Triplet Rydberg States of Aluminum Monofluoride

Aluminum monofluoride (AlF) is a suitable molecule for laser cooling and trapping. Such experiments require extensive spectroscopic characterization of the electronic structure. Two of the theoretically predicted higher-lying triplet states of AlF, the counterparts of the well-characterized D¹Δ and E¹Π states, had not been experimentally identified yet. We here report on the characterization of the d³Π (v = 0-6) and e³Δ (v = 0-2) states, confirming the predicted energetic ordering of these states (J. Chem. Phys. 1988, 88, 5715-5725), as well as of the f³Σ⁺ (v = 0-2) state. The transition intensity of the d³Π, v = 3 - a³Π, v = 3 band is negligibly small. This band gets its weak, unexpected rotational structure via intensity borrowing from the nearby e³Δ, v = 2 - a³Π, v = 3 band, made possible via spin-orbit and spin-rotation interaction between the d³Π and e³Δ states. This interaction affects the equilibrium rotational constants in both states; their deperturbed values yield equilibrium internuclear distances that are consistent with the observations. We determined the ionization potential of AlF to be 78,492(1) cm^-1 by ionization from the d³Π state.