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 D 1 Δ and E 1 Π states, had not been experimentally identified yet. We here report on the characterization of the d 3 Π ( v = 0-6) and e 3 Δ ( v = 0-2) states, confirming the predicted energetic ordering of these states ( J. Chem. Phys. 1988, 88, 5715-5725), as well as of the f 3 Σ + ( v = 0-2) state. The transition intensity of the d 3 Π, v = 3 - a 3 Π, v = 3 band is negligibly small. This band gets its weak, unexpected rotational structure via intensity borrowing from the nearby e 3 Δ, v = 2 - a 3 Π, v = 3 band, made possible via spin-orbit and spin-rotation interaction between the d 3 Π and e 3 Δ 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 d 3 Π state.