Laser cooling of BeCl and BeBr molecules in an ab initio method

Abstract

In this study, the feasibility of laser-cooling of BeCl and BeBr molecules is studied using ab initio quantum chemistry. The potential energy curves for the X²Σ⁺, A²Π, and 2²Π electronic states of BeCl and BeBr are plotted based on multi-reference configuration interaction plus Davidson corrections (MRCI + Q), and the spin–orbit coupling (SOC) effects are considered at the MRCI + Q level. The calculated spectroscopic parameters agree with the experimental data. Highly diagonally distributed Franck–Condon factors are determined for the A²Π(ν′ = 0) ← X²Σ⁺(ν′′ = 0) transition: f₀₀(BeCl) = 0.947 and f₀₀(BeBr) = 0.966. Moreover, the suitable radiative lifetimes τ of the A²Π(ν′ = 0) state are determined for rapid laser cooling: τ(BeCl) = 18.38 ns and τ(BeBr) = 27.09 ns. The proposed cooling wavelengths of both BeCl and BeBr are within the ultraviolet region at λ₀₀(BeCl) = 358.51 nm and λ₀₀(BeBr) = 379.38 nm. Laser cooling schemes for BeCl and BeBr molecules are also developed in consideration of the SOC effects. These results indicate that the inclusion of SOC effects does not affect the judgment of the feasibility of laser cooling of BeCl and BeBr molecules, even for the given BeBr molecules in which the SOC effect is significant.