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.
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.
We investigate atom-diatom reactive collisions, as a preliminary step,in order to assess the possibility of forming Rb(2) molecules in their lowest triplet electronic state by cold collisions of rubidium atoms on the surface of helium nanodroplets [corrected]. A simple model related to the well-known Rosen treatment of linear triatomic molecules [N. Rosen, J. Chem. Phys. 1, 319 (1933)] in relative coordinates is used, allowing to estimate reactive probabilities for different values of the total angular momentum. The best available full dimensional potential energy surface [Guillon et al., J. Chem. Phys. 136, 174307 (2012)] is employed through the calculations. Noticeable values of the probabilities in the ultracold regime, which numerically fulfill the Wigner threshold law, support the feasibility of the process. The rubidium dimer is mainly produced at high vibrational states, and the reactivity is more efficient for a bosonic helium partner than when the fermion species is considered.
We present a theoretical investigation of the chemical reaction SrF + SrF $\ensuremath{\rightarrow}$ products, focusing on reactions at ultralow temperatures. We find that bond swapping SrF + SrF $\ensuremath{\rightarrow}$ Sr2+F2 is energetically forbidden at these temperatures. Rather, the only energetically allowed reaction is SrF + SrF $\ensuremath{\rightarrow}$ SrF2+ Sr, and even then only singlet states of the SrF2 trimer can form. A calculation along a reduced reaction path demonstrates that this abstraction reaction is barrierless and proceeds by one SrF molecule ``handing off'' a fluorine atom to the other molecule.
We investigate the energetics of the atom exchange reaction in the SrF + alkali-metal atom and CaF + alkali-metal atom systems. Such reactions are possible only for collisions of SrF and CaF with the lithium atoms, while they are energetically forbidden for other alkali-metal atoms. Specifically, we focus on SrF interacting with Li, Rb, and Sr atoms and use ab initio methods to demonstrate that the SrF + Li and SrF + Sr reactions are barrierless. We present potential energy surfaces for the interaction of the SrF molecule with the Li, Rb, and Sr atoms in their energetically lowest-lying electronic spin states. The obtained potential energy surfaces are deep and exhibit profound interaction anisotropies. We predict that the collisions of SrF molecules in the rotational or Zeeman excited states most likely have a strong inelastic character. We discuss the prospects for the sympathetic cooling of SrF and CaF molecules using ultracold alkali-metal atoms.
It has recently become possible to form molecules in ultracold gases of trapped alkali metal atoms. Once formed, the molecules may undergo elastic, inelastic and reactive collisions. Inelastic and reactive collisions are particularly important because they release kinetic energy and eject atoms and molecules from the trap. The theory needed to handle such collisions is presented and recent quantum dynamics calculations on ultracold atom–diatom collisions of spin-polarized Li + Li2, Na + Na2 and K + K2 are described. All these systems have potential energy surfaces on which barrierless atom exchange reactions can occur, and both inelastic and reactive rates are very fast (typically k inel > 10-10 cm3 s−1 in the Wigner regime).
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.
Theoretical results for ultracold atom-molecule collisions involving exoergic reactions are presented. Systems with a reaction barrier are considered; specifically, we report the results of extensive computations for D + H 2 and Cl + H2. We analyzed the extreme situation of translationally cold collisions between an atom and an internally hot molecule; namely, for D + H2 we explored the role played by the internal vibrational excitation of the diatomic target for initial states H2(υ) with υ = 0,1,2,…, 8. The υ-dependence of the zero-temperature limit of the reaction rate coefficient shows two distinct regimes: a barrier dominated regime for υ < 4, and a barrierless regime for υ > 4. Also, for highly excited initial states, the distribution over the final states of the products shows an approximate conservation of the internal vibrational energy. For Cl + H2 we studied in detail the isotopic effect by varying continuously the mass of H, which allowed us to find resonance effects in the threshold behavior of cross sections. These resonances are caused by long-lived van der Waals complexes (Cl···H2) which have vanishingly small binding energy, or by virtual states of the Cl···H 2 complex. ^