their time evolution by ionization were performed

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

Double photoionization (DPI) and ionization-excitation (IE) of Li(2s) and Li(2p), 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 ([h-bar]omega=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

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

A reaction microscope (ReMi) has been combined with a magneto-optical trap (MOT) for the kinematically complete investigation of atomic break-up processes. With the novel MOTReMi apparatus, the momentum vectors of the fragments of laser-cooled and state-prepared lithium atoms are measured in coincidence and over the full solid angle. The first successful implementation of a MOTReMi could be realized due to an optimized design of the present setup, a nonstandard operation of the MOT, and by employing a switching cycle with alternating measuring and trapping periods. The very low target temperature in the MOT (~2 mK) allows for an excellent momentum resolution. Optical preparation of the target atoms in the excited Li 22P3/2 state was demonstrated providing an atomic polarization of close to 100%. While first experimental results were reported earlier, in this work, we focus on the technical description of the setup and its performance in commissioning experiments involving target ionization in 266 nm laser pulses and in collisions with projectile ions