(2017). Directly probing anisotropy in atom-molecule collisions through quantum scattering resonances. NATURE PHYSICS. 13:35-38. Abstract
Anisotropy is a fundamental property of particle interactions. It occupies a central role in cold and ultracold molecular processes, where orientation-dependent long-range forces have been studied in ultracold polar molecule collisions(1,2). In the cold collisions regime, quantization of the intermolecular degrees of freedom leads to quantum scattering resonances. Although these states have been shown to be sensitive to details of the interaction potential(3-8), the effect of anisotropy on quantum resonances has so far eluded experimental observation. Here, we directly measure the anisotropy in atom molecule interactions via quantum resonances by changing the quantum state of the internal molecular rotor. We observe that a quantum scattering resonance at a collision energy of k(B) x 270 mK appears in the Penning ionization of molecular hydrogen with metastable helium only if the molecule is rotationally excited. We use state-of-the-art ab initio theory to show that control over the rotational state effectively switches the anisotropy on or off, disentangling the isotropic and anisotropic parts of the interaction.
(2017). Molecular beam brightening by shock-wave suppression. SCIENCE ADVANCES. 3. Abstract
Supersonic beams are a prevalent source of cold molecules used in the study of chemical reactions, atom interferometry, gas-surface interactions, precision spectroscopy, molecular cooling, andmore. The triumph of this method emanates from the high densities produced in relation to othermethods; however, beam density remains fundamentally limited by interference with shock waves reflected from collimating surfaces. We show experimentally that this shock interaction can be reduced or even eliminated by cryocooling the interacting surface. An increase of nearly an order of magnitude in beam density was measured at the lowest surface temperature, with no further fundamental limitation reached. Visualization of the shock waves by plasma discharge and reproduction with direct simulation Monte Carlo calculations both indicate that the suppression of the shock structure is partially caused by lowering the momentum flux of reflected particles and significantly enhanced by the adsorption of particles to the surface. We observe that the scaling of beam density with source pressure is recovered, paving the way to order-of-magnitude brighter, cold molecular beams.
(2017). Adiabatic Variational Theory for Cold Atom-Molecule Collisions: Application to a Metastable Helium Atom Colliding with ortho- and para-Hydrogen Molecules. JOURNAL OF PHYSICAL CHEMISTRY A. 121:2194-2198. Abstract
We recently developed an adiabatic theory for cold molecular collision experiments. In our previous application of this theory (Pawlak, M.; et al. J. Chem. Phys. 2015, 143, 074114), we assumed that during the experiment the collision of an atom with a diatom takes place when the diatom is in the ground rotational state and is located in a plane. In this paper, we present how the variational approach of the adiabatic theory for low-temperature collision experiments can be used for the study a 5D collision between the atom and the diatomic molecule with no limitations on its rotational quantum states and no plane restrictions. Moreover, we show here the dramatic differences in the measured reaction rates of He(2(3)S(1)) + ortho/para-H2 -> He(1s(2)) + ortho/para-HI2+ + e(-) resulting from the anisotropic long-range interactions in the reaction. In collisions of metastable helium with molecular hydrogen in the ground rotational state, the isotropic potential term dominates the dynamics. When the collision is with molecular hydrogen in the first excited rotational state, the nonisotropic interactions play an important role in the dynamics. The agreement of our results with the latest experimental findings (Klein, A.; et al. Nat. Phys. 2017, 13, 35-38) is very good.
(2017). Trapping of Molecular Oxygen together with Lithium Atoms. PHYSICAL REVIEW LETTERS. 119. Abstract
We demonstrate simultaneous deceleration and trapping of a cold atomic and molecular mixture. This is the first step towards studies of cold atom-molecule collisions at low temperatures as well as application of sympathetic cooling. Both atoms and molecules are cooled in a supersonic expansion and are loaded into a moving magnetic trap that brings them to rest via the Zeeman interaction from an initial velocity of 375 m/s. We use a beam seeded with molecular oxygen, and entrain it with lithium atoms by laser ablation prior to deceleration. The deceleration ends with loading of the mixture into a static quadrupole trap, which is generated by two permanent magnets. We estimate 10(9) trapped O-2 molecules and 10(5) Li atoms with background pressure limited lifetime on the order of 1 sec. With further improvements to lithium entrainment we expect that sympathetic cooling of molecules is within reach.
