Session S09: Ultracold Collisions and Photoassociation Processes

Dynamical association of Efimov trimers and atom-dimers in cold gases

The dynamical excitation processes of three-dimensional harmonically trapped three-body systems subjected to time-dependent pulses of magnetic fields are investigated within the hyperspherical framework for zero-range interactions. The pulse characteristics are chosen such that we resonantly associate the first atom-dimer state (above the bare dimer threshold) as well as the Efimov trimer (directly below the threshold) at positive scattering lengths [1]. Following the Ramsey-like protocol of Ref.[1], a second pulse is employed to spectroscopically probe these energy states by measuring the oscillatory behavior of the remaining fraction of atoms during the free evolution in between the two pulses. To gain further insight and understand the various excitation pathways of the Efimov and first atom-dimer states during the entire sequence of pulses a three-level model is developed within perturbation theory.

References:

[1] Y. Yudkin, R. Elbaz, P. Giannakeas, C. H. Greene, and L. Khaykovich, Phys. Rev. Lett. 122, 200402 (2019)   

Propensity rules for product state distribution of ultracold Rb three-body recombination

In this talk, I will present our recent advances in understanding the dynamical processes relevant for state-to-state ultracold chemistry of Rb atoms. Our theoretical and experimental studies of three-body recombination have led to the determination of two types of propensity rules controlling the product state distribution of such chemical reaction [1,2]. The first propensity rule is associated to kinetic energy released in the reaction, making deeply bound molecular products increasingly unlikely to be formed. The second is associated to the spins of the atoms and molecules involved in the reaction, where the only molecular products formed are those whose share the same spin character of the reactant atoms. 

[1] S. Haze, J. P. D'Incao, D. Dorer, M. Deiss, E. Tiemann, P. S. Julienne, J. Hecker Denschlag, arXiv:2112.13714 

[2] J. Wolf, M. Deiss, A. Kurkow, E. Tiemann, B. P. Ruzic, Y. Wang, J. P. D’Incao, P. S. Julienne, and J. H. Denschlag, Science 358, 921 (2017)   

Coherent control of ultracold molecular collisions: Generalized Optical theorem

Coherent control of ultracold collisions relies on coherent superpositions of initial states of the collision partners to tune the scattering amplitudes to a final state via quantum interference of indistinguishable pathways. This method does not require external magnetic (or electric) fields and is complementary to current control methods based on external fields. In our previous work, we demonstrated highly efficient coherent control of state-to-state cross sections for spin exchange in ultracold molecule-molecule collisions in the s-wave regime for both the incident and final collision channels.

However, coherent control could be more easily observed by measuring the total (as opposed to state-to-state) scattering cross sections. In quantum scattering theory, the total cross section is related to the imaginary part of the elastic forward scattering amplitude via the optical theorem. We present a generalization of the multichannel optical theorem to include the scattering of molecules in coherent superpositions of their internal states.  Using the generalized optical theorem, we show extensive control of the total cross-section for O₂-O₂ and ⁸⁵Rb-⁸⁵Rb collisions.   

Photoassociation of atom-molecule pair and quantum Zeno based detection of a molecule

Ultracold collisions of neutral atoms and molecules have been of great interest since it became possible by advances in experimental techniques for cooling and trapping such species. We present an extensive theoretical study for low energy collision between an alkali atom and a diatomic molecule. The long-range interaction between the two species is investigated including the atomic spin-orbit and hyperfine splitting. Photoassociation (PA) rate of an atom and a dimer is calculated for different alkali atoms, namely Na, and Cs. Furthermore, we present a method to detect the formation of diatomic molecules in a tweezer. The atom-molecule interaction, relevant to PA, can be used to suppress tunneling of the molecule from the tweezer by utilizing the quantum Zeno effect. The laser-assisted process for detection enhances the decay rate significantly and therefore increases the detection efficiency.

    

Photoassociation of ultracold long-range polyatomic molecules

We theoretically investigate optical formation of long-range tetratomic and larger polyatomic molecules in the ground electronic state from ultracold pairs of polar molecules. Depending on their relative orientation, we predict that two types of complexes could be produced: a weakly bound and very extended halo state and a pure long-range molecule composed of nearly-collinear diatomic molecules. The second category represents a novel type of bound tetratomic molecules. These complexes are predicted to be stable at cold and ultracold temperatures.

We carried out specific numerical studies for pairs of ultracold KRb and RbCs molecules, leading to the production of (KRb)₂ and (RbCs)₂ complexes. The results and conclusions were generalized to larger complexes based on universal properties of long-range interactions between polar molecules. Thus, we identified several triatomic and tetratomic linear polar molecules with favorable ratios of dipole and quadrupole electric moments for which the proposed approach could be used. Such cold or ultracold complexes should also be stable enough to allow experimental confirmation.   

