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Ab Initio Investigation of Chemical Reaction Pathways in Ultracold NaBr and KBr Molecular Collisions: Diatomic Validation, Dissociation Energies, and the NaKBr2 Tetramer Intermediate

Marc E. Segovia, Oscar N. Ventura

Published Jun 16, 2026• 0 citations
Publisher

Abstract

Alkali bromides NaBr and KBr possess permanent electric dipole moments of approximately 9.1 and 10.5 D, among the largest of any diatomic molecule, making them exceptional candidates for dipolar quantum science and electric-field-controlled ultracold collisions. We report a systematic ab initio investigation of all two-body reaction channels for NaBr + NaBr, KBr + KBr, and NaBr + KBr collisions using CCSD(T)/aug-cc-pVQZ potential-energy curves validated against experimental spectroscopic constants, supplemented by phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) benchmarks. All bond-swap channels are endoergic by 38,000–43,000 cm−1 (109–122 kcal mol−1) and all atom-abstraction channels by 21,000–25,000 cm−1 (60–71 kcal mol−1), thermodynamically forbidding reactive product formation at ultracold collision energies. DLPNO-CCSD(T) approach potentials, validated against canonical CCSD(T), are purely attractive for all three collision pairs, with no entrance barriers. The NaBr + KBr system additionally supports a deep alternating NaKBr2 short-range complex (well depth −47.9 kcal mol−1 at the CCSD(T)/aug-cc-pVQZ level, −42.8 counterpoise-corrected; −50.3 ± 0.3 kcal mol−1 at the ph-AFQMC level), a rectangular rock-salt intermediate analogous to the M2X2 minimum characterized for calcium monofluoride (CaF + CaF) collisions. Owing to the large permanent dipoles, the isotropic long-range interaction is dominated by the rotational dispersion term, giving total C6 ≈ 4×107–1.3×108 a.u. and universal s-wave loss rates of ≈6–8 × 10−10 cm3 s−1, at or above the KRb value. Suppressing these losses requires electric-field shielding with the molecules prepared in their first rotationally excited state; the resonant mechanism is predicted to be highly effective, with critical fields of ≈1.6–3.4 kV cm−1, comparable to those for strongly dipolar bialkali molecules such as RbCs and within an order of magnitude of KRb (∼12 kV cm−1). These results establish alkali bromides as thermodynamically stable against exchange chemistry while identifying complex-mediated (“sticky”) loss and resonant electric-field shielding as the dominant processes governing ultracold NaBr and KBr collisions.

