Phase Cycling RT-TDDFT Simulation Protocol for Nonlinear XUV and X-ray Molecular Spectroscopy

Area of Science:

• Computational Chemistry
• Quantum Dynamics
• Theoretical Physics

Background:

• Simulating many-electron systems under external fields is computationally demanding.
• Traditional methods often require calculating expensive many-body states.
• Nonlinear optical signals arise from complex, multi-pathway interactions.

Purpose of the Study:

• To present a practical algorithm, real-time time-dependent density functional theory (RT-TDDFT), for simulating laser-driven electron dynamics.
• To enable direct computation of spectroscopic and diffraction signals, avoiding complex many-body state calculations.
• To implement a phase cycling protocol for separating contributing pathways in nonlinear optical signals.

Main Methods:

• Nonperturbative inclusion of external laser fields in the propagation.
• Direct computation of molecular reduced single-electron density operator.
• Application of a phase cycling protocol to disentangle signal pathways.
• Simulation of XUV four-wave mixing signals in CO molecules.

Main Results:

• RT-TDDFT provides a computationally tractable method for electron system propagation.
• The algorithm successfully computes signals directly, bypassing the need for many-body states.
• Phase cycling effectively separates contributions from multiple pathways in nonlinear optical signals.
• Simulations of XUV four-wave mixing in CO show good agreement with ab initio sum-over-states calculations.

Conclusions:

• RT-TDDFT is a powerful and efficient tool for studying laser-matter interactions.
• The developed phase cycling protocol enhances the analysis of complex nonlinear optical phenomena.
• This method offers a practical alternative for calculating spectroscopic and diffraction signals in driven quantum systems.