Characterisation of the $b^3{\mathrm{\Sigma }}^{+},v=0$ State and Its Interaction with the $A^1\mathrm{\Pi }$ State in Aluminium Monofluoride

Recently, we determined the detailed energy level structure of the $X^1{\mathrm{\Sigma }}^{+}$, $A^1\mathrm{\Pi }$ and $a^3\mathrm{\Pi }$ states of AlF that are relevant to laser cooling and trapping experiments. Here, we investigate the $b^3{\mathrm{\Sigma }}^{+},v=0$ state of the AlF molecule. A rotationally-resolved (1+2)-REMPI spectrum of the $b^3{\mathrm{\Sigma }}^{+},v^{\mathrm{\prime }}=0\leftarrow a^3\mathrm{\Pi },v^{\mathrm{\prime }\mathrm{\prime }}=0$ band is presented and the lifetime of the $b^3{\mathrm{\Sigma }}^{+},v=0$ state is measured to be 190(2)~ns. Hyperfine-resolved, laser-induced fluorescence spectra of the $b^3\Sigma^+, v’=0 \leftarrow X^1\Sigma^+, v’‘=1$andthe$b^3\Sigma^+, v’=0 \leftarrow a^3\Pi, v’'=0$ bands are recorded to determine fine- and hyperfine structure parameters. The interaction between the $b^3{\mathrm{\Sigma }}^{+},v=0$ and the nearby $A^1\mathrm{\Pi }$ 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 $A\to a$ loss from the main $A-X$ laser cooling cycle below the 10${}^{-6}$ level.