A Polarization Propagator for Nonlinear X-ray Spectroscopies

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• X-ray Spectroscopy

Background:

• Calculating nonlinear molecular properties, especially in the X-ray regime, is computationally demanding.
• Existing methods often require explicit calculation of excited states, increasing complexity.
• Understanding X-ray spectroscopies like two-photon absorption and absorption fine structure is crucial for materials science and chemistry.

Purpose of the Study:

• To develop and implement a computational approach for direct calculation of nonlinear molecular properties at X-ray wavelengths.
• To model coherent near-edge X-ray two-photon absorption (X2PA) using density functional theory (DFT).
• To compare X2PA with near-edge X-ray absorption fine structure (NEXAFS) to highlight spectroscopic differences.

Main Methods:

• Development of a complex polarization propagator approach to third order.
• Implementation within a density functional theory (DFT) framework.
• Direct calculation of nonlinear molecular properties without explicit excited-state computations.

Main Results:

• The propagator method successfully models coherent X-ray two-photon absorption.
• Comparison with NEXAFS spectra reveals distinct roles of symmetry, localization, and electron correlation in each technique.
• The method provides a direct route to nonlinear X-ray properties.

Conclusions:

• The developed third-order polarization propagator approach offers an efficient way to compute nonlinear X-ray molecular properties.
• This method simplifies the modeling of X-ray spectroscopies like X2PA.
• The findings pave the way for advancements in nonlinear X-ray research and spectroscopy.