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Characterisation of the $b^3Σ^+, v=0$ State and Its Interaction with the $A^1Π$ State in Aluminium Monofluoride

Recently, we determined the detailed energy level structure of the $X^1Σ^+$, $A^1Π$ and $a^3Π$ states of AlF that are relevant to laser cooling and trapping experiments. Here, we investigate the $b^3Σ^+, v=0$ state of the AlF molecule. A rotationally-resolved (1+2)-REMPI spectrum of the $b^3Σ^+, v'=0 \leftarrow a^3Π, v''=0$ band is presented and the lifetime of the $b^3Σ^+, v=0$ state is measured to be 190(2)~ns. Hyperfine-resolved, laser-induced fluorescence spectra of the $b^3Σ^+, v'=0 \leftarrow X^1Σ^+, v''=1$ and the $b^3Σ^+, v'=0 \leftarrow a^3Π, v''=0$ bands are recorded to determine fine- and hyperfine structure parameters. The interaction between the $b^3Σ^+, v=0$ and the nearby $A^1Π$ state is studied and the magnitude of the spin-orbit coupling between the two electronic states is derived using three independent methods to give a consistent value of 10(1)~cm$^{-1}$. The triplet character of the $A$ state causes an $A\rightarrow a$ loss from the main $A-X$ laser cooling cycle below the 10$^{-6}$ level.