Session S11: Focus Session: Collisions in Extreme Settings

Fragmentation of the helium dimer by the impact of relativistic highly charged ions.

We study the fragmentation of He₂ dimers into He⁺ ions by relativistic highly charged projectiles. We demonstrate that the interaction between an ultrafast projectile with an extremely extended object—the helium dimer—possesses interesting features that are absent in collisions with “normal” molecules. We also show that such projectiles, due to their enormous interaction range, can accurately probe the ground state of the dimer and even be used for a determination of its binding energy.   

Control of He*-Li chemi-ionization

Ultracold mixtures of different atomic species are used to obtain dense samples of ultracold heteronuclear molecules which may feature long-range and anisotropic interactions. Such interactions allow for new physics and chemistry studies in a regime purely dominated by quantum effects. To achieve the co-trapping of ultracold atoms, reactive collisions must be efficiently suppressed.

As a first step towards co-trapping, we study the chemi-ionization of ultracold Li by metastable He (He*). For this, we combine a supersonic-beam source for He* with a magneto-optical trap for Li [1]. To distinguish in between the contributions of He(2³S₁) and He(2¹S₀) to the ionization rate, we deplete the He* population in the 2¹S₀ state using a novel laser-excitation scheme [2]. We also use laser-optical pumping to prepare both He(2³S₁) and Li(2²S_1/2) in selected magnetic sub-levels prior to the collision [3].

Here, we demonstrate the efficient control of He*-Li chemi-ionization at thermal energies using spin- and quantum-state preparation. Our results imply a strong suppression (enhancement) of chemi-ionization for non-spin-conserving (spin-conserving) reaction channels. These results are in good agreement with a model based on spin angular momentum coupling of the prepared atomic states to the molecular reaction channels. Small deviations from the model are indicative for a violation of spin-conservation rules. The ionization rate also decreases when Li is laser-excited to the 2²P_1/2,3/2 states. This is due to the conservation of the projection of the total molecular orbital angular momentum along the internuclear axis [4].

[1] Grzesiak, J. et al., J. Chem. Phys. 150, 034201 (2019).

[2] Guan, J. et al., Phys. Rev. Appl. 11, 054037 (2019).

[3] Sixt, T. et al., Rev. Sci. Instrum. 92, 073203 (2021).

[4] Dulitz, K. et al., Phys. Rev. A 102, 022818 (2020).   

Feshbach Resonance in NaLi-NaLi collision

Richness of molecular systems due to large number of molecular rovibrational bound states, anisotropic and long-range interactions has drawn attention for new platforms of quantum control. In recent years, new ideas emerged to explain fast losses in non-reactive molecular systems, and studies on intermediate complexes is envisioned as frontier for controlled chemistry. Feshbach resonance is one of the great tools to learn about collision complexes as they happen when a molecular bound state in the closed channel approaches the threshold of the scattering state in the open channel. ²³Na⁶Li is a fermionic molecule that has weak singlet-triplet mixing making it a suitable system to study the triplet ground state. It is notable for its both non-zero electric and magnetic dipole moments and small two-body scattering rate, as predicted by universal model for cold collision. We report our recent results on magnetic field dependent collision rate between two NaLi molecules that are highly reactive. A narrow p-wave Feshbach resonance is observed where collisional loss rate is enhanced by two orders of magnitude from the p-wave universal value at the background and approaching the 2D unitarity limit near the peak.

    

Feshbach resonances between a single ion and ultracold atoms

The fields of ultracold atoms and trapped ions are important pillars of experimental quantum optics. Recently, the expertise of both fields has been combined in hybrid trapping setups [1] with the aim to prepare atom-ion mixtures at low temperatures where new quantum phenomena have been predicted. Reaching the ultracold regime however, is a challenging task, as intrinsic micromotion heating effects of conventional radio-frequency (rf) traps limit most experiments. In our setup, we follow two pathways to overcome these limitations. On the one hand, we choose an atom-ion mixture with high mass imbalance – ⁶Li atoms and ¹³⁸Ba⁺ ions – as comparably heavy ions are subject to less heating. On the other hand, we have developed optical dipole traps for ions allowing us to probe atom-ion interactions in complete absence of rf fields [2].

