Narrow-Band Pulsed Electron Source Based on Near-Threshold Photoionization of Cs in a Magneto-Optical Trap

TL;DR

This paper presents a novel time-of-flight momentum microscopy method to analyze cold electron emission from a Cs magneto-optical trap, revealing detailed ionization pathways and electron distributions under near-threshold photoionization.

Contribution

The study introduces a combined ToF microscopy and resonant photoionization technique to investigate cold electron emission with high spectral and spatial resolution.

Findings

Identification of ionization pathways via characteristic dependencies

Observation of transition between direct photoemission and Rydberg state ionization

Disentanglement of ionization processes based on polarization and field dependence

Abstract

The newly developed method of time-of-flight (ToF) momentum microscopy was used to analyse the cold electron emission from a Cs 3D magneto-optical trap (MOT). Three-step resonant photoionization was implemented via two intermediate states (6P3/2 pumped with 852 nm laser and 7S1/2 with 1470 nm) and a tuneable femtosecond Ti:sapphire laser for the final ionization step. The magnetic field of the MOT is switched off during the photoionization step. The natural bandwidth of the fs-laser is reduced to 4 meV using optical spectral filters. Precise tuning of the photon energy makes it possible to observe the transition regime between direct photoemission into the open continuum and field induced ionization of highly-excited Rydberg states. The paths can be identified by their characteristic dependency on the extraction field and on the Ti:sapphire polarization. ToF analysis allowed us to disentangle the ionization paths and the dependence of the spatio-temporal distribution of the cold electrons on the polarization of the ionizing laser.