Theoretical Study Of Optical And Vibrational Molecular Spectroscopies

Optical spectroscopies are key characterization techniques in modern science. In general, measured optical spectra have three features, namely:energy, as well as spec-tral intensity and profile. In this thesis, the intensity of three types of commonly used linear spectroscopies, including ultraviolet-visible (UV-Vis), infrared (IR), and Raman spectroscopy, are studied in detail at the first-principles level.UV-Vis absorption, which is the initial process of dye-sensitized solar cells, is usu-ally applied in the study of molecular excited states. In this dissertation, we focus on the optical properties of the Pt complexes in the form of donor-metal-acceptor (D-M-A). These complexes are widely used in dye-sensitized solar cells and luminescence spec-troscopy, because their charge transfer (CT) states have clear direction. Based on the wavefunctions of the CT states obtained at the time-dependent density functional theory level, a direct decomposition method for transition dipole moment has been proposed. In this method, the elements of the transition matrix can be divided into contributions of arbitrary atomic groups. With this method, our calculated results for D-M-A systems show that, when acceptors have small electron affinity, not only the local excitations in acceptor have important contributions for the final results, but the local excitations in donor also have significant contributions, The latter contribution was assumed to be less important in the pioneer CT theory. Furthermore, the calculated results also demonstrate that, by tuning the dihedral angle between donor and acceptor, the tran-sition probability between ground and excited state can be precisely controlled, which would have unique applications in luminescence spectroscopy.In this dissertation, the intensity of two conventional vibrational spectroscopies, i.e., IR and Raman, are also addressed. For IR, within the framework of Mulliken pop-ulation, a method for decomposition of dynamic IR dipole moment into individual basis sets has been proposed, which allows us to investigate the convergence of theoretical IR spectra with respect to diffuse basis sets for a special system, i.e., the dipole bounded anionic water clusters. Because MP2 and B3LYP are widely used in the study of an-ionic water clusters, we choose them as the representations for wavefunction theory and density functional theory, respectively. Calculated results reveal that MP2 can give the converged IR spectra with the addition of diffuse functions. However, the more widely used B3LYP fails completely for the convergence. The detailed analysis of dynamic IR dipole moment manifests that the reason of above results should be attributed to the fact that MP2 can give correct descriptions for the electronic structure of the extra electron but B3LYP cannot. Therefore, for the dipole bounded systems, we recommand MP2 rather than the commonly used B3LYP.For Raman, we focus our attention on the intensity of nonresonant Raman pro-cesses based on the general Albrecht’s Raman theory. In Albrecht’s Raman theory, the intensity consists of A (Franck-Condon) and B (Herzberg-Teller) terms. In con-ventional treatments, the A term was considered to be negligible for far off-resonant Raman processes. However, we demonstrate that this widely accepted long-standing argument fails for totally symmetric vibrational modes. Take water monomer as an ex-ample, first-principles calculations reveal that, for the symmetric OH stretching mode, A term together with the strong constructive interference between A and B terms could contribute around 50% for the final result. Meanwhile, for the bending mode, a par-tially destructive interference is observed. Furthermore, a state-to-state mapping be-tween the Albrecht’s theory and perturbation theory has also been established. By this mapping, we could examine the accuracy of the simply extended perturbation theory for dynamic/resonant Raman processes. Calculated results for the bending mode of water on the first excited state indicate that the simply extended perturbation theory is a good approximation only when the absolute difference between the energy and the incident light is more than five times greater than the vibrational energy in the ground state.All results presented here manifest that, even for the simple linear optical pro-cesses, detailed theoretical investigations are necessary. We expect that the theory pre-sented in this thesis would be further developed and have widely applications in future works.