Linear and nonlinear optical response in 3-nitroaniline: A DFT study for comparison between molecular and bulk phases

We have studied the electronic structure and optical responses of both molecular and bulk phases of 3-nitroaniline (m-NA) using density functional theory (DFT). We have developed our calculations within the framework of band structure theory for both phases. The results of current simulation are also useful for comparison between solids and vapors. Findings show that the crystalline m-NA has an indirect bandgap which is smaller than its molecular counterpart. Due to the weak intermolecular interactions, the crystalline band structure shows very low dispersions and hence the crystalline spectra are very similar to those of molecules, especially at lower energies. This resemblance is extended to lower wavelengths in linear regime, but limited to higher wavelengths in nonlinear one. This study shows that the substituent groups play major roles in the band structure and charge transfer excitations. The optical spectra show higher intensity and more splitting, especially in nonlinear regime, when we go from molecular phase to bulk phase. Findings show that the electron energy loss structures in vapor phase are very close to epsilon(2), hence energy loss spectroscopy is a direct way for measuring dielectric function of vapors. According to our results, the crystalline phase exhibits plasmon resonances at much higher energies compared to those of vapor phase. For example, the energy loss spectrum of m-NA molecule shows plasmon peak around 17 eV which is about 10 eV lower than the crystalline counterpart. The comparison between nonlinear spectra and the linear spectra (as functions of both omega and 2 omega) reveals the significant resemblance between linear and nonlinear structures in both phases. This study shows that the molecular-based models for investigating optical properties of organic crystals are only suitable for low energy regions. Finally, our simulation reproduces the experimental results very well.