AlF-AlF sticking time and prospects for ultracold dimers

We report on the sticking time of the AlF dimer in the ultracold regime. We employ a full-dimensional potential energy surface for AlF-AlF, constructed using a machine learning approach [X. Liu et al., J. Chem. Phys. 159, 144103 (2023)], to compute the density of states using a semiclassical counting method. Next, using the Rice-Ramsperger-Kassel-Marcus (RRKM) theory, we determine an RRKM sticking time of 216.0 $\ifmmode\pm\else\textpm\fi{}$ 4.8 ns, which is shorter than that of other previously reported nonreactive dimers. Then, based on the available potential energy surfaces and RRKM sticking time data for a few dimers, we observe that the RRKM sticking time correlates with the ratio of the dimer dissociation energy to the dissociation energy of its parent molecule. Thus, we propose this ratio as a parameter to estimate RRKM sticking times. Furthermore, using ab initio methods, we optimize and calculate the energies of the equilibrium geometries of AlCl-AlCl, CaH-CaH, and MgF-MgF, thereby determining the ratio of the dimer dissociation energy to the parent molecule dissociation energy. Based on this ratio, we propose a hierarchy to order the sticking lifetimes of those dimers in the ultracold regime.