Efimovian Fano-Feshbach resonances in mass-imbalanced atom-dimer gases

Three-body collisions in a two-species ultracold gas facilitate an ideal platform for the exploration of unique attributes in the Efimov physics. For example, theoretical studies on mass-imbalanced three-body collisions have demonstrated that the opposite sign of the intra- and inter-species scattering lengths results into recombination spectra which exhibit Efimov resonances intertwined with Stueckelberg destructive interference effects. In this work, we exploit these unique attributes of Efimov physics in mass-imbalanced systems by investigating the relevant scattering processes in atom-dimer gases. In particular, our analysis shows that the corresponding atom-dimer collisions are strongly enhanced when an Efimov bound state from the energetically closed three-body channel lies in the atom-dimer continuum, i.e. energetically open channel. Namely, our study demonstrates that in mass-imbalanced atom-dimer gases exists a series of atom-dimer resonances which fulfill a Fano-Feshbach scenario. In addition, we highlight the pivotal role of Stueckelberg physics on the width of the atom-dimer resonances where by adjusting the intra-species scattering length the atom-dimer continuum fully decouples from the Efimov states. Finally, our analysis addresses the universal aspects of this type of atom-dimer resonances, as well as, the importance of the Van der Waals physics.   

Quantum interference effects in stereodynamically prepared cold He+D2 collisions

In a recent experiment using the Stark-induced Adiabatic Raman Passage (SARP) technique, Zhou et al. [J. Chem. Phys. 154, 104309 (2021); Science 374, 960 (2021)] reported angular distributions for  pure rotational quenching of D₂ from j=2 to 0 in the v=2 vibrational level. The differential cross sections for this process for different initial stereodynamic preparation of the molecular bond-axis alignment relative to the SARP laser polarization revealed that the angular distribution is dominated by a l=2 partial wave, suggesting the possibility of a shape resonance near 1 K that controls the collision dynamics. Here we report explicit quantum scattering calculations of He+D₂ collisions on a highly accurate interaction potential that reveal the presence of a weak l=2 resonance near 1 K in agreement with the experiment. However, our results uncover a much more intense l=1 shape resonance around 0.01-0.1 K that dominates the angular distribution when it is averaged  to reproduce the experimental velocity distribution.  Our results reproduce key features of the experimentally measured angular distributions when contributions from the l = 1 resonance are artificially suppressed.   

Quantum kinetic isotope effect in He(23S)-H2/HD/D2 collision pairs at subkelvin temperatures

We demonstrate very accurate theoretical results of cold reaction rate coefficients of excited metastable helium and H₂/HD/D₂, confirming the recently observed strong quantum kinetic isotope effect in subkelvin collisions [1]. The calculations are carried out using our newly developed high-quality ab initio interaction energy surface [2]. This surface includes information about the nonrigidity effects of the molecule that play an important role in low-energy molecular anisotropic collisions. The obtained rate of the Penning ionization process can differ even by an order of magnitude, depending on the used isotopologue at certain collision energy. We demonstrate the origin of observed resonances and the mechanism of their appearing and disappearing. Since our unique calculations include, besides the rotational excitation, the vibrational excitation of the molecule, we are able to show that the low-energy resonances (below 1 K) can be effectively controlled by the rovibrational state of the molecule. The change of the rotational and vibrational quantum numbers of the molecule can strongly enhance or suppress the Penning ionization reaction like a quantum switch. 

[1] E. Lavert-Ofir et al., Nature Chem. 6, 332 (2014)

[2] M. Pawlak et al., J. Chem. Theory Comput. 16, 2450 (2020)   

Empirical LiK excited state potentials: connecting short-range and near-dissociation expansions

We report on a high-resolution spectroscopic survey of ⁶Li⁴⁰K molecules near the 2S+4P dissociation threshold and produce a fully empirical representation for the B¹Π potential by connecting available short- and long-range data. The purpose is to identify a suitable intermediate state for a coherent Raman transfer to the absolute ground state, and the creation of a molecular gas with dipolar interactions. Starting from weakly-bound ultracold Feshbach molecules, the transition frequencies to twenty-six vibrational states are determined. Our data are combined with long-range measurements¹, and near-dissociation expansions for the spin-orbit coupled potentials are fitted to extract the van der Waals C₆ dispersion coefficients. A suitable vibrational level is identified by resolving its Zeeman structure and by comparing the experimentally attained g-factor to our theoretical prediction. Using mass-scaling of the short-range data² for the B¹Π and an updated value for its depth, we model the short- and the long-range data simultaneously and produce a Rydberg-Klein-Rees curve covering the entire range. This work was recently published in PCCP³. 

¹Ridinger et al., EPL, 2011, 96, 33001

²Pashov et al., Chem. Phys. Lett., 1998, 292, 615-620

³Botsi et al., PCCP, 2022, 24, 3933 - 3940   

Towards a coherent control of the cold H + D2 and D + HD chemical reactions through stereodynamic preparation of the reactants

Stereodynamic control of the inelastic scattering of ro-vibrationally excited molecules, in cold collisions, has been demonstrated recently by combining a co-expansion of the colliding species in a single molecular beam and the Stark-induced adiabatic Raman Passage  (SARP) technique for molecular state preparation. We have recently explored this approach for the title reactions, in which specific alignments of the bond-axis for both reactants are considered. We have found strong evidence that supports a nearly-total stereodynamic control of the reaction, from significant enhancement to near complete suppression for molecules prepared in the v=4 vibrational level for which the reaction occurs along a barrierless pathway. In this work, we explore quantum coherence effects in this reaction,  considering  two simultaneous alignments of the bond-axis using a cross polarizing scheme as in the X-SARP experiment.