(2016). Photoassociation Spectroscopy in Penning Ionization Reactions at Sub-Kelvin Temperatures. JOURNAL OF PHYSICAL CHEMISTRY A. 120. Abstract
Penning ionization reactions in merged beams with precisely controlled collision-energies have been shown to accurately probe quantum mechanical, effects in reactive collisions. A complete microscopic understanding of the reaction is) however, faced with two major challenges the highly excited character of the reaction's entrance channel and the limited precision of even the best state-of-the-art ab initio potential energy surfaces. Here, we suggest photoassociation spectroscopy as a tool to identify the character of orbiting resonances in the entrance channel and probe the ionization width as a function of interparticle separation. We introduce the basic concept, using the example of metastable helium and argon, and discuss the general conditions under which this type of spectroscopy will be successful.
(2015). Adiabatic theory for anisotropic cold molecule collisions. The Journal of chemical physics. 143:74114. Abstract
We developed an adiabatic theory for cold anisotropic collisions between slow atoms and cold molecules. It enables us to investigate the importance of the couplings between the projection states of the rotational motion of the atom about the molecular axis of the diatom. We tested our theory using the recent results from the Penning ionization reaction experiment (4)He(1s2s (3)S) + HD(1s(2)) --> (4)He(1s(2)) + HD(+)(1s) + e(-) [Lavert-Ofir et al., Nat. Chem. 6, 332 (2014)] and demonstrated that the couplings have strong effect on positions of shape resonances. The theory we derived provides cross sections which are in a very good agreement with the experimental findings.
(2015). Molecular hydrogen interacts more strongly when rotationally excited at low temperatures leading to faster reactions. Nature chemistry. 7:921-6. Abstract
The role of internal molecular degrees of freedom, such as rotation, has scarcely been explored experimentally in low-energy collisions despite their significance to cold and ultracold chemistry. Particularly important to astrochemistry is the case of the most abundant molecule in interstellar space, hydrogen, for which two spin isomers have been detected, one of which exists in its rotational ground state whereas the other is rotationally excited. Here we demonstrate that quantization of molecular rotation plays a key role in cold reaction dynamics, where rotationally excited ortho-hydrogen reacts faster due to a stronger long-range attraction. We observe rotational state-dependent non-Arrhenius universal scaling laws in chemi-ionization reactions of para-H2 and ortho-H2 by He(2(3)P2), spanning three orders of magnitude in temperature. Different scaling laws serve as a sensitive gauge that enables us to directly determine the exact nature of the long-range intermolecular interactions. Our results show that the quantum state of the molecular rotor determines whether or not anisotropic long-range interactions dominate cold collisions.
(2015). Simultaneous deceleration of atoms and molecules in a supersonic beam. NEW JOURNAL OF PHYSICS. 17. Abstract
A unique property of Zeeman effect based manipulation of paramagnetic particle's motion is the ability to control velocities of both atoms and molecules. In particular the moving magnetic trap decelerator is capable of slowing and eventually trapping mixtures of both cold atoms and cold molecules generated in a supersonic expansion. Here we report the deceleration of molecular oxygen together with metastable argon atoms. The cold mixture with temperature below 1 K is slowed from an initial velocity of 430 m s(-1) down to 100 m s(-1). Our decelerator spans 2.4 m and consists of 480 quadrupole traps. Our results pave the way for the study of sympathetic cooling of molecules by laser cooled atoms.
(2014). Observation of the isotope effect in sub-kelvin reactions. Nature Chemistry. 6:332-335. Abstract
Quantum phenomena in the translational motion of reactants, which are usually negligible at room temperature, can dominate reaction dynamics at low temperatures. In such cold conditions, even the weak centrifugal force is enough to create a potential barrier that keeps reactants separated. However, reactions may still proceed through tunnelling because, at low temperatures, wave-like properties become important. At certain de Broglie wavelengths, the colliding particles can become trapped in long-lived metastable scattering states, leading to sharp increases in the total reaction rate. Here, we show that these metastable states are responsible for a dramatic, order-of-magnitude-strong, quantum kinetic isotope effect by measuring the absolute Penning ionization reaction rates between hydrogen isotopologues and metastable helium down to 0.01 K. We demonstrate that measurements of a single isotope are insufficient to constrain ab initio calculations, making the kinetic isotope effect in the cold regime necessary to remove ambiguity among possible potential energy surfaces.
