Combination bands in vibronic spectra of molecular crystals

Abstract

The molecular crystals’ vibronic spectra with combination bands have been studied using the complete vibronic approach. Those vibronic spectra consist of one electronic excitation (Frenkel exciton) plus vibrational quanta of two different intramolecular vibrational modes. The linear absorption spectra of one-dimensional models of some aromatic crystals have been calculated applying the methods of canonical transformations and Green functions (at T = 0) in the following cases: (i) vibronic spectra of naphthalene and benzene which consist of Frenkel exciton plus one phonon of non-totally symmetrical vibration plus one/two phonons of totally symmetrical vibrations; (ii) vibronic spectra of an anthracene-like model and other two models with combination bands with two different totally symmetrical vibrations (one of them is weakly coupled with Frenkel exciton). In this paper, we show that the linear absorption demonstrates considerable changes in the one-phonon and two-phonon vibronic spectra caused by the indirect coupling between vibrational modes (appearing because of their coupling with a Frenkel exciton). Our simulations of absorption spectra prove the opportunity to observe some peculiarities like the manifestation of various molecular configurations and edge anomalies in the shape of the absorption bands.