The Role of Rydberg–Valence Coupling in the Ultrafast Relaxation Dynamics of Acetone

Abstract

The electronic structure of excited states of acetone is represented by a Rydberg manifold that is coupled to valence states which provide very fast and efficient relaxation pathways. We observe and characterize the transfer of population from photoexcited Rydberg states (6p, 6d, 7s) to a whole series of lower Rydberg states (3p to 4d) and a simultaneous decay of population from these states. We obtain these results with time-resolved photoelectron–photoion coincidence (PEPICO) detection in combination with the application of Bayesian statistics for data analysis. Despite the expectedly complex relaxation behavior, we find that a simple sequential decay model is able to describe the observed PEPICO transients satisfactorily. We obtain a slower decay (∼320 fs) from photoexcited states compared to a faster decay (∼100 fs) of states that are populated by internal conversion, demonstrating that different relaxation dynamics are active. Within the series of Rydberg states populated by internal conversion, the decay dynamics seem to be similar, and a trend of slower decay from lower states indicates an increasingly higher energy barrier along the decay pathway for lower states. The presented results agree all in all with previous relaxation studies within the Rydberg manifold. The state-resolved observation of transient population ranging from 3p to 4d can serve as reference for time-dependent simulations.

Analysis of femtosecond pump-probe photoelectron-photoion coincidence measurements applying Bayesian probability theory

“…Moreover, the populations in the excited state can decay to energetically lower states by fast and efficient nonadiabatic processes [3,7,30]. The channel 2 signal in a pump-probe measurement can therefore become significantly smaller than the channel 1 background, in particular for long delay times.…”

“…For the measurements we use a femtosecond pumpprobe setup, which has been described in detail previously [7,30]. Acetone molecules are excited by a threephoton transition to high-lying Rydberg states (6p, 6d, 7s) at about 9.30 eV [30].…”

“…For the measurements we use a femtosecond pumpprobe setup, which has been described in detail previously [7,30]. Acetone molecules are excited by a threephoton transition to high-lying Rydberg states (6p, 6d, 7s) at about 9.30 eV [30]. Pump and probe pulses are obtained from a commercial Ti:sapphire laser system (Coherent Vitara oscillator and Legend Elite Duo amplifier) and frequency doubled in BBO crystals to obtain 3.1 eV photon energy (395-nm center wavelength).…”

Bayesian Analysis of Femtosecond Pump-Probe Photoelectron-Photoion Coincidence Spectra with Fluctuating Laser Intensities

“…For example, the assignment of the photoelectron kinetic energy to an excited electronic state of the unfragmented molecule and coincidence detection of an ion fragment show that the molecule was intact at the moment of ionization and that fragmentation must have occurred afterwards. Moreover, the population in the excited state can decay to energetically lower states quite quickly, e.g., on a femtosecond timescale [4,7,28]. It is due to this decay that the Channel 2 signal can become significantly smaller than the Channel 1 background, in particular for long delay times, causing a poor signal-to-noise ratio.…”

“…The energy level diagram shows how the electron kinetic energy, given the energy of the states and the photons, identifies the state the system was in at the moment of ionization. A detailed description of the setup can be found in our previous publications [7,28].…”