Determining the nature of quantum resonances by probing elastic and reactive scattering in cold collisions
Scattering resonances play a central role in collision processes in physics and chemistry. They help build an intuitive understanding of the collision dynamics due to the spatial localization of the scattering wavefunctions. For resonances that are localized in the reaction region, located at short separation behind the centrifugal barrier, sharp peaks in the reaction rates are the characteristic signature, observed recently with state-of-the-art experiments in low-energy collisions. If, however, the localization occurs outside of the reaction region, mostly the elastic scattering is modified. This may occur due to above-barrier resonances, the quantum analogue of classical orbiting. By probing both elastic and inelastic scattering of metastable helium with deuterium molecules in merged-beam experiments, we differentiate between the nature of quantum resonances—tunnelling resonances versus above-barrier resonances—and corroborate our findings by calculating the corresponding scattering wavefunctions.
Improved design for a highly efficient pulsed-valve supersonic source with extended operating frequency range
We present a new design for a pulsed supersonic-beam source, inspired by the Even-Lavie valve, which is about four times more energy efficient than its predecessor and can run at more than double the repetition rate without experiencing resonances. Its characteristics make it a better candidate as a source for cryogenic-related experiments as well as spectroscopy with rapidly pulsed lasers. The new design is also simpler to build and is more robust, making it accessible to a larger portion of the scientific community.
(2021) Physical Chemistry Chemical Physics. 23, 2, p. 846-858 Abstract
We describe the setup and the performance of a new pulsed Stern-Gerlach deflector and present results for small sodium-doped ammonia clusters Na(NH3)n (n = 1-4) in a molecular beam. NaNH3 shows the expected deflection of a spin ½ system, while all lager clusters show much smaller deflections. Experimental deflection ratios are compared with the values calculated from molecular dynamics simulations. The comparison reveals that intracluster spin relaxation in NaNH3 takes place on a time scale significantly longer than 200 μs. Assuming that intracluster relaxation is the cause of the reduced deflection, relaxation times seem to be on the order of 200 μs for all larger clusters Na(NH3)n (n = 2-4). Our work is a first attempt to understand the magnetic properties of isolated, weakly-bound clusters with relevance to the variety of diamagnetic and paramagnetic species expected in solvated electron systems.
(2020) New Journal of Physics. 22, 10, 103055. Abstract
Trapping of atoms and molecules in electrostatic, magnetic and optical traps has enabled studying atomic and molecular interactions on a timescale of many seconds, allowing observations of ultra-cold collisions and reactions. Here we report the first magnetic deceleration and trapping of neutral carbon atoms in a static magnetic trap. When co-trapping the carbon atoms with oxygen molecules in a superconducting trap, the carbon signal decays in a non-exponential manner, consistent with the decay model describing losses resulting from atom-molecule collisions. Our findings pave the way to studying both elastic and inelastic collisions of species that cannot be laser cooled, and specifically may facilitate the observation of reactions at low temperatures, such as C + O2 → CO + O, which is important in interstellar chemistry.
Direct observation of a Feshbach resonance by coincidence detection of ions and electrons in Penning ionization collisions
Observation of molecular dynamics with quantum state resolution is one of the major challenges in chemical physics. Complete characterization of collision dynamics leads to the microscopic understanding and unraveling of different quantum phenomena such as scattering resonances. Here we present an experimental approach for observing molecular dynamics involving neutral particles and ions that is capable of providing state-to-state mapping of the dynamics. We use Penning ionization reaction between argon and metastable helium to generate argon ion and ground state helium atom pairs at separation of several angstroms. The energy of an ejected electron carries the information about the initial electronic state of an ion. The coincidence detection of ionic products provides a state resolved description of the post-ionization ion-neutral dynamics. We demonstrate that correlation between the electron and ion energy spectra enables us to directly observe the spin-orbit excited Feshbach resonance state of HeAr+. We measure the lifetime of the quasi-bound HeAr(+)A(2) state and discuss possible applications of our method.
(2020) Journal of Physics: Conference Series. 1412, 12, 122001. Abstract
We decelerate and trap molecular oxygen using time-dependent magnetic fields and superconducting magnets. The density-dependent, non-exponential decay in particle number provides a clear proof of moleculemolecule collisions within the trapped ensemble. The spatial distribution of molecules in the trap is found to change over time, allowing to set limits on the ratio between the elastic and inelastic collision cross sections. Our experimental scheme opens up new possibilities for studying quantum effects in chemistry as well as for evaporative cooling of molecules.
