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arXiv:2211.05909 · Physical Review A

Four-body singlet potential-energy surface for reactions of calcium monofluoride

Dibyendu Sardar, Arthur Christianen, Hui Li, John L. Bohn

Published Mar 24, 2023• 14 citations
arXivHTMLPDF

Abstract

A full six-dimensional Born-Oppenheimer singlet potential-energy surface is constructed for the reaction CaF $+$ CaF $\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}{\mathrm{CaF}}_{2} +$ Ca using a multireference configuration-interaction electronic structure calculation. The ab initio data thus calculated are interpolated by Gaussian process regression. The four-body potential-energy surface features one ${D}_{2h}$ global minimum and one ${C}_{s}$ local minimum, connected by a barrierless transition state that lends insight to the reaction mechanism. This surface is intended to be of use in understanding ultracold chemistry of CaF molecules.

DOI
10.1103/physreva.107.032822

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A full six-dimensional Born-Oppenheimer singlet potential-energy surface is constructed for the reaction CaF +++ CaF \ensuremath→CaF2+\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}{\mathrm{CaF}}_{2} +\ensuremath→CaF2​+ Ca using a multireference configuration-interaction electronic structure calculation. The ab initio data thus calculated are interpolated by Gaussian process regression. The four-body potential-energy surface features one D2h{D}_{2h}D2h​ global minimum and one Cs{C}_{s}Cs​ local minimum, connected by a barrierless transition state that lends insight to the reaction mechanism. This surface is intended to be of use in understanding ultracold chemistry of CaF molecules.
Accurate ab initio potential energy surfaces are essential to understand and predict collisional outcomes in ultracold molecular systems. In this study, we explore the intermolecular interactions between two laser-cooled CaF molecules, in both their ground and excited electronic states, aiming to understand the mechanisms behind the observed collisional losses on the nonreactive, spin-polarized surface of the CaF+CaF\mathrm{CaF}+\mathrm{CaF}CaF+CaF system. Using state-of-the-art ab initio methods, we compute 12 electronic states of the Ca2F2{\mathrm{Ca}}_{2}{\mathrm{F}}_{2}Ca2​F2​ complex within the rigid rotor approximation applied to CaF. Calculating the potential energy surfaces for the excited electronic states of Ca2F2{\mathrm{Ca}}_{2}{\mathrm{F}}_{2}Ca2​F2​ is challenging and computationally expensive. Our approach employs the multireference configuration interaction method, restricted to single and double excitations, along with a reasonably large active space to ensure the convergence in the excited states. We also compute the spin-orbit coupling between the ground state and the lowest spin-polarized triplet state, as well as the spin-spin coupling within the lowest triplet state (1) $^{3}{\mathrm{A}}^{\ensuremath{'}}$. Additionally, we determine the electric transition dipole moments for the (1) $^{3}{\mathrm{A}}^{\ensuremath{'}}$--(2) $^{3}{\mathrm{A}}^{\ensuremath{'}}$ and (1) $^{3}{\mathrm{A}}^{\ensuremath{'}}$--(1) $^{3}{\mathrm{A}}^{\ensuremath{''}}$ transitions. Notably, we find that the lowest spin-polarized state (1) $^{3}{\mathrm{A}}^{\ensuremath{'}}$, shifted by 1064 nm of laser light from the optical dipole trap, intersects several electronically excited states. Finally, by analyzing the potential energy surfaces, we discuss two plausible pathways that may account for the observed collisional losses on the spin-polarized surface of the CaF+CaF\mathrm{CaF}+\mathrm{CaF}CaF+CaF system.
electronic structure and full-dimensional quantum dynamical calculations of the Li + CaF → LiF + Ca chemical reaction. The electronic structure calculations are performed using the internally contracted multi-reference configuration-interaction method with Davidson correction (MRCI + Q). An analytic fit of the interaction energies is obtained using a many-body expansion method. A coupled-channel quantum reactive scattering approach implemented in hyperspherical coordinates is adopted for the scattering calculations under cold conditions. Results show that the Li + CaF reaction populates several low-lying vibrational levels and many rotational levels of the product LiF molecule and that the reaction is inefficient in the 1-100 mK regime allowing sympathetic cooling of CaF by collisions with cold Li atoms.
