Vibrational branching ratios and radiative lifetimes in the laser cooling of AlBr

Abstract

The feasibility of laser cooling of the AlBr molecule is investigated using ab initio quantum chemistry. Potential energy curves, permanent dipole moments, and transition dipole moments for the ground state X¹Σ⁺ and the first two excited states (a³Π and A¹Π) are calculated using the multi-reference configuration interaction plus Davidson corrections (MRCI+Q) method with the ACVQZ basis set; the spin–orbit coupling effects are also taken into account in electronic structure calculations at the MRCI level. Based on the acquired potential energy curves and transition dipole moments, highly diagonally distributed Franck–Condon factors (f₀₀ = 0.9540, f₁₁ = 0.8172) and vibrational branching ratios (R₀₀ = 0.9708, R₁₁ = 0.8420) for the transition are determined. Radiative lifetime calculations of the A¹Π₁ (ν′ = 0–4) state are found to be short (9.16–11.48 ns) enough for rapid laser cooling. The proposed main cycling laser drives the transition at the wavelength λ₀₀ = 279.19 nm. The vibrational branching loss ratios of the A¹Π₁ (ν′) state to the intervening states a³Π_0⁺ and a³Π₁ are small (<5.2 × 10⁻⁶) enough to be negligible. The present theoretical results indicate that the AlBr molecule is a promising candidate for laser cooling.