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The results of a detailed rotational analysis involving the assignment of some 14 500 lines are reported. Observed bands have been ascribed to transitions between nine singlet states (including the ground state) and between seven triplet states. Singlet-triplet systems have not yet been observed, but the relative energies of the singlet and triplet states have been determined by the analysis of spin-forbidden perturbations. The ground state is π4σ2, X 1Σ+, and the first excited states π4σπ*, a 3Πr, A 1Π. Above these lie two further non-Rydberg states, B 1Σ+ and b 3Σ+, which seem to arise from a configuration of the kind π4σσ*. The remaining eleven states seem to be Rydberg states, π4σ(nlλ) 3Λ, 1Λ, which converge on the 2Σ+ ground state of AlF+. However, no series have yet been recognized so that neither is the ionization potential of AlF well determined nor are there certain assignments of the Rydberg states, although suggestions are made. The triplet splittings in all but the a 3Π state are very small (and in some systems there may be cancellation), so that the 3Σ+-3Σ+ transitions and a 3Π-3Σ+ system simulate, even at fairly high resolution, 1Σ-1Σ and 1Π-1Σ systems respectively. At high resolution lines of the 3Σ+-a 3Π systems show small splittings, some of which are of the kind expected for nuclear magnetic hyperfine structure in transitions of the type case (bβ-S)-case (aβ); however some of the observations are not yet understood.

Lines in the b 3Σ+-a 3II and c 3Σ+-a 3II systems of AlF show partially resolved hyperfine structure. This structure has been attributed to magnetic interactions involving the 27Al nucleus for which I = 5/2. About twenty unblended lines from the (0, 0) bands of the two systems have been analysed in detail, using a computer programme to simulate the profiles by calculating the frequencies and intensities of individual hyperfine transitions. For low values of the rotational quantum number, the lines show a simple triplet structure, which reflects the case (bβS) coupling in the 3Σ states. Analysis of these lines gives values for the Fermi contact parameters in the two states involved (bF = 0.049 cm-1 and 0.057 cm-1 in the b 3Σ+ and c 3Σ+ states respectively). As the rotational quantum number increases, the hyperfine patterns become less regular, indicating a departure from case (bβS) coupling in the 3Σ states and from case (aβ) coupling in the 3II state. By reproducing the details of these line-shapes, it has also proved possible to determine values for the electron spin-spin and spin-rotation parameters in the two 3Σ states: b 3Σ+ λ = -0.025(10)cm-1 γ = 0.0000(3)cm-1 c 3Σ+ λ = 0.00(1)cm-1 γ = 0.0015(10)cm-1 The experimental values for the Fermi contact parameter of AlF in the a 3II and b 3Σ+ states are well produced by a simple single configuration molecular orbital calculation but the value for the c 3Σ state is rather larger than expected on this basis.

Three topics concerning [Formula: see text] states are discussed: (1) The magnitude of the Ω-type splitting in a [Formula: see text] state arising from a Σ state of even multiplicity has been considered and is found to be given to a first approximation by [Formula: see text]. This result leads to the introduction and discussion of a coupling case (a′) for Σ states. (2) Expressions for the Λ-type splitting in a 2Π state and the spin splitting in a 2Σ state (caused by their mutual interaction via spin-orbit coupling) are derived. These expressions are valid when the rotational intervals are small compared to both the spin-orbit interaction and the 2Π−2Σ energy separation, but do not place any restriction on the relative size of the latter two quantities. (3) Branch intensity expressions are presented which apply to any [Formula: see text] transition in which the [Formula: see text] states are not contaminated (due to uncoupling phenomena) by states having a different value of Ω.