Interactions and cold collisions of AlF in the ground and excited electronic states with He

TL;DR

This work addresses how AlF molecules in their ground and excited electronic states interact with He, with the goal of informing buffer-gas cooling and laser cooling strategies. The authors construct high-accuracy two-dimensional potential energy surfaces using CCSD(T)-F12b with CCSDT corrections for $X^{1}A'$, $a^{3}A''$, and $b^{3}A'$ states, and MR-CI for the higher excited $A^{1}A'$ and $B^{1}A''$ states; the interaction is expressed as $V_{ ext{int}}(R,\theta)=\sum_{\lambda=0}^{\lambda_{\max}} V_{\lambda}(R) P_{\lambda}(\cos\theta)$ and interpolated with RP-RKHS. Collision dynamics are treated with coupled-channel scattering to obtain elastic and inelastic cross sections and bound rovibrational states, yielding a predominantly weak but strongly anisotropic interaction landscape with global minima that are largely linear (except for the $a^{3}A''$ state) and a favorable elastic-to-inelastic ratio across the studied temperature range. The results indicate promising prospects for buffer-gas cooling of AlF, quantify uncertainties (approximately $0.3\,\mathrm{cm}^{-1}$ in the global minima), and provide numerical PES data in the Supplement, contributing valuable benchmarks for designing ultracold-molecule experiments and guiding future refinements. Overall, the state-dependent AlF+He interactions offer a controllable pathway to optimize cooling and trapping of AlF molecules in ultracold environments.

Abstract

Aluminium 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 Frank-Condon factors. In this study, we examine the interactions and collisions of AlF in its $X^1Σ^+$, $a^3Π$, and $A^{1}Π$ electronic states with ground-state He using state-of-the-art \textit{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.

Figures (8)

• Figure 1: The Jacobi coordinates for the AlF+He system.
• Figure 2: One-dimensional cuts through the potential energy surfaces for the (a) $X^{1} A'$, (b) $a^{3}A"$ and $b^{3}A'$, and (c) $A^{1}A'$ and $B^{1}A"$ electronic states of AlF+He in linear and perpendicular orientations.
• Figure 3: Two-dimensional potential energy surfaces and corresponding Legendre components for the (a,d) $X^{1}A'$, (b,e) $b^{3}A'$, and (c,f) $a^{3}A"$ electronic states of AlF+He.
• Figure 4: Two-dimensional potential energy surfaces and corresponding Legendre components for the (a,c) $A^{1}A'$ and (b,d) $B^{1}A"$ electronic states of AlF+He.
• Figure 5: The convergence of the interaction energy of AlF+He in the $X^{1}A'$ state at $\theta=180^{\circ}$ (a) for different ab initio methods using the aug-cc-pV5Z basis set and (b) for the aug-cc-pV$X$Z basis set with increasing cardinal number $X$ using the CCSD(T) method.