EMA-based fitting of optical transitions in CoPc: insights into carrier dynamics

Optical measurements have been performed in a wide frequency range between 400 cm⁻¹ and 52 000 cm⁻¹ on partially oxidized cobalt phthalocyanine (CoPc). The mid-IR band for the electric field E parallel to the stacking axis, centered at about 4008 for [CoPc]⁺ ions, respectively, has been discussed in terms of resonance absorption based on the effective medium approximation (EMA) model. The band in the reflectivity and conductivity spectra has been fitted using the normal Lorentz function and the general form of the Maxwell–Garnett equation. Various fitting features, which are explored for free electron metals, are obtained by inserting one Drude term in the Maxwell–Garnett equation, such as the plasma frequency and the time scattering rate. Based on the parameters extracted from fitting the mid-IR band according to the EMA model, we were able to obtain the effective conductivity σ_eff (ω) as a function of frequency.