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Conference on Lasers and Electro-Optics

Cold free radical molecules in the laboratory frame

Eric Hudson, Jason R. Bochinski, Heather J. Lewandowski, Jun Ye

Published Jan 1, 2004• 0 citations
Publisher

Abstract

For scientifically interesting diatomic free radical molecules, which typically possess large electric dipole moments, Stark deceleration of weak-field seeking states is an ideal method for cold sample production. We utilize a seeded, pulsed supersonic beam in combination with an electric discharge to realize an efficient free radical source that is well-suited for use with Stark deceleration. The source apparatus creates an intense OH molecular beam whose subsequent longitudinal phase-space manipulation by precisely sequenced, pulsed inhomogeneous electric fields is directly observed by in-situ laser-induced fluorescence (LIF) detection.

DOI
10.1364/iqec.2004.imi5

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For scientifically interesting diatomic free radical molecules, which typically possess large electric dipole moments, Stark deceleration of weak-field seeking states is an ideal method for cold sample production. We utilize a seeded, pulsed supersonic beam in combination with an electric discharge to realize an efficient free radical source that is well-suited for use with Stark deceleration. The source apparatus creates an intense OH molecular beam whose subsequent longitudinal phase-space manipulation by precisely sequenced, pulsed inhomogeneous electric fields is directly observed by in-situ laser-induced fluorescence (LIF) detection.
A special class of molecules that are important to many subfields in molecular dynamics and chemical physics, namely free-radical molecules, now enjoy a significant degree of center-of-mass motion control in the laboratory frame. The example reported in this paper concerns the hydroxyl radical (OH)(\mathrm{OH})(OH), which, after the internal degrees of freedom are cooled in a supersonic expansion, has been bunched, accelerated, and slowed using time-varying inhomogeneous electric fields. In situ observations of laser-induced fluorescence along the beam propagation path allows for detailed characterization of the longitudinal phase-space manipulation of OH\mathrm{OH}OH molecules by the electric fields. The creation of a pulse containing 103−−106{10}^{3}--{10}^{6}103−−106 molecules possessing a longitudinal velocity spread from 2 to 80m∕s80\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}80m∕s around a mean laboratory velocity variable from 550m∕s550\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}550m∕s to rest with only a few mm spatial extent represents an exciting and useful experimental capability for exploring free-radical dynamics.
Samples of cold and ultracold polar molecules have the potential to revolutionize physical chemistry, precision measurement, and few-body quantum physics. This thesis describes experimental results for the production of cold samples of OH radicals by Stark deceleration of a supersonic beam. Since Stark deceleration cannot increase the phase space density of the sample, the initial production stage of the OH molecule is critical. The first set of experiments describes a general methodology for the production of OH beams with maximal phase space density, as well as the subsequent coupling to a Stark decelerator. Additionally, we describe the redesign of our electrostatic trap, optimized for future collision experiments of OH with co-trapped ultracold Rb atoms. The new design resulted in a 15-fold increase in total number of trapped molecules over the previous design when tested with ND3 molecules. The second set of experiments focuses on laser-based detection of OH molecules at the exit of the decelerator based on laser induced fluorescence (LIF) and resonance enhanced multiphoton ionizaton (REMPI). The latter method uses vacuum-ultraviolet light at 118nm produced by third-harmonic generation in Xe/Ar gas mixtures as the ionizing step. The sensitivity of this latter technique is limited by the attainable photon flux of the ionizing radiation at 118nm. We present detailed measurements of the conversion efficiencies as well as absolute photon fluxes. Strategies to overcome these limitations as well as prospects for detection of OH molecules in a trap are discussed.
Deceleration of supersonic beams of SD radicals by a linear Stark decelerator was demonstrated for the first time. SD radicals produced by an electric discharge of supersonic beams mainly occupied the lowest two rotational states of and in the 2Π3/2 state. Laser-induced fluorescence of the decelerated beam showed that, due to the proximity of the Stark shift between these two rotational states, the final radical packet still contained both the and rotational states when the decelerator was operated at a constant phase below 50°. We show that by controlling the timing sequence of the Stark decelerator, it is possible to create a decelerated molecular packet containing only a single rotational state of either or of the SD radicals with a tunable velocity. The creation of a cold radical beam containing a single rotation–vibration state will be a useful tool for the study of cold collisions and reactions in the quantum regime.
The motion of polar molecules can be controlled by time-varying inhomogeneous electric fields. In a Stark decelerator, this is exploited to accelerate, transport, or decelerate a fraction of a molecular beam. When combined with a trap, the decelerator provides a means to store the molecules for times up to seconds. Here, we review our efforts to produce cold molecules via this technique. In particular, we present a new generation Stark decelerator and electrostatic trap that selects a significant part of a molecular beam pulse that can be loaded into the trap. Deceleration and trapping experiments using a beam of OH radicals are discussed.