An Analysis of Hyperfine Interactions in the Electronic Spectrum of AIF

Lines in the b 3 Σ + - a 3 II and c 3 Σ + - a 3 II systems of AlF show partially resolved hyperfine structure. This structure has been attributed to magnetic interactions involving the 27 Al 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 (b βS ) coupling in the 3 Σ states. Analysis of these lines gives values for the Fermi contact parameters in the two states involved ( b F = 0.049 cm -1 and 0.057 cm -1 in the b 3 Σ + and c 3 Σ + states respectively). As the rotational quantum number increases, the hyperfine patterns become less regular, indicating a departure from case (b βS ) coupling in the 3 Σ states and from case (a β ) coupling in the 3 II 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 3 Σ states: b 3 Σ + λ = -0.025(10)cm -1 γ = 0.0000(3)cm -1 c 3 Σ + 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 3 II and b 3 Σ + states are well produced by a simple single configuration molecular orbital calculation but the value for the c 3 Σ state is rather larger than expected on this basis.