Ab initio mixed polarizabilities responsible for the Raman-forbidden ν2 transition in carbon dioxide molecule

Area of Science:

• Quantum Chemistry
• Molecular Spectroscopy
• Computational Physics

Background:

• Centrosymmetric molecules can exhibit broken vibrational parity in Raman spectra.
• This phenomenon occurs when electric-dipole (E1) transitions are coupled with electric-quadrupole (E2) or magnetic-dipole (M1) transitions.

Purpose of the Study:

• To accurately calculate the intensity of the previously undetected ν2 CO2 Raman band.
• To determine the mixed E1-E2 (Â) and E1-M1 (Ĝ) polarizabilities for a bent CO2 molecule.
• To investigate the contribution of different transition types to the Raman band intensity.

Main Methods:

• Utilized the DALTON program suite for ab initio calculations.
• Employed extended basis sets and coupled-cluster with singles and doubles (CCSD) for electron correlation.
• Applied the symmetry-adapted perturbation theory (SOPPA) approach to compute derivatives of the Ĝ polarizability tensor.
• Derived irreducible spherical tensors (ISTs) to characterize the intensity contributions.

Main Results:

• Obtained refined ab initio values for  polarizability tensor components.
• Calculated Ĝ polarizability tensor derivatives for the first time.
• Identified specific ISTs (A1(1), A1(2), A1(3), A3(3) for E1-E2; G1(1), G1(2) for M1) governing the ν2 band intensity.
• Calculated integrated ν2 band intensities, highlighting the dominance of E1-M1 terms.

Conclusions:

• The E1-M1 interaction significantly contributes to the ν2 CO2 Raman band intensity.
• The calculated intensities provide a basis for detecting this parity-changing Raman process.
• Results align with recent findings on the ν3 band, emphasizing the importance of M1 contributions.