Protocol for Optically Pumping AlH$^+$ to a Pure Quantum State
We propose an optical pumping scheme to prepare trapped $\mathrm{AlH}^+$ molecules in a pure state, the stretched hyperfine state $\lvert F=\frac{7}{2},\, m_F=\frac{7}{2}\rangle$ of the rovibronic ground manifold $\lvert \mathrm{X}^2Σ^+,\, v=0,\, N=0\rangle$. Our scheme utilizes linearly-polarized and circularly-polarized fields of a broadband pulsed laser to cool the rotational degree of freedom and drive the population to the hyperfine state, respectively. We simulate the population dynamics by solving a representative system of rate equations that accounts for the laser fields, blackbody radiation, and spontaneous emission. In order to model the hyperfine structure, new hyperfine constants of the $\mathrm{A}^2Π$ excited state were computed using a RASSCF wavefunction. We find that adding an infrared laser to drive the $1 \,-\; 0$ vibrational transition within the $ \mathrm{X}^2Σ^+$ manifold accelerates the cooling process. The results show that under optimum conditions, the population in the target state of the rovibronic ground manifold can reach 63 $\%$ after 68 $\mathrmμ$s (330 ms) and 95 $\%$ after 25 ms (1.2 s) with (without) the infrared laser.