Measurements of the energy distribution of an ultracold rubidium ion beam

TL;DR

This study measures the energy distribution of an ultracold rubidium ion beam using a retarding field analyzer, achieving a narrow energy spread and comparing results with detailed numerical simulations to understand ionization effects.

Contribution

It provides the first detailed measurement and analysis of the energy distribution of an ultracold rubidium ion beam, including the effects of ionization position and laser parameters.

Findings

Achieved a full width 50% energy spread of 0.205 eV.

Found good agreement between measurements and optical Bloch equation simulations.

Identified ionization position variation as a key factor in energy spread.

Abstract

The energy distribution of an ultracold rubidium ion beam, which is intended to be used as the source for a focused ion beam instrument, is measured with a retarding field analyzer. The ions are created from a laser-cooled and compressed atomic beam by two-step photoionization in which the ionization laser power is enhanced in a build-up cavity. Particle tracing simulations are performed to ensure the analyzer is able to resolve the distribution. The lowest achieved full width 50% energy spread is $\left(0.205\pm0.006\right)$ eV. The energy spread originates from the variation in the ionization position of the ions which are created inside an extraction electric field. This extraction field is essential to limit disorder-induced heating which can decrease the ion beam brightness. The ionization position distribution is limited by a tightly focused excitation laser beam. Energy distributions are measured for various ionization and excitation laser intensities and compared with calculations based on numerical solutions of the optical Bloch equations including ionization. A good agreement is found between measurements and calculations.