Forbidden electronic transitions in XeF₂ and XeF₄

Transition strengths have been measured for the weak 2330Å band in XeF₂ (f= 0.002) and for the two weak bands in XeF₄ at 2280Å (f= 0.009) and 2580Å (f= 0.003). To investigate the origins of these weak transitions, the possibilities of vibronic and singlet-triplet transitions in XeF₂ and XeF₄ were examined. Using the Herzberg-Teller theory of vibronic transitions and a molecular orbital treatment of excited electronic states, estimated strengths of the relevant vibronic transitions have been calculated to be f= 0.001 for both XeF₂ and XeF₄. The vibronic band in XeF₂ borrows intensity from the symmetry allowed ¹A_1g→¹A_2u transition at 1580Å (f=0.45), while in XeF₄ the major contribution to the vibronic band is from the symmetry allowed 1A _1g→₁E_u transition at 1325 Å (f=0.8). A temperature dependence of the intensity of the 2330 Å band in XeF ₂ has been observed and found to be less than that predicted by the Herzberg-Teller theory. The estimated strength of the singlet-triplet transition in XeF₂ corresponding to the singlet-singlet transition at 1580 Å is shown to be small (f≦ 10^-4) in spite of a heavy atom effect; the small transition strength persists because of the lack of nearby excited states of the required symmetry. In XeF₄ the triplet excited state ³E_u corresponding to the singlet-singlet transition 1A_1g→¹E_u at 1840Å (f=0.22) is permitted by group theoretical selection rules to mix with its own singlet state. Using an intermediate coupling scheme the estimated intensity of this singlet-singlet transition is calculated to be f=0.007. The theoretical estimates of the symmetry and spin forbidden transition strengths are used for the assignment of the weak electronic transitions in the xenon fluorides.