 

Using these techniques, we demonstrate the first observation of Feshbach resonances between atoms and ions [3]. We find 11 resonances by magnetic-field dependent ion loss spectroscopy and identify four of them as s-wave resonances. Operating with lower densities we suppress inelastic three-body recombination, allowing us to apply Feshbach resonances to control the ion’s sympathetic cooling rate -  a first step towards reaching atom-ion s-wave scattering.   

Quantum Tomography of Feshbach Resonance States

Quantum phenomena that lead to a formation of long-lived collision complexes, such as scattering resonances play a central role to the outcome of cold molecular collisions. These resonances are fundamental probes of the fine details of internuclear interactions and serve as a benchmark for current computational methods.

Here we present a joint experimental and theory study where we are able to generate and investigate the multi-channel decay of a Feshbach resonance state with quantum state-to-state resolution. Our method is based on the coincidence detection of electron/ion momenta in Penning ionization collisions between metastable noble gas atoms and neutral molecules. At the ionization step of the dynamics, the molecular ion-neutral atom system is generated in a specific Feshbach resonance state which is identified by the kinetic energy of the ejected electron. The kinetic energy of the product ions provides information about the decay of each resonance state to a manifold of states which differ by the final vibrational state of the molecular ion. Here, in a single measurement, we obtain both the energy and the composition at the continuum of each resonance state. Such a tomography of the Feshbach resonance states provides several tens of quantum numbers per measurement.

The experimental results pose a formidable challenge to current computational methods. We show that Feshbach state tomography allows us to probe both the short range interactions that are responsible for rovibrational and reactive dynamics as well as the long range part of the potential affecting the energy location of resonances states.

We also present an experimental scheme for control of tomography of the Feshbach states which is based on the initial constraint of total angular momentum at the Ionization step of the dynamics. The latter is motivated by our recent observation of a partial wave resonance at the lowest state of relative angular momentum.   

Reaction kinetics of SiO+ and H2 in an ion trap at low rotational states and at super-rotor energies

One among many long-standing goals in chemical physics has been to control chemical reactions and understand reaction pathways. To get to this goal, the strategy in the scientific community has been multi-pronged. Ion traps are a great place to study chemical reactions due to the long storage times. Furthermore, recent work in our group has enabled preparing SiO⁺ molecules in a target rotational state from the ground state all the way to “super-rotor” energies. Taking advantage of this technique, I will present our latest results on the reaction kinetics of SiO⁺ + H₂ ---> SiOH⁺ + H in an ion trap and discuss the striking difference between reaction rates at low rotational energies and at “super-rotor” energies (molecules with energy that greatly exceed kT or exceed the bond energy). I will also briefly discuss the results of our collaborators that explain our observations.    

Observation of trap-assisted bound states in ultracold atom-ion collisions

Classically, a collision between two freely moving bodies cannot lead to the creation of a bound state. In many ultracold experiments, however, atoms and ions collide while being trapped. In principle, the presence of the trap, the relative and center-of-mass motion are no longer uncoupled. In atom-ion collisions, in which the ion is deeply trapped in a Paul trap, and an ultracold atom collides with it, energy can be transferred from the relative frame of reference to that of the center-of-mass. This energy transfer can leave the atom bound to the ion for a few 100’s nsec during which multiple short-range collisions occur while this weakly-bound molecule is oscillating in the trap. Here, we report on the detection of these bound molecular states in a linear Paul trap by observing the electronic spin-exchange rates between a Sr⁺ ion and Rb atoms at various magnetic fields. We estimate the bond energy and lifetime of these bound states from our results. We compare these results with numerical simulations and find good agreement.