Estudo numérico do espectro Raman ressonante anarmônico de moléculas diatômicas

Abstract

In this study, the resonance Raman spectra of H2 and O2 molecules were computed by the numerical resolution of the vibrational Schrödinger equation employing fully anharmonic potentials, obtained by Multiconfigurational ab initio methods. The vibrational problem was solved through the Modified Variational Quantum Monte Carlo (MVQMC) simulation and the Vibrational Configurations Interaction (VCI) methods. The time independent framework of the RR effect was used to compute the RR intensities. The Potential Energy Surfaces (PESs) were in good agreement with the experimental values of the spectroscopic constants (Re, Te, De, Be, We e WeXe), except for the excited electronic state B3Σ− u PES of the O2 molecule, which could not be fully described due to an avoided crossing between electronic states. It was verified through the theoretical-experimental deviation values that the vibrational energies generated by MVQMC simulations are, in general, very accurate for the lowest energy vibrational states. However, as it was necessary to optimize higher energy states, required in the calculation of the RR polarizability, the computational time of MVQMC method was greatly increased, motivating the use of VCI method in order to try to reduce processing time and achieve results that are more accurate. Regarding to calculating the RR cross sections of the H2 and O2 molecules, results have showed that the convergence is faster when using the harmonic potential instead of the anharmonic potential. The addition of vibronic coupling effects was quite impactful for both molecular systems evaluated, and tended to attenuate the RR transition intensities relative to the values obtained with the Franck–Condon contribution. It was verified that the bandwidth variation of the excited electronic state (Γ parameter) pose little influence in the RR cross sections of the addressed molecular systems, therefore being able to vary Γ in more than an order of magnitude without substantially affecting the RR intensities. Ultimately, the target property is largely influenced by the number of vibrational wave functions, requiring considerable quantities of intermediate vibrational states (dozens) for a good convergence of the anharmonic RR cross sections, which is a much larger quantity compared to the harmonic calculations.