Nonadiabatic and Time-Resolved Photoelectron Spectroscopy for Molecular Systems

Area of Science:

• Quantum chemistry
• Spectroscopy
• Materials science

Background:

• Vibronic sidebands in photoelectron spectra are crucial for understanding molecular dynamics.
• The Born-Oppenheimer approximation is a common simplification, but its limitations in describing electron-phonon coupling are not fully understood.
• Time-resolved photoelectron spectroscopy offers insights into ultrafast processes.

Purpose of the Study:

• To quantify nonadiabatic contributions to vibronic sidebands in photoelectron spectra.
• To investigate the validity of the Born-Oppenheimer approximation for vibronic systems.
• To analyze time-resolved nonequilibrium photoelectron spectra and trace wavepacket motion.

Main Methods:

• Exact diagonalization was used to evaluate sum-over-states expressions for linear-response photocurrent.
• Calculations were performed for a vibronic model system of trans-polyacetylene.
• Both equilibrium and explicitly time-resolved nonequilibrium spectra were analyzed.

Main Results:

• Spurious peaks were identified in the Born-Oppenheimer approximation of the vibronic spectral function.
• These spurious features arise from the factorized nature of initial and final photoemission states.
• Correlated vibronic states were shown to suppress these artifacts and reveal system dynamics.
• Wavepacket motion was successfully traced in time-resolved spectra for different excitation schemes.

Conclusions:

• Accurate quantification of nonadiabatic effects is essential for interpreting vibronic photoelectron spectra.
• The Born-Oppenheimer approximation introduces artifacts that are mitigated by including correlated vibronic states.
• Time-resolved photoelectron spectroscopy, when analyzed with accurate theoretical methods, can effectively monitor molecular dynamics.