Collisions of room-temperature helium with ultracold lithium and the van der Waals bound state of HeLi.

We have computed the thermally averaged total, elastic rate coefficient for the collision of a room-temperature helium atom with an ultracold lithium atom. This rate coefficient has been computed as part of the characterization of a cold-atom vacuum sensor based on laser-cooled 6Li or 7Li atoms that will operate in the ultrahigh-vacuum (p < 10-6 Pa) and extreme-high-vacuum (p < 10-10 Pa) regimes. The analysis involves computing the X 2 Σ+ HeLi Born-Oppenheimer potential followed by the numerical solution of the relevant radial Schrodinger equation. The potential is computed using a single-reference-coupled-cluster electronic-structure method with basis sets of different completeness in order to characterize our uncertainty budget. We predict that the rate coefficient for a 300 K helium gas and a 1 μK Li gas is 1.467(13) × 10-9 cm3/s for 4He + 6Li and 1.471(13) × 10-9 cm3/s for 4He + 7Li, where the numbers in parentheses are the one-standard-deviation uncertainties in the last two significant digits. We quantify the temperature dependence as well. Finally, we evaluate the s-wave scattering length and binding of the single van der Waals bound state of HeLi. We predict that this weakly bound level has a binding energy of -0.0064(43) × hc cm-1 and -0.0122(67) × hc cm-1 for 4He6Li and 4He7Li, respectively. The calculated binding energy of 4He7Li is consistent with the sole experimental determination.