Time-Resolved Photoelectron Imaging of Acetone with 9.3 eV Photoexcitation

Abstract

Ultrafast electronic relaxation following 9.3 eV photoexcitation of gaseous acetone was investigated with time-resolved photoelectron imaging spectroscopy. An intense photoionization signal due to a transition from the 4 1 A 1 (π,π*) state to the D 1 (π −1 ) cationic state diminishes within 50 fs, owing to vibrational wave packet motion leaving our observation energy window. Additional photoionization signals were assigned to transitions from Rydberg states with principal quantum numbers of 3−8 to the D 0 (n −1 ) cationic state, created by strong vibronic couplings with the bright 4 1 A 1 (π,π*) state. The deactivation processes of the 4 1 A 1 (π,π*) and Rydberg states are discussed based on their decay profiles obtained from a time−energy map of photoelectron kinetic energy distributions.

Vacuum Ultraviolet Excited State Dynamics of the Smallest Ketone: Acetone

“…As shown in the Supporting Information, this plateau is even more extended at lower pump photon energies (ℏω pump = 7.78 eV). Analogous “flat top” behavior was seen in a TRPES study of acetone photoexcited at the considerably higher pump photon energy of 9.3 eV . Based on our MCTDH simulations, we suggest that these plateaux are due to the complex dynamics of multiple (Rydberg and valence) channels feeding population into the lower-lying A 2 (nπ*) state on differing time scales, thus giving the appearance of a “flat top” time dependent behavior.…”

“…Its strongly allowed transitions are in the vacuum ultraviolet (VUV) range and its spectra and dynamics comprise complex interactions between Rydberg and valence states. Powerful experimental and theoretical techniques have been applied to the study of VUV-induced dynamics in acetone. − It was variably proposed that the A 1 ( ππ *) state played a key role in these dynamics, perhaps in the initial excitation, or as the repository of complex Rydberg dynamics, or both. The detailed nature of the initially excited state and the role of the A 1 ( ππ *) state thus remains unclear.…”

“…As depicted in Figure , we probed the VUV induced wavepacket dynamics via photoionization with a time-delayed fs UV probe pulse (200.5 nm, ℏω probe = 6.18 eV). All prior studies of VUV-excited acetone − employed probe photon energies <4.8 eV, thus limiting the total ionization energy. Importantly, our higher 6.18 eV probe photon energy allowed us to track multiple nonadiabatic transitions between lower-lying states, thus following the excited state dynamics to lower-lying valence states.…”

Revealing Ultrafast Population Transfer between Nearly Degenerate Electronic States

“…Specifically, we are resolving the population transfer dynamics in the n3p Rydberg manifold of the acetone molecule after two-photon excitation. The photophysical and photochemical processes involving acetone have been in the spotlight since the early days of femtochemistry, − with a recent renaissance ,− motivated by the importance of acetone as the simplest aliphatic ketone. The difficulty of resolving light-induced processes in acetone derives from its electronic structure, characterized by series of Rydberg states that are strongly coupled to valence states, with the consequence of complex nonadiabatic relaxation dynamics where the electronic population is cascading down the ladder of Rydberg states. ,,− As shown schematically in Figure a, the n3p manifold, energetically located at the lower end of the Rydberg series, consists of the three states n3p x (A 2 , 7.34 eV), n3p y (A 1 , 7.40 eV), and n3p z (B 2 , 7.45 eV), lying within about 100 meV in the Franck–Condon region of the ground state.…”

“…The photophysical and photochemical processes involving acetone have been in the spotlight since the early days of femtochemistry, − with a recent renaissance ,− motivated by the importance of acetone as the simplest aliphatic ketone. The difficulty of resolving light-induced processes in acetone derives from its electronic structure, characterized by series of Rydberg states that are strongly coupled to valence states, with the consequence of complex nonadiabatic relaxation dynamics where the electronic population is cascading down the ladder of Rydberg states. ,,− As shown schematically in Figure a, the n3p manifold, energetically located at the lower end of the Rydberg series, consists of the three states n3p x (A 2 , 7.34 eV), n3p y (A 1 , 7.40 eV), and n3p z (B 2 , 7.45 eV), lying within about 100 meV in the Franck–Condon region of the ground state. This manifold of states was investigated in time-resolved experiments and is considered to play an important role in the population transfer to lower states. ,, The energetic proximity of the three n3p states leads to significant vibrational coupling andtogether with the presence of symmetry-dependent couplings with the ππ * (A 1 ) valence statea complex interaction among all these states. − As a consequence, the dynamics of the individual n3p states within this dense region has evaded detailed observation to date, despite the importance of the n3p manifold as the bottleneck for the radiationless deactivation pathways of the higher-lying Rydberg states.…”