Time-dependent photoionization of azulene: Competition between ionization and relaxation in highly excited states

TL;DR

This study uses pump-probe photoionization to investigate the relaxation and ionization processes in highly excited azulene, revealing a time-invariant photoelectron spectrum driven by an unstable electronic state and quantifying internal conversion rates.

Contribution

It demonstrates that the photoelectron spectra are invariant to time delay and wavelength, indicating a dominant ionization pathway via an unstable state and provides quantitative analysis of internal conversion rates.

Findings

Photoelectron spectra are invariant to pump-probe delay and wavelength.

Ionization is driven by an unstable electronic state below the ionization potential.

Internal conversion rate from S2 to S0 follows an exponential energy gap law.

Abstract

Pump-probe photoionization has been used to map the relaxation processes taking place from highly vibrationally excited levels of the S2 state of azulene, populated directly or via internal conversion from the S4 state. Photoelectron spectra obtained by 1+2[prime] two-color time-resolved photoelectron imaging are invariant (apart from in intensity) to the pump-probe time delay and to the pump wavelength. This reveals a photoionization process which is driven by an unstable electronic state (e.g., doubly excited state) lying below the ionization potential. This state is postulated to be populated by a probe transition from S2 and to rapidly relax via an Auger-like process onto highly vibrationally excited Rydberg states. This accounts for the time invariance of the photoelectron spectrum. The intensity of the photoelectron spectrum is proportional to the population in S2. An exponential energy gap law is used to describe the internal conversion rate from S2 to S0. The vibronic coupling strength is found to be larger than 60$\pm$5 $\mu$eV.