I.J. Lalov

Model of mixed Frenkel and charge-transfer excitons in donor?acceptor molecular crystals: investigation of vibronic spectra

The mixing of Frenkel excitons (FEs) and charge-transfer excitons (CTEs) in a molecular stack of regularly arranged donor (D) and acceptor (A) molecules is considered a model case and its vibronic line shapes have been calculated for several parameter sets. The two types of excitons (FE and CTE) are coupled linearly and quadratically with one vibrational mode of the D molecule (or of the A molecule). Using the methods of canonical transformation and of Greens functions (at T=0), as well as the vibronic approach which is applicable in the case of a narrow exciton band, the linear optical susceptibility is calculated for the three spectral regions: (a) excitonic, (b) one-phonon vibronics, and (c) two-phonon vibronics. As the study is directed to centrosymmetrical stacks, the cases of mixing of gerade excitons and of ungerade excitons have been treated separately in the calculation of the linear absorption coefficients. Because until now experimental observations of FE–CTE mixing in DA charge-transfer systems are absent, the numerical calculations have been performed for hypothetical sets of parameters which include: the parameters of CTEs in DA stacks (like anthracene–PMDA) and the parameters of FE–CTE mixing in a one-component stack (like that of PTCDA). The simulations establish the main features of the excitonic and vibronic spectra in the case of FE–CTE mixing, namely (i) the mutual influence on the positions and on the absorption intensities of all terms of the vibronic progressions stemming from FE and CTE levels; (ii) in the case of vibration of an A molecule (if the FE is assumed to be an excited electronic state of the D molecule), only one vibronic progression is manifest and the vibronic levels closer to the FE will be most enhanced; (iii) the redistribution of the absorption intensities depends on the sign of the mixing constant and may be so strong that the terms of the two vibronic progressions could have comparable absorption; (iv) spectral lines of different shape correspond to the bound and unbound exciton–phonon states; and (v) in the case of mixing of gerade excitons its possible impact on the absorption of the ungerade CTE-combination connected with the noncentral part k≠0 of the Brillouin zone was established. The simulation of the FE–CTE mixing can be useful in the assignment of the linear absorption spectra and in the description of the FE–CTE–vibration coupling.

Excitonic and vibronic spectra of Frenkel excitons in a two-dimensional simple lattice

Abstract Excitonic and vibronic spectra of Frenkel excitons (FEs) in a two-dimensional (2D) lattice with one molecule per unit cell have been studied and their manifestation in the linear absorption is simulated. We use the Green function formalism, the vibronic approach, see Lalov and Zhelyazkov [I.J. Lalov, I. Zhelyazkov, Phys. Rev. B 75 (2007) 245435], and the nearest-neighbor approximation to find expressions of the linear absorption lineshape in closed form (in terms of the elliptic integrals) for the following 2D models: (a) vibronic spectra of polyacenes (naphthalene, anthracene, tetracene); (b) vibronic spectra of a simple hexagonal lattice. The two 2D models include both linear and quadratic FE–phonon coupling. Our simulations concern the excitonic density of state (DOS), and also the position and lineshape of vibronic spectra (FE plus one phonon, FE plus two phonons). The positions of many-particle (MP-unbound) FE–phonon states, as well as the impact of the Van Hove singularities on the linear absorption have been established by using typical values of the excitonic and vibrational parameters. In the case of a simple hexagonal lattice the following types of FEs have been considered: (i) non-degenerate FEs whose transition dipole moment is perpendicular to the plane of the lattice, and (ii) degenerate FEs with transition dipole moments parallel to the layer. We found a cumulative impact of the linear and quadratic FE–phonon coupling on the positions of vibronic maxima in the case (ii), and a compensating impact in the case (i).

Interference by multiple reflections in crystal plates of point group symmetries C 3 v, C 4 v, C 6 v

Crystals of symmetry point groups C 3v , C 4v , C 6v possess an antisymmetric tensor of linear spatial dispersion (LSD) and thus they do not rotate the plane of light polarization. The antisymmetric LSD (ALSD) influences the polarization of the electromagnetic eigenmodes of the crystal and it may be studied through reflection only. In this paper we study the interference by multiple reflection in anisotropic crystal plates of the aforementioned point groups (the optical axis is perpendicular to the plane of incidence). The amplitudes of the reflected and transmitted electromagnetic beams are calculated both in the case of single reflection and in the case of multiple reflection. Differential reflection methods have been proposed and analysed in the case of elliptically and linearly polarized incident beams. The dependences of the multiple reflection intensity and of the differential reflected and transmitted signals on the angle of incidence is calculated for the crystals Ag3AsSe3, ZnO and CdS. It is shown that in the case of total reflection in an anisotropic plate surrounded by an optically denser medium the effects of the ALSD on reflection are enhanced several times.