Calculation of the exciton states in molecular crystals

Experimentally, crystals of polyacenes have been studied by absorption and reflection spectroscopy. The pioneering work by Clark and his students, using a microtome to deliberately cut specific crystal faces of organic crystals which were then studied by reflection spectroscopy was very important. The corresponding absorption spectra have been derived by Kramers-Kronig analysis (63) (equa- 

Reflection spectra of the (001) face. The solid and dotted curves were obtained with the incident light polarized parallel to the b and a crystallographic axes, respectively. Reprinted with permission from Clark et al. (63). Copyright 1970, American Institute of Physics. 

Molecular transition Naphthalene Anthracene Tetracene Pentacene 

The anthracene crystal is of monoclinic type. The elementary cell of this crystal is spanned by the primitive lattice vectors a, b, and c, where the vector b is oriented along the monoclinic axis, containing two molecules (cf. Fig. 3.6). The point group of the anthracene crystal is Cf. The vectors a and b are orthogonal, and the vectors a and c form an angle such that ac = ac cos 125°. The lengths of the primitive vectors are a = 0.856 nm, b = 0.604 nm, and c = 1.116 nm. 

As given in Ch. 2, the position of the molecular center-of-mass within the elementary cell is determined by the vectors rna = n + pa, where n is an integervalued lattice vector. In the case of anthracene 

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