(2019) Journal of Physics B: Atomic, Molecular and Optical Physics. 52, 20, 202001. Abstract[All authors]
STIRAP (stimulated Raman adiabatic passage) is a powerful laser-based method, usually involving two photons, for efficient and selective transfer of populations between quantum states. A particularly interesting feature is the fact that the coupling between the initial and the final quantum states is via an intermediate state, even though the lifetime of the latter can be much shorter than the interaction time with the laser radiation. Nevertheless, spontaneous emission from the intermediate state is prevented by quantum interference. Maintaining the coherence between the initial and final state throughout the transfer process is crucial. STIRAP was initially developed with applications in chemical dynamics in mind. That is why the original paper of 1990 was published in The Journal of Chemical Physics. However, from about the year 2000, the unique capabilities of STIRAP and its robustness with respect to small variations in some experimental parameters stimulated many researchers to apply the scheme to a variety of other fields of physics. The successes of these efforts are documented in this collection of articles. In Part A the experimental success of STIRAP in manipulating or controlling molecules, photons, ions or even quantum systems in a solid-state environment is documented. After a brief introduction to the basic physics of STIRAP, the central role of the method in the formation of ultracold molecules is discussed, followed by a presentation of how precision experiments (measurement of the upper limit of the electric dipole moment of the electron or detecting the consequences of parity violation in chiral molecules) or chemical dynamics studies at ultralow temperatures benefit from STIRAP. Next comes the STIRAP-based control of photons in cavities followed by a group of three contributions which highlight the potential of the STIRAP concept in classical physics by presenting data on the transfer of waves (photonic, magnonic and phononic) between respective waveguides. The works on ions or ion strings discuss options for applications, e.g. in quantum information. Finally, the success of STIRAP in the controlled manipulation of quantum states in solid-state systems, which are usually hostile towards coherent processes, is presented, dealing with data storage in rare-earth ion doped crystals and in nitrogen vacancy (NV) centers or even in superconducting quantum circuits. The works on ions and those involving solid-state systems emphasize the relevance of the results for quantum information protocols. Part B deals with theoretical work, including further concepts relevant to quantum information or invoking STIRAP for the manipulation of matter waves. The subsequent articles discuss the experiments underway to demonstrate the potential of STIRAP for populating otherwise inaccessible high-lying Rydberg states of molecules, or controlling and cooling the translational motion of particles in a molecular beam or the polarization of angular-momentum states. The series of articles concludes with a more speculative application of STIRAP in nuclear physics, which, if suitable radiation fields become available, could lead to spectacular results.
(2019) Nature. 572, 7768, p. 189-193 Abstract
Collisions between cold molecules are essential for studying fundamental aspects of quantum chemistry, and may enable the formation of quantum degenerate molecular matter by evaporative cooling. However, collisions between trapped, naturally occurring molecules have not been directly observed so far owing to the low collision rates of dilute samples. Here we report the direct observation of collisions between cold trapped molecules, without the need for laser cooling. We magnetically capture molecular oxygen in an 800-millikelvin-deep superconducting trap and set bounds on the ratio between the elastic- and inelastic-scattering rates-the key parameter determining the feasibility of evaporative cooling. We further co-trap atoms and molecules and identify collisions between them, paving the way for studies of cold interspecies collisions in a magnetic trap.
(2019) Molecular Physics. 117, 15-16, p. 2128-2137 Abstract
In many chemical reactions with more than one possible outcome, the branching ratio, which is the ratio between the different reaction paths, is nearly constant over a wide range of collision energies. In barrierless systems governed by long-range interactions, however, the branching ratio is more sensitive to collision energy, and its dependence on it can be useful for better understanding the dynamics and reconstructing interaction potentials. Here we present the reaction rates of Penning and associative ionisation of metastable neon and helium with argon atoms. We obtain reaction rates in merge beam experiments, over a wide range of collision energies corresponding to that of room temperature, all the way down to a few millikelvins. We observe a change of two orders of magnitude in the branching ratio in the measured collision energy range and explain these changes using theoretical calculations.
(2019) Journal of Physical Chemistry Letters. 10, 4, p. 855-863 Abstract
The quantum phenomena of electronic and nuclear resonances are associated with structures in measured cross sections. Such structures were recently reported in a cold chemistry experiment of ground-state hydrogen isotopologues (H-2/HD) colliding with helium atoms in the excited triplet P-state (He(2(3)P)) [Shagam et al. Nature Chem. 2015, 7, 921], but a theoretical explanation of their appearance was not given. This work presents a quantum explanation and simulation of this experiment, which are strictly based on ab initio calculations. We incorporate complex potential energy surfaces into adiabatic variational theory, thereby reducing the multidimensional scattering process to a series of uncoupled 1D scattering "gedanken experiments". Our theoretical result, which is in remarkable agreement with the experimental data, manifests that the structures in the observed reaction rate coefficient are due to the spatial arrangement of the excited He p-orbitals with respect to the interaction axis, consequently changing the system from a normal two-rotor model to a three-rotor one. This theoretical scheme can be applied to explain and predict cross sections or reaction rate coefficients for any resonance-related phenomenon.
