Intensities in polarized light: near ultraviolet absorption spectrum of 1,2,4,5 tetramethylbenzene

The absorption anisotropy of 1,2,4,5 tetramethylbenzene crystals in the region 33 000–40 500 cm–1 is investigated at room and liquid nitrogen temperature. The f-values for both directions of propagation of the light vector in the (001) face were measured and corrected for reflectivity losses. Frequency shifts of the absorption peaks and changes in f-values with temperature are discussed. Comparison of crystal absorption with oriented gas intensities leads to the conclusion that the transition is long-axis polarized. The assignment is discussed in relation to a calculation on crystal shift and Davydov splitting. The probable orientation and magnitude of the transition moment, induced by the crystal field in the unit cell, is indicated.