Velocity map imaging of a slow beam of ammonia molecules inside a quadrupole guide

TL;DR

This paper demonstrates velocity map imaging inside an electrostatic quadrupole guide, enabling the characterization of slow ammonia molecules with high velocity resolution and improved signal-to-noise ratio.

Contribution

It introduces a method for velocity map imaging within a quadrupole guide, including a compensation technique for astigmatism, and applies it to analyze Stark decelerated ammonia molecules.

Findings

Achieved a velocity resolution of 35 m/s.

Successfully eliminated thermal background signals.

Characterized the phase-space distribution of a Stark decelerated ammonia beam.

Abstract

Velocity map imaging inside an electrostatic quadrupole guide is demonstrated. By switching the voltages that are applied to the rods, the quadrupole can be used for guiding Stark decelerated molecules and for extracting the ions. The extraction field is homogeneous along the axis of the quadrupole while it defocuses the ions in the direction perpendicular to both the axis of the quadrupole and the axis of the ion optics. To compensate for this astigmatism, a series of planar electrodes with horizontal and vertical slits is used. A velocity resolution of 35 m/s is obtained. It is shown that signal due to thermal background can be eliminated, resulting in the detection of slow molecules with an increased signal-to-noise ratio. As an illustration of the resolving power, we have used the velocity map imaging system to characterize the phase-space distribution of a Stark decelerated ammonia beam.