The effect of large autoionization decay rates (resonance widths) on cold molecular cross-sections and the reflection phenomenon
Autoionization resonance states are characterized by a complex potential energy surfaces, where the imaginary part is proportional to the decay rate. Sharp changes in the autoionization decay rates, with respect to the reaction coordinates, introduce reflections. That is, when the autoionization decay rate is very small outside the interaction region and large within it, the reactants will be reflected away from the interaction region. Consequently, the reaction is expected to be suppressed. However, no reflections was observed in the collision of H-2 with He in the P-3 state [Nature Chemistry 7 (11) (2015) 921-926], for which the autoionization decay rate is very large in the interaction region and it is small elsewhere. In this paper, we theoretically investigate the reflection phenomenon within cold-chemistry collisions. Moreover, we present the conditions for which such reactions will not be suppressed due to reflections.
(2018) Physical review letters. 121, 17, 173402. Abstract
We present a joint experimental and theoretical study of spin dynamics of a single Sr-88(+) ion colliding with an ultracold cloud of Rb atoms in various hyperfine states. While spin exchange between the two species occurs after 9.1(6) Langevin collisions on average, spin relaxation of the Sr+ ion Zeeman qubit occurs after 48(7) Langevin collisions, which is significantly slower than in previously studied systems due to a small second-order spin-orbit coupling. Furthermore, a reduction of the endothermic spin-exchange rate is observed as the magnetic field is increased. Interestingly, we find that while the phases acquired when colliding on the spin singlet and triplet potentials vary largely between different partial waves, the singlet-triplet phase difference, which determines the spin-exchange cross section, remains locked to a single value over a wide range of partial waves, which leads to quantum interference effects.
(2018) Journal of Chemical Physics. 148, 11, 114101. Abstract
We present an ab initio theory and computational method for Penning ionization widths. Our method is based on the Fano theory of resonances, algebraic diagrammatic construction (ADC) scheme for many-electron systems, and Stieltjes imaging procedure. It includes an extension of the Fano-ADC scheme [V. Averbukh and L. S. Cederbaum, J. Chem. Phys. 123, 204107 (2005)] to triplet excited states. Penning ionization widths of various He∗-H
2 states are calculated as a function of the distance R between He∗ and H
2. We analyze the asymptotic (large-R) dependences of the Penning widths in the region where the well-established electron transfer mechanism of the decay is suppressed by the multipole- and/or spin-forbidden energy transfer. The R
-12 and R
-8 power laws are derived for the asymptotes of the Penning widths of the singlet and triplet excited states of He∗(1s2s
1,3S), respectively. We show that the electron transfer mechanism dominates Penning ionization of He∗(1s2s
2 up until the He∗-H
2 separation is large enough for the radiative decay of He∗ to become the dominant channel. The same mechanism also dominates the ionization of He∗(1s2s
2 when R < 5 Å. We estimate that the regime of energy transfer in the He∗-H
2 Penning ionization cannot be reached by approaching zero collisional temperature. However, the multipole-forbidden energy transfer mechanism can become important for Penning ionization in doped helium droplets.
(2017) Physical Review Letters. 119, 7, 073204. 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.
(2017) Science Advances. 3, 3, e1602258. 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.
Adiabatic Variational Theory for Cold Atom-Molecule Collisions: Application to a Metastable Helium Atom Colliding with ortho- and para-Hydrogen Molecules
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) Nature Physics. 13, 1, p. 35-38 Abstract[All authors]
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.
(2016) Journal of Physical Chemistry A. 120, 19, p. 3309-3315 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.
Molecular hydrogen interacts more strongly when rotationally excited at low temperatures leading to faster reactions.
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) The Journal of chemical physics. 143, 7, 074114. 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) New Journal of Physics. 17, 065015. 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) Optics Letters. 39, 15, p. 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
(2014) Nature Chemistry. 6, 4, p. 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.
(2013) Journal of Physical Chemistry C. 117, 43, p. 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.
Observation of Resonances in Penning Ionization Reactions at Sub-Kelvin Temperatures in Merged Beams
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) Physical Review A. 85, 5, 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) New Journal of Physics. 13, 103030. 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.
(2011) Physical Chemistry Chemical Physics. 13, 42, p. 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.
(2009) 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) 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, .Non Hermitian Quantum Mechanics: Formalism and Applications