Low temperature spectra of the¹B1u←¹A1gtransition of C₆H₆, sym-C₆H₃D₃and C₆D₆

The low temperature thin-film absorption spectra of C6H6, sym-C6H3D3 and C6D6 in the 1 B 1u ←1 A 1g transition region are analysed in terms of a 0, 0 crystal-field-induced origin, and v 18(e 2g ), v 7(b 2g ) and v 16(e 2g ) false origins. The 0, 0 + v 2(α1g ) band is clearly resolved in sym-C6H3D3 whilst it merges with 0, 0 + v 7(b 2g ) in C6H6. Recent polarization measurements of the C6H6 2150 Å band are shown to be consistent with a 0, 0 assignment. The 0, 0 origin is also seen in the fluorescence excitation spectrum of crystal C6H6.