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

Abstract

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 (<i>p</i> < 10^-6 Pa) and extreme-high-vacuum (<i>p</i> < 10^-10 Pa) regimes. The analysis involves computing the <i>X</i> 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 <i>μ</i>K 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 <i>s</i>-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) × <i>hc</i> cm^-1 and -0.0122(67) × <i>hc</i> cm^-1 for ⁴He⁶Li and ⁴He⁷Li, respectively. The calculated binding energy of ⁴He⁷Li is consistent with the sole experimental determination.