Two-photon absorption in the relativistic four-component Hartree-Fock approximation

Area of Science:

• Quantum Chemistry
• Relativistic Calculations
• Molecular Spectroscopy

Background:

• The quadratic response function is crucial for understanding molecular properties.
• Relativistic effects become significant in heavy atoms and molecules.
• Previous work established a framework for quadratic response functions.

Purpose of the Study:

• To implement the single residue of the quadratic response function within the four-component Hartree-Fock approximation.
• To investigate two-photon absorption cross sections in noble gases using this new implementation.
• To explore the impact of relativistic effects on spectral transitions.

Main Methods:

• Utilized a Kramers paired molecular orbital basis.
• Employed time and spatial symmetry reductions via quaternion formulation.
• Performed sample calculations for monochromatic and coherent two-photon absorption cross sections.

Main Results:

• Successfully implemented the single residue of the quadratic response function.
• Demonstrated strong two-photon absorption for a spin-forbidden transition in neon (X (1)S(0)-->2 (3)P(2)).
• Showcased the relativistic two-photon selection rule (DeltaJ = {0,+/-2}) enabling non-relativistic spin-forbidden transitions.

Conclusions:

• The relativistic framework correctly predicts transitions not observable with non-relativistic methods.
• Relativistic calculations are essential for accurate two-photon absorption predictions in noble gases.
• Comparisons between relativistic and non-relativistic calculations should focus on integrated absorption cross sections.