Excitons in 2H-WSe₂ and 3R-WS₂

The A exciton series in the liquid-nitrogen-temperature epsilon ₂ spectra of 2H-WSe₂ and 3R-WS₂, derived from Kramers-Kronig analyses of normal-incidence, basal-plane reflectivity spectra, have been fitted to a Lorentzian oscillator model. It has been found that the energy levels and oscillator strengths of the excitons obey a three-dimensional theoretical model for 2H-WSe₂ and a two-dimensional model for 3R-WS₂. The experimental observations are consistent with Toyozawa's (1958) theory for weak exciton-phonon interactions and small exciton effective mass.