(2014). Magneto-optical cooling of atoms. Optics Letters. 39:4502-4505. Abstract
We propose an alternative method to laser cooling. Our approach utilizes the extreme brightness of a supersonic atomic beam, and the adiabatic atomic coilgun to slow atoms in the beam or to bring them to rest. We show how internal-state optical pumping and stimulated optical transitions, combined with magnetic forces, can be used to cool the translational motion of atoms. This approach does not rely on momentum transfer from photons to atoms, as in laser cooling. We predict that our method can surpass laser cooling in terms of flux of ultracold atoms and phase-space density, with lower required laser power. (C) 2014 Optical Society of America
(2013). Sub-Kelvin Collision Temperatures in Merged Neutral Beams by Correlation in Phase-Space. Journal Of Physical Chemistry C. 117:22454-22461. Abstract
Recent merged neutral beam experiments have introduced the possibility of measuring reactive collisions in the cold regime down to 10 mK. The lowest temperature attained in these experiments cannot be explained using the standard formalism developed for crossed molecular beam scattering. These low temperatures become accessible because pulsed supersonic beams develop a correlation in velocity-position space during free propagation such that the local velocity standard deviation decreases. This effect is responsible for a reduction in the attainable collision energy by more than 2 orders of magnitude along with an order of magnitude improvement in the resolution. We show that supersonic nozzles with short pulsed opening durations compared to the time-of-flight, such as the Even-Lavie valve, have a clear advantage in achieving low collision energies with improved resolution. We discuss possible improvements in the energy resolution by varying the detection time duration.
(2012). Observation of Resonances in Penning Ionization Reactions at Sub-Kelvin Temperatures in Merged Beams. Science. 338:234-238. Abstract
Experiments have lagged theory in exploring chemical interactions at temperatures so low that translational degrees of freedom can no longer be treated classically. The difficulty has been to realize in the laboratory low-enough collisional velocities between neutral reactants to access this regime. We report here the realization of merged neutral supersonic beams and the manifestation of clear nonclassical effects in the resulting reactions. We observed orbiting resonances in the Penning ionization reaction of argon and molecular hydrogen with metastable helium, leading to a sharp absolute ionization rate increase in the energy range corresponding to a few degrees kelvin down to 10 millikelvin. Our method should be widely applicable to many canonical chemical reactions.
(2012). Density and phase-space compression of molecular gases in magneto-electrostatic traps. Physical Review A. 85. Abstract
We introduce, analyze, and compare two methods of single-photon cooling that generically cool and compress molecular gases. The first method compresses the molecular gas density by 3 orders of magnitude and increases collision frequency in trapped samples. The second method compresses the phase-space density of the gas by at least 2 orders of magnitude. Designed with combinations of electric and magnetic fields, these methods cool the molecules from similar to 100 to 1 mK using a single irreversible state change. They can be regarded as generic cooling schemes applicable to any molecule with a magnetic and electric dipole moment. The high efficiency calculated, compared to schemes involving cycling, is a result of cooling the molecules in a single step.
(2011). Stopping paramagnetic supersonic beams: the advantage of a co-moving magnetic trap decelerator. Physical Chemistry Chemical Physics. 13:18948-18953. Abstract
The long standing goal of chemical physics is finding a convenient method to create slow and cold beams intense enough to observe chemical reactions in the temperature range of a few Kelvin. We present an extensive numerical analysis of our moving magnetic trap decelerator showing that a 3D confinement throughout the deceleration process enables deceleration of almost all paramagnetic particles within the original supersonic expansion to stopping velocities. We show that the phase space region containing the decelerating species is larger by two orders of magnitude as compared to other available deceleration methods.
(2011). A moving magnetic trap decelerator: a new source of cold atoms and molecules. New Journal Of Physics. 13. Abstract
We present an experimental realization of a moving magnetic trap decelerator, where paramagnetic particles entrained in a cold supersonic beam are decelerated in a co-moving magnetic trap. Our method allows for an efficient slowing down of both paramagnetic atoms and molecules to near stopping velocities. We show that under realistic conditions we will be able to trap and decelerate a large fraction of the initial supersonic beam. We present our first results on deceleration in a moving magnetic trap by bringing metastable neon atoms to near rest. Our estimated phase space volume occupied by decelerated particles at a final velocity of 50 m s(-1) shows an improvement of two orders of magnitude as compared to currently available deceleration techniques.
(2009). Single-photon molecular cooling. New Journal Of Physics. 11. Abstract
We propose a general method to cool the translational motion of molecules. Our method is an extension of single photon atomic cooling which was successfully implemented in our laboratory. Requiring a single event of absorption followed by a spontaneous emission, this method circumvents the need for a cycling transition and can be applied to any paramagnetic or polar molecule. In our approach, trapped molecules would be captured near their classical turning points in an optical dipole or RF-trap following an irreversible transition process.
(2003). Non Hermitian Quantum Mechanics: Formalism and Applications. editors E. J. Brandas and E. S. Kryachko, Fundamental World of Quantum Chemistry a Tribute Volume to the Memory of Per-Olov Lowdin, Kluwer, Dordrecht, .