Electronic transition oscillator strengths in solids: An extended Huckel tight-binding approach

A method for calculating electronic dipole-induced transitions in solids based on extended Huckel tight-binding (EHTB) wave functions is presented, The proposed computational treatment relies on the position formulation for intensity calculations. It is compared with the well-known velocity method that involves differentiation of the wave functions with respect to the electron positions, The described CEDiT (crystal electronic dipole-induced transitions) computations are applied to interpret the low-energy absorption spectrum of polyacetylene and the group VIA transition metal dichalcogenide MoS2. Good agreement between calculation and experimental absorption spectra is obtained. In the case of polyacetylene the first prominent absorption band is due to pi* <--- pi transitions about the Z point of the irreducible Brillouin zone. The high density of states (DOS) encountered at Z can be identified as responsible for the characteristic shape of this band, For MoS2 we plot the oscillator strengths of the first four prominent electronic transitions as a function of the k vector in the irreducible Brillouin zone. The excitonic transitions A and B are due to resonance of two almost degenerate interband absorptions confined to a relatively well-defined region in k space st about k(A) = (22/57, 2/57) and k(B) = (1/3, 2/57), respectively. The origin in k space of the interband absorptions C and D is also discussed.