DOI
10.26434/chemrxiv.15004795/v1

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Alkali bromides NaBr and KBr possess permanent electric dipole moments of approximately 9.1 and 10.5 D, among the largest of any diatomic molecule, making them exceptional candidates for dipolar quantum science and electric-field-controlled ultracold collisions. We report a systematic ab initio investigation of all two-body reaction channels for NaBr + NaBr, KBr + KBr, and NaBr + KBr collisions using CCSD(T)/aug-cc-pVQZ potential-energy curves validated against experimental spectroscopic constants, supplemented by phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) benchmarks. All bond-swap channels are endoergic by 38,000–43,000 cm−1 (109–122 kcal mol−1) and all atom-abstraction channels by 21,000–25,000 cm−1 (60–71 kcal mol−1), thermodynamically forbidding reactive product formation at ultracold collision energies. DLPNO-CCSD(T) approach potentials, validated against canonical CCSD(T), are purely attractive for all three collision pairs, with no entrance barriers. The NaBr + KBr system additionally supports a deep alternating NaKBr2 short-range complex (well depth −47.9 kcal mol−1 at the CCSD(T)/aug-cc-pVQZ level, −42.8 counterpoise-corrected; −50.3 ± 0.3 kcal mol−1 at the ph-AFQMC level), a rectangular rock-salt intermediate analogous to the M2X2 minimum characterized for calcium monofluoride (CaF + CaF) collisions. Owing to the large permanent dipoles, the isotropic long-range interaction is dominated by the rotational dispersion term, giving total C6 ≈ 4×107–1.3×108 a.u. and universal s-wave loss rates of ≈6–8 × 10−10 cm3 s−1, at or above the KRb value. Suppressing these losses requires electric-field shielding with the molecules prepared in their first rotationally excited state; the resonant mechanism is predicted to be highly effective, with critical fields of ≈1.6–3.4 kV cm−1, comparable to those for strongly dipolar bialkali molecules such as RbCs and within an order of magnitude of KRb (∼12 kV cm−1). These results establish alkali bromides as thermodynamically stable against exchange chemistry while identifying complex-mediated (“sticky”) loss and resonant electric-field shielding as the dominant processes governing ultracold NaBr and KBr collisions.
Progress in ultracold experiments with polar molecules requires a clear understanding of their interactions and reactivity at ultralow collisional energies. Two important theoretical steps in this process are the characterization of interaction potentials between molecules and the modeling of the reactive scattering mechanism. Here, we report on the ab initio calculation of isotropic and anisotropic van der Waals interaction potentials for polar KRb and RbCs colliding with each other or with ultracold atoms. Based on these potentials and two short-range scattering parameters, we then develop a single-channel scattering model with flexible boundary conditions. Our calculations show that, at low temperatures (and in the absence of an external electric field), the reaction rates between molecules or between molecules and atoms have a resonant character as a function of the short-range parameters. We also find that both the isotropic and anisotropic van der Waals coefficients have significant contributions from dipole coupling to excited electronic states. Their values can differ dramatically from those solely obtained from the permanent dipole moment. A comparison with recently obtained reaction rates of fermionic 40 K 87 Rb shows that the experimental data cannot be explained by a model where the short-range scattering parameters are independent of the relative orbital angular momentum or partial wave.
Abstract We performed ab initio calculations in order to assess the reactivity of ultracold RbSr ( Σ + 2 ) + RbSr ( Σ + 2 ) collisions occurring on the singlet as well as the triplet potential. At ultracold energies reactions are energetically possible if they release energy, i.e., they are exoergic. The exoergicity of reactions between RbSr molecules producing diatomic molecules are known experimentally. We extend this to reactions producing triatomic molecules by calculating the binding energy of the triatomic reaction products. We find that, in addition to the formation of Rb2 + 2Sr and Rb2 + Sr2 in singlet collisions, also the formation of Sr2Rb + Rb and Rb2Sr + Sr in both singlet and triplet collisions is exoergic. Hence, the formation of these reaction products is energetically possible in ultracold collisions. For all exoergic reactions the exoergicity is larger than 1000 cm−1. We also find barrierless qualitative reaction paths leading to the formation of singlet Rb2 + 2Sr and both singlet and triplet Rb2Sr + Sr and Sr2Rb + Rb reaction products and show that a reaction path with at most a submerged barrier exists for the creation of the singlet Rb2 + Sr2 reaction product. Because of the existence of these reactions we expect ultracold RbSr collisions to result in almost-universal loss even on the triplet potential. Our results can be contrasted with collisions between alkali-diatoms, where the formation of triatomic reaction products is endoergic, and with collisions between ultracold SrF molecules, where during triplet collisions only the spin-forbidden formation of singlet SrF2 is exoergic.
Studying ultracold chemical reactions with quantum state resolution reveals details of the reaction processes, especially with single quantum state preparation of the reactants and state-selective detection of reaction products. In this thesis, we investigate two main processes: KRb + KRb → K2 + Rb2 reactions, and Rb and KRb atom-molecule collisions. In the KRb bimolecular reaction, we demonstrate the preservation of quantum coherence in bimolecular reactions for the first time, which is surprising in a reaction considered to be largely chaotic. Additionally, we proposed a coherent control scheme to manipulate product yields across different product channels. These results represent a critical step toward probing quantum coherence and entanglement in the chemical reaction. In atom-molecule collisions between Rb and KRb, we discovered an exceptionally long-lived intermediate complex, KRb2, which can be photo-excited by trapping light. The observed complex lifetime was orders of magnitude longer than theoretical predictions, underscoring the limitations of the current theory. We further explored the dependence of complex lifetimes on the initial quantum states and external electric and magnetic fields, providing additional experimental benchmarks to guide future theoretical work. Separately, we found that inelastic collisions between hyperfine-excited Rb atoms and KRb molecules can lead to rotation excitation of KRb post-collision. By probing the product state distribution, the result suggests that mechanical angular momentum is coupled to the spins. Such couplings are too weak in the current theory models to explain the result. Moreover, our result contradicts state-of-the-art coupled-channel calculations. These suggest that some subtle effects, such as molecular vibration and conical intersections, play a critical role in the reaction dynamics. The final piece of the thesis describes the observation of resonant interaction between a Rydberg atom and an ensemble of polar molecules. Such hybrid quantum systems have been proposed for a wide range of applications. Our work provides an experimental demonstration of resonant dipolar interactions between Rydberg atoms and ultracold polar molecules, paving the way for the realization of hybrid systems for quantum computation and simulation.
We investigate the ultracold collisions of rotationally excited dipolar molecules in free-space, taking the hetero-nuclear bi-alkali molecule of KRb as an example. We show that we can sharply tune the elastic, inelastic and reactive rate coefficients of lossy molecular collisions when a second rotationally excited colliding channel crosses the threshold of the initial colliding channel, with the help of an applied electric field, as found by Avdeenkov et al for non-lossy molecules ( Phys. Rev. A 73 022707 ). We can increase or decrease the inelastic and reactive processes whether the second channel is above or below the initial channel. This is seen for both bosonic and fermionic molecules. Additionally, we include the electric quadrupole and octopole moment to the dipole moment in the expression of the long-range multipole–multipole interaction. We found that for processes mediated by the incident channel, such as elastic and reactive collisions, the inclusion of quadrupole and octopole moments is not important at ultralow energies. The moments are important for processes mediated by state-to-state transitions like inelastic collisions.
Universal collision rate constants are calculated for ultracold collisions of two like bosonic or fermionic heteronuclear alkali-metal dimers involving the species Li, Na, K, Rb, or Cs. Universal collisions are those for which the short range probability of a reactive or quenching collision is unity such that a collision removes a pair of molecules from the sample. In this case, the collision rates are determined by universal quantum dynamics at very long range compared to the chemical bond length. We calculate the universal rate constants for reaction of the reactive dimers in their ground vibrational state v = 0 and for vibrational quenching of non-reactive dimers with v ≥ 1. Using the known dipole moments and estimated van der Waals coefficients of each species, we calculate electric field dependent loss rate constants for collisions of molecules tightly confined to quasi-two-dimensional geometry by a one-dimensional optical lattice. A simple scaling relation of the quasi-two-dimensional loss rate constants with dipole strength, trap frequency and collision energy is given for like bosons or like fermions. It should be possible to stabilize ultracold dimers of any of these species against destructive collisions by confining them in a lattice and orienting them with an electric field of less than 20 kV cm(-1).