Surface, subsurface, and bulk exciton transitions of crystalline anthracene

Abstract

The polarized reflection spectrum of the first singlet transition of crystalline anthracene at 2 °K is reported for light at near normal angles of incidence on the (001) face. The effect on the spectra of condensing small amounts of gases, transparent in the 400 nm region, has been studied and found to be a useful technique for differentiating surface from bulk exciton transitions. The optical properties of the bulk crystal have been calculated by a Kramers–Kronig transformation of the reflectivity data. The complex refractive index so obtained was used to calculate the effect of surface transitions on the bulk reflection spectrum using equations given by a microscopic theory of surface and bulk exciton states. The agreement between the calculated and experimental spectra is excellent. Model calculations show that the two‐dimensional sums of excited state van der Waals interactions are of sufficient range to create a series of excitons localized on surface planes. It is also demonstrated that the reflectivity formulas of thin film optics and the microscopic surfaceexciton theory are equivalent. The reflection spectra of crystals coated with frozen deposits of air show the presence of a satellite minimum that upon further absorption of gas first grow more prominent at the expense of the original minimum and then are replaced by a single minimum with an even larger red shift. These observations appear to correspond to nucleation, growth, and coalescence of islands of the adsorbed molecules. The higher vibronic transitions are not effected by frozen gas films, except for one peak in the aspectrum that disappears. This peak is located at the position expected of the a‐polarized factor group component of the surface state and as such provides new evidence for this assignment. A partial analysis of some of the higher vibronic transitions is presented based on the concepts of single‐particle transitions embedded in bands of two‐particle states.