Shake-Down Spectroscopy as State- and Site-Specific Probe of Ultrafast Chemical Dynamics

Area of Science:

• Physical Chemistry
• Chemical Physics
• Molecular Dynamics

Background:

• Tracking ultrafast molecular changes in photochemistry is challenging.
• Valence and core electronic states are typically studied separately.
• Time-resolved techniques have advanced the study of molecular transformations.

Purpose of the Study:

• To demonstrate a unified approach for measuring valence and core electronic states simultaneously.
• To investigate the controversial predissociation dynamics of photoexcited carbon disulfide (CS2).
• To elucidate the roles of internal conversion (IC) and intersystem crossing (ISC) in CS2 photochemistry.

Main Methods:

• Utilizing seeded free-electron laser (FEL) X-ray pulses for high-resolution, time-resolved X-ray photoelectron spectroscopy (XPS).
• Measuring weak shake-down satellite states in valence-excited molecules.
• Performing accurate multireference quantum chemical calculations for spectral comparison.

Main Results:

• Developed an FEL-based XPS method combining valence and core electronic state analysis.
• Observed shake-down satellite channels highly sensitive to electronic and geometric changes in CS2.
• Derived a spin-selective propensity rule for shake-down processes.

Conclusions:

• The FEL-XPS technique provides unprecedented insight into molecular photochemistry.
• Spin-selective shake-downs unequivocally assign contributions from singlet excited states in CS2.
• This method clarifies predissociation dynamics and excited-state branching ratios.