Fully Anharmonic Vibrational Resonance Raman Spectrum of Diatomic Systems.

Area of Science:

• * Theoretical Chemistry
• * Molecular Spectroscopy
• * Quantum Mechanics

Background:

• * Resonance Raman (RR) spectroscopy is a powerful technique for studying molecular vibrations.
• * Accurate theoretical models are crucial for interpreting complex RR spectra.
• * Anharmonic effects in molecular vibrations can significantly influence spectral properties.

Purpose of the Study:

• * To investigate the impact of anharmonicity on resonance Raman spectra of diatomic molecules.
• * To compare spectral predictions using anharmonic versus harmonic approximations.
• * To evaluate the accuracy of computational methods for RR spectrum simulation.

Main Methods:

• * Calculation of ab initio multireference potential energy curves for diatomic systems.
• * Numerical solution of the vibrational Schrödinger equation using variational stochastic and Cooley-Numerov methods.
• * Evaluation of anharmonic Franck-Condon and Herzberg-Teller integrals.
• * Computation of RR polarizability within the time-independent RR theory framework.

Main Results:

• * Anharmonic wave functions and energies were obtained for diatomic systems.
• * RR spectra were calculated for H2, C2, and O2 molecules, incorporating anharmonic effects.
• * Anharmonic spectra showed significant deviations from harmonic predictions in peak positions and intensities.
• * Faster convergence of differential cross sections was observed at the harmonic level compared to anharmonic calculations.

Conclusions:

• * Anharmonic effects play a critical role in determining the accurate resonance Raman spectra of diatomic molecules.
• * The inclusion of anharmonicity is essential for precise spectral interpretation and prediction.
• * Computational methods employed provide reliable simulations of anharmonic RR spectra.