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arXiv:physics/0508120 · Physical Review A

Production of cold formaldehyde molecules for study and control of chemical reaction dynamics with hydroxyl radicals

Eric R. Hudson, Christopher Ticknor, Brian C. Sawyer, Craig A. Taatjes, Heather J. Lewandowski, Jason R. Bochinski, John L. Bohn, Jun Ye

Published Jun 6, 2006• 118 citations
arXivHTMLPDF

Abstract

We propose a method for controlling a class of low temperature chemical reactions. Specifically, we show the hydrogen abstraction channel in the reaction of formaldehyde $({\mathrm{H}}_{2}\mathrm{CO})$ and the hydroxyl radical $(\mathrm{OH})$ can be controlled through either the molecular state or an external electric field. We also outline experiments for investigating and demonstrating control over this important reaction. To this end, we report the first Stark deceleration of ${\mathrm{H}}_{2}\mathrm{CO}$. We have decelerated a molecular beam of ${\mathrm{H}}_{2}\mathrm{CO}$ essentially to rest, producing molecules at $100\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$ with a density of $\ensuremath{\sim}{10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$.

DOI
10.1103/physreva.73.063404

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Production of cold formaldehyde molecules for study and control of chemical reaction.

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We propose a method for controlling a class of low temperature chemical reactions. Specifically, we show the hydrogen abstraction channel in the reaction of formaldehyde (H{sub 2}CO) and the hydroxyl radical (OH) can be controlled through either the molecular state or an external electric field. We also outline experiments for investigating and demonstrating control over this important reaction. To this end, we report the first Stark deceleration of H{sub 2}CO. We have decelerated a molecular beam of H{sub 2}CO essentially to rest, producing molecules at 100 mK with a density of {approx} 10{sup 6} cm{sup -3}.

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The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Hudson, Eric R. (Ph.D., Physics) Experiments on Cold Molecules Produced via Stark Deceleration Thesis directed by Prof. Jun Ye The work described in this thesis is the construction and operation of a Stark deceleration apparatus. Specifically, this work represents only the second successful im-plementation of the method1 and the first deceleration of the hydroxyl radical (OH) and the formaldehyde molecule (H2CO). Experimentally, much of the work in the beginning of our experiments was the construction of a suitable molecular beam. Though molecular beams have been used for decades in physical chemistry, the design goals of the typical physical chemistry experiment are very different than ours, and as such, much work was required to adapt these sources for Stark deceleration. Since the maximum-attainable,

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Cold free-radical molecules in the laboratory frame

Jason R. Bochinski, Eric R. Hudson, Heather J. Lewandowski +1 more

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 $(\mathrm{OH})$, 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 $\mathrm{OH}$ molecules by the electric fields. The creation of a pulse containing ${10}^{3}--{10}^{6}$ molecules possessing a longitudinal velocity spread from 2 to $80\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}$ around a mean laboratory velocity variable from $550\phantom{\rule{0.3em}{0ex}}\mathrm{m}∕\mathrm{s}$ to rest with only a few mm spatial extent represents an exciting and useful experimental capability for exploring free-radical dynamics.

