Multiphoton Double Ionization of Helium at 394nm - a Fully Differential Experiment

TL;DR

This study provides a comprehensive, fully differential analysis of helium's double ionization at 394nm, revealing detailed electron and ion momentum distributions, energy sharing, and photon absorption quantization, advancing understanding of strong field ionization processes.

Contribution

It presents the most complete differential measurements of helium double ionization at this wavelength, including angular, energy, and momentum correlations, with insights into photon quantization effects.

Findings

Double to single ionization ratio varies from 2e-4 to 1.5e-3.

Electron emission patterns depend on intensity, from back-to-back to same direction.

Discrete energy structures reveal photon absorption quantization.

Abstract

We report on a kinematically complete experiment on strong field double ionization of helium using laser pulses with a wavelength of 394\,nm and intensities of $3.5-5.7\times10^{14}\,W/cm^2$. Our experiment reaches the most complete level of detail which previously has only been reached for single photon double ionization. We give an overview over the observables on many levels of integration, starting from the ratio of double to single ionization, the individual electron and ion momentum distributions over joint momentum and energy distributions to fully differential cross sections showing the correlated angular momentum distributions. Within the studied intensity range the ratio of double to single ionization changes from $2\times 10^{-4}$ to $1.5\times 10^{-3}$. We find the momentum distributions of the $\rm{He}^{2+}$ ions and the correlated two electron momentum distributions to vary substantially. Only at the highest intensity both electrons are emitted to the same direction while at the lowest intensity back-to-back emission dominates. The joint energy distribution of the electrons shows discrete structures from the energy quantization of the photon field which allows us to count the number of absorbed photons and thus access the parity of the final state. We find the energy of the individual electron to show a peak structure indicating a quantized sharing of the overall energy absorbed from the field. The joint angular momentum distributions of the two electrons show a highly directed emission of both electrons along the polarization axis as well as clear imprints of electron repulsion. They strongly change with the energy sharing between the electrons. The aspect of selection rules in double ionization which are also visible in the presented dataset has been subject to a preceding publication [1].