High Resolution Image Download MS PowerPoint Slide Aluminum monofluoride (AlF) is a promising candidate for laser cooling and the production of dense ultracold molecular gases, thanks to its relatively high chemical stability and diagonal Franck–Condon factors. In this study, we examine the interactions and collisions of AlF in its X 1 Σ +, a 3 Π, and A 1 Π electronic states with ground-state He using state-of-the-art ab initio quantum chemistry techniques. We construct accurate potential energy surfaces (PESs) employing either the explicitly correlated coupled-cluster CCSD(T)-F12 method augmented by the CCSDT correction or the multireference configuration-interaction method for higher-excited electronic states. Subsequently, we employ these PESs in coupled-channel calculations to determine the scattering cross sections for AlF + He collisions and bound states of the complex. We estimate the uncertainty of the calculated PESs and apply it to assess the uncertainty of the scattering results. We find a relatively low sensitivity of the cross sections to the variation of the PESs, but the positions of shape resonances remain uncertain. The present results are relevant for further improvements and optimizations of buffer-gas cooling of AlF molecules.
Constructing accurate global potential energy surfaces (PESs) describing chemically reactive molecule-molecule collisions of alkali metal dimers presents a major challenge. To be suitable for quantum scattering calculations, such PESs must represent accurately three- and four-body interactions, describe conical intersections, and have a proper asymptotic form at the long range. Here, we demonstrate that such global potentials can be obtained by Gaussian Process (GP) regression merged with the analytic asymptotic expansions at the long range. We propose an efficient sampling technique, which allows us to construct an accurate global PES accounting for different chemical arrangements with <2500 ab initio calculations. We apply this method to (NaK)2 and obtain the first global PES for a system of four alkali metal atoms. The resulting surface exhibits a complex landscape including a pair and a quartet of symmetrically equivalent local minima and a seam of conical intersections. The dissociation energy found from our ab initio calculations is 4534 cm−1. This result is reproduced by the GP models with an error of less than 3%. The GP models of the PES allow us to analyze the features of the global PES, representative of general alkali metal four-atom interactions. Understanding these interactions is of key importance in the field of ultracold chemistry.
A full-dimensional global potential energy surface (PES) for the KRb + KRb → K 2 Rb 2 * → K 2 + Rb 2 reaction is reported based on high-level ab initio calculations. The short-range part of the PES is fit with the permutationally invariant polynomial-neural network method, while the long-range parts of the PES in both the reactant and product asymptotes are represented by an asymptotically correct form. The long- and short-range parts are connected with intermediate-range parts to make them smooth. Within a statistical quantum model, this PES reproduces both the measured loss rates of ultracold KRb molecules and the K 2 and Rb 2 product state distributions, underscoring the important role of tunneling in ultracold chemistry. The PES also correctly predicts the lifetime of the K 2 Rb 2 * intermediate complex within the Rice–Ramsperger–Kassel–Marcus limit. It thus provides a reliable platform for future dynamical studies of the prototypical reaction.
A six-dimensional potential energy surface is constructed for the spin-polarized triplet state of CaF–CaF by ab initio calculations at the CCSD(T) level of theory, followed by Gaussian process interpolation. The potential is utilized to calculate the density of states for this bialkaline-earth-halogen system where we find the value 0.038 μK –1, implying a mean resonance spacing of 26 μK in the collision complex. This value implies an associated Rice–Ramsperger–Kassel–Marcus lifetime of 1.8 μs, thus predicting long-lived complexes in collisions at ultracold temperatures.