Kinematically complete experimental study of Compton scattering at helium atoms near the ionization threshold

TL;DR

This study experimentally investigates Compton scattering at helium atoms near the ionization threshold, revealing complex electron ejection patterns and linking the process to other ionization phenomena.

Contribution

It provides the first kinematically complete experimental data on Compton scattering below the ionization threshold, showing unexpected electron ejection directions.

Findings

Electrons are emitted both in the direction of momentum transfer and backward.

The experiment links Compton scattering to ultrashort optical pulse ionization and impact ionization processes.

New momentum correlation patterns were observed in the scattering process.

Abstract

Compton scattering is one of the fundamental interaction processes of light with matter. Already upon its discovery [1] it was described as a billiard-type collision of a photon kicking a quasi-free electron. With decreasing photon energy, the maximum possible momentum transfer becomes so small that the corresponding energy falls below the binding energy of the electron. Then ionization by Compton scattering becomes an intriguing quantum phenomenon. Here we report a kinematically complete experiment on Compton scattering at helium atoms below that threshold. We determine the momentum correlations of the electron, the recoiling ion, and the scattered photon in a coincidence experiment finding that electrons are not only emitted in the direction of the momentum transfer, but that there is a second peak of ejection to the backward direction. This finding links Compton scattering to processes as ionization by ultrashort optical pulses [2], electron impact ionization [3,4], ion impact ionization [5,6], and neutron scattering [7] where similar momentum patterns occur.