Theoretical Study On The Optical Properties Of Molecules And Noble Metal Nanoparticles

The current thesis presents the major works that I have done during my PhD study. The main work includes two parts: one part is concerned with the theory and applica-tions of time-dependent approaches to one/two photon electronic and scattering spectra with inclusion of Herzberg-Teller (HT) contribution and mode-mixing (Duschinsky) effect. Another part is to study the special optical properties of isolated noble metal nanoparticle and nanoparticle pairs in small size. These works not only provide the detailed mathematical expressions of differential cross sections of spectra in the time domain, and elaborate the hybrid mechanism of plasmon for noble metal nanoparti-cle pairs, but also present a microscopic description to the optical properties of se-ries molecules, such as the luminescent groups of biological molecules, free radical molecules, porphyrin, as well as metal nanoparticles. It helps people to more easily un-derstand photophysical and photochemical processes in these systems, and facilitates the development of the life sciences, medicine, solar cells, and materials science. The specific work mainly includes the following aspects:(1) To effectively describe the mode-mixing and vibronic coupling effects of complex molecules, the mathematical expressions of linear and nonlinear spectra have been constructed in time-domain based on the time-correlation functions.(2) Studying the realistic system with the develope-d theoretical methods and models. For example, we have investigated the influence of substitution and vibronic coupling effects on the absorption, fluorescence and reso-nance Raman spectroscopy in free radicals; examined the role of the excited potential energy surfaces in two-photon and resonance hyper-Raman spectroscopy of π conju-gated molecules and biological chromophores.(3) Based on the first-principles meth-ods, and then combined with exciton coupling theory model and appropriate exchange-correlation functional, we explain the mechanism of interaction and generation in the plasmon resonance of metal nanoparticles and its aggregates from the perspective of quantum mechanics. The main results are presented as follows:1. Developing and consummating the detailed mathematical expressions of differ- ential cross sections for a series of vibrationally-resolved electric spectra and resonance raman scattering spectra with inclusion of Duschinsky rotation and Herzberg-Teller ef-fect in the time-domain. These include the linear absorption, emission, circular dichro-ism and resonance Raman spectroscopy (RRS) as well as the nonlinear two-photon absorption (TPA) and resonance hyper-Raman spectroscopy (RHRS). Unlike sum-of-state (SOS) method, the analytical expressions for these spectroscopies avoid trunca-tion and summation over the large number of intermediate vibrational states, and thus have higher accuracy and efficiency. Consequently, the established theoretical models and program are used to study a series of free radical molecules, chromophores and π conjugated molecular systems.2. The calculation of resonance Raman spectra for molecules in the gas phase and solution proves that, our developed theoretical approach is correct, it can well reproduce the results originate from the experiment and other theoretical approaches. The basis sets, theoretical model and reference configuration play a significant role on the electronic and geometric structure of the molecule. Tamm-Dancoff approximation to TDDFT is not suitable to describe the vibrationally resolved spectroscopy.3. From the studies of the free radicals, we find that TD-B3LYP seriously un-derestimates the calculated excitation energies compared with the experimental values, however, it provides a good description for the ground state and excited state poten-tial energy surface, because we get the perfectly consistent spectral lineshapes between theoretical calculations and experiments for the absorption, fluorescence and resonance Raman spectra. In additional, the vibronic coupling significantly varies from the sub-stituent groups, but it has a little influence on the vibrational frequency.4. The investigation on the calculated TPA and RHRS show that, the correct resonance-states potential energy surface is essential to the simulation of the spectrum, and some importantly vibronic information will be missed by vertical gradient approx-imation. On the other hand, mode-mixing, frequency difference and Herzberg-Teller effects play an important role on the TPA and RHRS.5. The calculation to metal nanoparticles illustrate that, only the long-range-corrected functional (LRC-wPBE) can produce the correct plasmon characteristics of gold nanoparticles, plasmon absorption band is significantly influenced by the different particle size and interparticle spacing, and even new peaks will be yielded for nanopar-ticle pairs in smaller interparticle distance. The localized exciton coupling model can not describe the spectrum of closely packed nanoparticles. Furthermore, the coupled form between nanoparticles can adjust position and intensity of the plasmon absorption bands in aggregates, as well as its electron transfer.