An Analysis of Hyperfine Interactions in the Electronic Spectrum of AIF

Lines in the b³Σ⁺-a³II and c³Σ⁺-a³II 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 (b_βS) coupling in the ³Σ 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 b³Σ⁺ and c³Σ⁺ states respectively). As the rotational quantum number increases, the hyperfine patterns become less regular, indicating a departure from case (b_βS) coupling in the ³Σ states and from case (a_β) 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:

b³Σ⁺ λ = -0.025(10)cm^-1 γ = 0.0000(3)cm^-1

c³Σ⁺ λ = 0.00(1)cm^-1 γ = 0.0015(10)cm^-1

The experimental values for the Fermi contact parameter of AlF in the a³II and b³Σ⁺ states are well produced by a simple single configuration molecular orbital calculation but the value for the c³Σ state is rather larger than expected on this basis.