Two-dimensional x-ray correlation spectroscopy of remote core states.

Area of Science:

• Atomic and Molecular Physics
• Physical Chemistry
• Spectroscopy

Background:

• The dipole approximation is commonly used for nonlinear X-ray signals.
• Large core-atom separations can introduce phase factors not accounted for by the dipole approximation.
• Accurate theoretical descriptions are needed for complex molecular systems.

Purpose of the Study:

• To develop expressions for 2D X-ray photon echo signals that incorporate position-dependent phase factors.
• To extend the applicability of theoretical models to systems with large core-atom separations relative to X-ray wavelengths.
• To apply these expressions to experimental data from a selenium dipeptide system.

Main Methods:

• Derivation of rotationally averaged 2D X-ray photon echo signal expressions.
• Inclusion of position-dependent phase factors for arbitrary core separation to wavelength ratios.
• Application to the Selenium K-edge (Se K-edge) of a selenium dipeptide.

Main Results:

• Formulation of theoretical expressions accounting for non-dipole effects in 2D X-ray photon echo spectroscopy.
• Demonstration of the importance of phase factors in systems with significant core-atom separation.
• Successful application to a specific molecular system (selenium dipeptide).

Conclusions:

• The presented expressions provide a more accurate description of nonlinear X-ray signals in systems with large core-atom separations.
• This work advances the theoretical understanding and interpretation of advanced X-ray spectroscopic techniques.
• The findings are relevant for studying the structure and dynamics of complex molecules using X-ray methods.