Inelastic collisions are observed between magnetically trapped neutral hydroxyl (OH•) radicals at a temperature of 45 mK in the presence of an electric field. The collision rate is measured over a range of electric fields from 0.2 to 10 kV/cm. However, the two-body collision rates must be deconvolved from a novel electric-field induced non-adiabatic single particle loss, which arises from Landau–Zener crossings between energy surfaces at the plane where the electric field vector is transverse to the trapping magnetic field. The observed inelastic collision rate follows an approximate quadratic power law in the effective dipole moment.Inelastic collisions are observed between magnetically trapped neutral hydroxyl (OH•) radicals at a temperature of 45 mK in the presence of an electric field. The collision rate is measured over a range of electric fields from 0.2 to 10 kV/cm. However, the two-body collision rates must be deconvolved from a novel electric-field induced non-adiabatic single particle loss, which arises from Landau–Zener crossings between energy surfaces at the plane where the electric field vector is transverse to the trapping magnetic field. The observed inelastic collision rate follows an approximate quadratic power law in the effective dipole moment.

Bulletin of the American Physical SocietyJun 6, 20131 citation

Molecular Beam Collisions with a Magnetically Trapped Target

Brian C. Sawyer, Benjamin Stuhl, Dajun Wang +2 more

Cold, neutral hydroxyl radicals are Stark decelerated and efficiently loaded into a permanent magnetic trap. The OH molecules are trapped in the rovibrational ground state at a density of $\ensuremath{\sim}{10}^{6}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$ and temperature of 70 mK. Collision studies between the trapped OH sample and supersonic beams of atomic He and molecular ${\mathrm{D}}_{2}$ determine absolute collision cross sections. The He-OH and ${\mathrm{D}}_{2}\mathrm{\text{\ensuremath{-}}}\mathrm{OH}$ center-of-mass collision energies are tuned from $60\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ to $230\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ and $145\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ to $510\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$, respectively, yielding evidence of quantum threshold scattering and resonant energy transfer between colliding particles.Cold, neutral hydroxyl radicals are Stark decelerated and efficiently loaded into a permanent magnetic trap. The OH molecules are trapped in the rovibrational ground state at a density of $\ensuremath{\sim}{10}^{6}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}3}$ and temperature of 70 mK. Collision studies between the trapped OH sample and supersonic beams of atomic He and molecular ${\mathrm{D}}_{2}$ determine absolute collision cross sections. The He-OH and ${\mathrm{D}}_{2}\mathrm{\text{\ensuremath{-}}}\mathrm{OH}$ center-of-mass collision energies are tuned from $60\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ to $230\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ and $145\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ to $510\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$, respectively, yielding evidence of quantum threshold scattering and resonant energy transfer between colliding particles.

arXiv:0806.2624Physical Review LettersNov 14, 2008113 citations

Achieving a Large Density of Hydroxyl Radicals for Cold Collisions

Hao Wu

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.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.

Jan 1, 20190 citations

Cold heteromolecular dipolar collisions

Brian C. Sawyer, Benjamin Stuhl, Mark Yeo +7 more

Cold molecules promise to reveal a rich set of novel collision dynamics in the low-energy regime. By combining for the first time the techniques of Stark deceleration, magnetic trapping, and cryogenic buffer gas cooling, we present the first experimental observation of cold collisions between two different species of state-selected neutral polar molecules. This has enabled an absolute measurement of the total trap loss cross sections between OH and ND(3) at a mean collision energy of 3.6 cm(-1) (5 K). Due to the dipolar interaction, the total cross section increases upon application of an external polarizing electric field. Cross sections computed from ab initio potential energy surfaces are in agreement with the measured value at zero external electric field. The theory presented here represents the first such analysis of collisions between a (2)Π radical and a closed-shell polyatomic molecule.Cold molecules promise to reveal a rich set of novel collision dynamics in the low-energy regime. By combining for the first time the techniques of Stark deceleration, magnetic trapping, and cryogenic buffer gas cooling, we present the first experimental observation of cold collisions between two different species of state-selected neutral polar molecules. This has enabled an absolute measurement of the total trap loss cross sections between OH and ND(3) at a mean collision energy of 3.6 cm(-1) (5 K). Due to the dipolar interaction, the total cross section increases upon application of an external polarizing electric field. Cross sections computed from ab initio potential energy surfaces are in agreement with the measured value at zero external electric field. The theory presented here represents the first such analysis of collisions between a (2)Π radical and a closed-shell polyatomic molecule.

arXiv:1008.5127Physical Chemistry Chemical PhysicsJan 1, 201195 citations

Cold Polar Molecules for Novel Collision Experiments at Low Energies

Brian C. Sawyer

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.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.

Jan 1, 20102 citations

Hyperfine structure of the hydroxyl free radical (OH) in electric and magnetic fields

Kenji Maeda, Michael L Wall, Lincoln D. Carr

We investigate single-particle energy spectra of the hydroxyl free radical (OH) in the lowest electronic and rovibrational level under combined static electric and magnetic fields, as an example of heteronuclear polar diatomic molecules. In addition to the fine-structure interactions, the hyperfine interactions and centrifugal distortion effects are taken into account to yield the zero-field spectrum of the lowest 2 Π 3 / 2 manifold to an accuracy of less than 2 kHz. We also examine level crossings and repulsions in the hyperfine structure induced by applied electric and magnetic fields. Compared to previous work, we found more than 10% reduction of the magnetic fields at level repulsions in the Zeeman spectrum subjected to a perpendicular electric field. In addition, we find new level repulsions, which we call Stark-induced hyperfine level repulsions, that require both an electric field and hyperfine structure. It is important to take into account hyperfine structure when we investigate physics of OH molecules at micro-Kelvin temperatures and below.We investigate single-particle energy spectra of the hydroxyl free radical (OH) in the lowest electronic and rovibrational level under combined static electric and magnetic fields, as an example of heteronuclear polar diatomic molecules. In addition to the fine-structure interactions, the hyperfine interactions and centrifugal distortion effects are taken into account to yield the zero-field spectrum of the lowest 2 Π 3 / 2 manifold to an accuracy of less than 2 kHz. We also examine level crossings and repulsions in the hyperfine structure induced by applied electric and magnetic fields. Compared to previous work, we found more than 10% reduction of the magnetic fields at level repulsions in the Zeeman spectrum subjected to a perpendicular electric field. In addition, we find new level repulsions, which we call Stark-induced hyperfine level repulsions, that require both an electric field and hyperfine structure. It is important to take into account hyperfine structure when we investigate physics of OH molecules at micro-Kelvin temperatures and below.

arXiv:1410.3849New Journal of PhysicsApr 22, 201531 citations

{\mathrm{D}}_{2}

determine absolute collision cross sections. The He-OH and ${\mathrm{D}}_{2}\mathrm{\text{\ensuremath{-}}}\mathrm{OH}$ center-of-mass collision energies are tuned from $60\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ to $230\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ and $145\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ to $510\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$, respectively, yielding evidence of quantum threshold scattering and resonant energy transfer between colliding particles.