Broken Patterns - Anomalies in the Spectra of Aluminum Monofluoride

Usually, rotational spectra of diatomics can be described by a handful of spectroscopic parameters. In some cases, the observed spectra clearly differ from what one would expect: Lines that should be there are suddenly missing, some additional ones appear out of the blue, and some others are observed at unexpected positions. Such anomalies are caused by the interactions among different electronic states.

In this talk, various anomalies that have been observed in the UV- and VIS-spectra of aluminum monofluoride are discussed: The spin-orbit interaction among the metastable a³Π state with distant singlet states brings intensity to the spin-forbidden a³Π ← X¹Σ⁺ band, that can be used for laser cooling.[1] The interaction among the nearly iso-energetic A¹Π, v = 6 and b³Σ⁺, v = 5 states gives rise to levels with a strongly mixed spin-character, that can be used as a singlet-triplet doorway.[2] The interaction among the closely d³Π and e³∆ Rydberg states affects the equilibrium rotational constants in both states and causes a very unexpected rotational structure of one particular band.[3]

[1] Walter et al: Spectroscopic characterization of the a³Π state of aluminum monofluoride, Journal of Chemical Physics 156, 124306 (2022)

[2] Walter et al: Spectroscopic characterization of singlet–triplet doorway states of aluminum monoflu- oride, Journal of Chemical Physics 156, 184301 (2022)

[3] Walter et al: Triplet Rydberg states of aluminum monofluoride, The Journal of Physical Chemistry A, 128, 2752-2762 (2024)