Aluminum monofluoride (AlF) has favorable properties for laser cooling and trapping experiments. The strong A¹Π ← X¹Σ⁺ band near 227 nm and the narrow a³Π ← X¹Σ⁺ band near 367 nm have rotationally and vibrationally closed transitions, making them well-suited for efficient photon scattering. Among the states relevant to laser cooling, the a³Π state is the least characterized, as its differing spin multiplicity from the ground state makes it difficult to access experimentally. This thesis reports on the spectroscopic characterization of the a³Π state and higher triplet states of AlF. Special emphasis is placed on the interpretation of anomalies in the molecular spectra, which occur due to interactions between different electronic states. The presented experiments are performed on supersonically cooled AlF molecules in a molecular beam. Pulsed and continuous-wave dye lasers (270–570 nm) and an RF transmission line (1–500 MHz) are used for resonant excitation.
The rotationally resolved spectra of the spin-forbidden a³Π ← X¹Σ⁺ band reveal that this band is weakly allowed due to the spin–orbit interaction of the a³Π state with distant singlet states. The vibrational, rotational, and spin–orbit coupling parameters of the a³Π, v = 0–7 states are extracted from these spectra. This allows the construction of an accurate electronic potential for the a³Π state and determination of the Franck–Condon matrix of the a³Π – X¹Σ⁺ system. All Λ-doublet transitions within the a³Π₁, v = 5, J = 1 level are recorded, and the hyperfine parameters of this level are determined. The observation of a spectral line with a full width at half maximum of 1.27 kHz, belonging to a hyperfine transition within the a³Π₁, v = 0 state, confirms the absence of lifetime broadening for transitions between two metastable levels.
The lowest rotational levels of the A¹Π, v = 6 and b³Σ⁺, v = 5 states are nearly isoenergetic and interact via spin–orbit coupling. These strongly mixed levels provide a doorway between the singlet and triplet manifolds. These perturbed levels are characterized from hyperfine-resolved spectra. The lifetimes of selected hyperfine levels, which range from 2 ns to 200 ns depending on the degree of singlet–triplet mixing, are determined by time-delayed ionization, Lamb-dip spectroscopy, and precise analysis of transition lineshapes.
Two theoretically predicted triplet states of AlF, the counterparts of the well-characterized D¹Δ and E¹Π states, had not been experimentally identified prior to this work. This thesis reports on the first characterization of the d³Π (v = 0–6) and e³Δ (v = 0–2) states, confirming the predicted energetic ordering of these states. In addition, the f³Σ⁺ (v = 0–2) states are characterized. Interestingly, the spectra of the d³Π, v = 3 ← a³Π, v = 3 band show an intensity distribution that is clearly different from that of all other diagonal bands. This band gets its weak, unexpected rotational structure due to intensity borrowing from the nearby e³Δ, v = 2 ← a³Π, v = 3 band. Analysis of this effect allows for the detailed characterization of the spin–orbit and spin–rotation interactions between the d³Π and e³Δ states.
