Abstract
The magnetic circular dichroism (MCD) spectrum of the 1 B 2u ←1 A 1g transition of benzene has been measured in the vapour phase. Many of the bands due to transitions between single vibronic levels display A terms. It has been shown that the angular momentum arises by vibronic mixing both of the 1 E 1u state with the 1 B 1u state and of the 1 E 2g states with the 1 A 1g ground state by e 2g vibrations. The magnitudes and signs of the experimental and calculated ratios, A/D, for the A 0 0 vibronic origin are in excellent agreement. Two strong MCD progressions of opposite sign with B-term dispersion have been observed in regions of low absorption. These are identified with vibronic origins due to the v 8 and v 9 e 2g modes. By contrast the MCD spectrum of hexadeuterobenzene vapour has a much lower magneto-rotational strength and displays none of the striking features of the benzene MCD spectrum.