Ground and excited potential energy surfaces for CaF+Ca interactions and isotope exchange reactions

TL;DR

This study computes high-level ab initio potential energy surfaces for CaF+Ca across nine electronic states arising from the three lowest CaF+Ca asymptotes, treating CaF as a rigid rotor and using Jacobi coordinates to build 1D and 2D PES. The ground-state surface X2A' is deeply bound and highly anisotropic, with a bent global minimum, while excited surfaces, including the (2)2A' state correlating to CaF(^2Σ^+)+Ca(^3P), are also strongly bound and can cross or approach the ground-state asymptote, implying possible nonadiabatic pathways. Ground-state atom exchange CaF+Ca is found to be barrierless and exothermic for isotope exchange, with reaction energetics sensitive to Ca isotopes, and long-range coefficients C6,0 and C6,2 computed to characterize ultracold collisions; for the metastable Ca(3P) channel, the 2Σ excited surface lies well below the ground-state asymptote, suggesting spin-orbit driven rovibronic mixing could enable nonadiabatic transitions, though no direct surface crossings were observed. Overall, the work provides detailed PES benchmarks and insights into isotope-controlled and excited-state ultracold chemistry for CaF+Ca, guiding future spectroscopy and reaction studies in CaF-containing systems.

Abstract

We investigate the intermolecular interactions between laser-cooled CaF and Ca, in their ground and excited electronic states, aiming to understand atom-exchange reaction pathways. Using state-of-the-art \textit{ab initio} quantum chemistry methods, we compute potential energy surfaces for nine electronic states arising from the lowest three asymptotes of Ca$_2$F trimer, within the rigid rotor approximation applied to CaF. Two-dimensional potential energy surfaces are computed for the ground state and one of the excited states. We use a combination of the coupled cluster method restricted to single, double, and perturbative triple excitations, and the multireference configuration interaction method with single and double excitations. The ground (X)~$^2\mathrm{A}'$ electronic state of the trimer is significantly deep and highly anisotropic. The excited electronic states are also strongly bound. Notably, the potential energy surface of one of the excited states, (2)~$^2\mathrm{A}'$, lies below the ground-state asymptote of the trimer. By analyzing the potential energy surfaces, we discuss atom-exchange reaction pathways involving both the ground-state interaction between CaF and Ca and the excited metastable state of Ca.

Figures (5)

• Figure 1: A schematic diagram of the CaF+Ca system in the Jacobi coordinates.
• Figure 2: One-dimensional cuts of the potential energy surfaces for CaF+Ca as a function of $R$. Panels (a) and (b) show the 1D PESs for the linear configurations at $\theta = 0^\circ$ and $\theta = 180^\circ$ within the C$_{2\mathrm{v}}$ point group, while panel (c) shows the T-shaped configuration under the C$_\mathrm{s}$ point group.
• Figure 3: 2D PES and the corresponding Legendre components for the ground (X) $^2\mathrm{A}'$ state of CaF+Ca in panel (a) and (b). The contour colorbar indicates the interaction energy in cm$^{-1}$.
• Figure 4: 2D PES for atom exchange in the ground (X) $^2\mathrm{A}'$ state of CaF+Ca. The bond angle Ca-F-Ca is fixed at the global equilibrium geometry. The interaction energy (in cm$^{-1}$) is calculated with respect to the ground-state asymptote.
• Figure 5: 2D PES for the (2) $^2\Sigma$ state of CaF+Ca. Energies are referenced to the ground-state asymptote of the Ca$_2$F trimer and expressed in cm$^{-1}$.