arXiv:physics/0403126•Physical Review A•Oct 19, 2004•60 citations
We propose a method for controlling a class of low temperature chemical reactions. Specifically, we show the hydrogen abstraction channel in the reaction of formaldehyde (H2CO)({\mathrm{H}}_{2}\mathrm{CO})(H2​CO) and the hydroxyl radical (OH)(\mathrm{OH})(OH) can be controlled through either the molecular state or an external electric field. We also outline experiments for investigating and demonstrating control over this important reaction. To this end, we report the first Stark deceleration of H2CO{\mathrm{H}}_{2}\mathrm{CO}H2​CO. We have decelerated a molecular beam of H2CO{\mathrm{H}}_{2}\mathrm{CO}H2​CO essentially to rest, producing molecules at 100mK100\phantom{\rule{0.3em}{0ex}}\mathrm{mK}100mK with a density of \ensuremath∼106cm\ensuremath−3\ensuremath{\sim}{10}^{6}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}\ensuremath∼106cm\ensuremath−3.
We propose a method for controlling a class of low temperature chemical reactions. Specifically, we show the hydrogen abstraction channel in the reaction of formaldehyde (H{sub 2}CO) and the hydroxyl radical (OH) can be controlled through either the molecular state or an external electric field. We also outline experiments for investigating and demonstrating control over this important reaction. To this end, we report the first Stark deceleration of H{sub 2}CO. We have decelerated a molecular beam of H{sub 2}CO essentially to rest, producing molecules at 100 mK with a density of {approx} 10{sup 6} cm{sup -3}.
The final copy of this thesis has been examined by the signatories, and we find that both the content and the form meet acceptable presentation standards of scholarly work in the above mentioned discipline. iii Hudson, Eric R. (Ph.D., Physics) Experiments on Cold Molecules Produced via Stark Deceleration Thesis directed by Prof. Jun Ye The work described in this thesis is the construction and operation of a Stark deceleration apparatus. Specifically, this work represents only the second successful im-plementation of the method1 and the first deceleration of the hydroxyl radical (OH) and the formaldehyde molecule (H2CO). Experimentally, much of the work in the beginning of our experiments was the construction of a suitable molecular beam. Though molecular beams have been used for decades in physical chemistry, the design goals of the typical physical chemistry experiment are very different than ours, and as such, much work was required to adapt these sources for Stark deceleration. Since the maximum-attainable,
Molecular physics has experienced groundbreaking progress in the fields of precision spectroscopy, chemical reaction kinetics, quantum state engineering and many-body physics. In order to better observe these phenomena, there is an insatiable pursuit of larger trapped molecular densities and longer lifetime. In this thesis, several key milestones that we have recently achieved towards these goals for hydroxyl radicals (OH) are reported. First, we discovered an enhanced spin-flip behavior of dipolar molecules due to the existence of dual (electric and magnetic) dipole moments and obtained a better understanding of complex spin-dynamics for both Hund's case (a) and (b) molecules in mixed electric and magnetic fields. Second, we demonstrated that the skimmer cooling technique can be applied to radicals and the production of OH can be enhanced by factor of 30 due to both clogging mitigation and more favorable skimmer-valve distance. Finally, we showed some preliminary results that it is possible to use a newly built Stark decelerator to study electric field controlled sub-kelvin collisions in an intra-beam.
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.
Research in the field of cold polar molecules is progressing rapidly. An array of interesting\ntopics is being developed including precision measurement and fundamental tests, quantum phase transitions, and ultracold chemistry. In particular, dipolar molecules in well-defined quantum states enable exquisite control of their interactions via applied electric fields. The long-range, anisotropic interaction between dipolar molecules leads to new collision dynamics that could be used for novel collective effects, quantum state engineering, and information processing. The focus of this Thesis is the production of cold samples of neutral OH molecules via Stark deceleration and magnetic trapping for novel collision experiments. This novelty results from our combination of cold external beams and trapped target molecules which facilitates observation of dipolar effects as well as low temperature collision resonances. The large permanent electric dipole moment of OH allows us to precisely tune the lab-frame velocity of the molecular packets from ∼ 500 m/s to rest. We have magnetically trapped OH packets at the terminus of our Stark decelerator at a temperature of 70 mK and density of 106 cm−3. With improved understanding of Stark deceleration, we optimize the decelerator efficiency and its coupling to the magnetic trap. Our latest trap design uses permanent ring magnets to create a three-dimensional magnetic quadrupole field. Use of magnetically trapped OH molecules for collision experiments with external beams allow us the unique opportunity of observing both elastic and inelastic collisions. In addition, the trap confinement yields sensitivity to differential elastic cross sections at low collision energies. This open magnetic trap has allowed measurement of collision cross sections between trapped OH and external supersonic beams of He and D2, the latter of which is of astrophysical interest due to the role of H2-OH collisions in pumping interstellar OH megamasers. The combination of trapped OH molecules and temperature tuned beams of He and D2 has facilitated measurement of the lowest-energy D2-OH collision cross sections yet reported. More recently, we report the first observation of electric-field dependent cross sections between two different species of cold polar molecules - OH and ND3 - thereby demonstrating control over molecular scattering in the cold regime. By combining for the first time the production techniques of Stark deceleration and buffer gas cooling, we increase the molecular interaction time by∼ 105 over traditional crossed-beam experiments to gain enhanced sensitivity at the characteristic densities of cold molecule production.
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.