Revealing Ultrafast Population Transfer between Nearly Degenerate Electronic States

Abstract

The response of a molecule to photoexcitation is governed by the coupling of its electronic states. However, if the energetic spacing between the electronically excited states at the Franck–Condon window becomes sufficiently small, it is infeasible to selectively excite and monitor individual states with conventional time-resolved spectroscopy, preventing insight into the energy transfer and relaxation dynamics of the molecule. Here, we demonstrate how the combination of time-resolved spectroscopy and extensive surface hopping dynamics simulations with a global fit approach on individually excited ensembles overcomes this limitation and resolves the dynamics in the n3p Rydberg states in acetone. Photoelectron transients of the three closely spaced states n3p x , n3p y , and n3p z are used to validate the theoretical results, which in turn allow retrieving a comprehensive kinetic model describing the mutual interactions of these states for the first time.

Surface Hopping Dynamics on Vibronic Coupling Models

“…Acetone was studied in the gas phase with SH/LVC in order to resolve the population dynamics among the set of the three near-degenerate n3p Rydberg states. 40 The main challenge stems from the many different internal conversion processes (n3p x ⇄ n3p y ⇄ n3p z , n3p x /n3p y /n3p z → ππ*) that take place simultaneously (Figure 3a), preventing one from obtaining all of the involved time constants from experimental data, e.g., from time-resolved photoelectron spectroscopy.…”

“…The population dynamics was investigated using an LVC model with all 24 DOFs and 49 diabatic states fitted with spin- opposite-scaled algebraic diagrammatic construction to second order [SOS-ADC(2)]. 40 We simulated three independent ensembles of roughly 1000 trajectories each, starting in the three different n3p Rydberg states. After excitation, nonadiabatic transitions quickly equilibrate the populations of the three Rydberg states while the population more slowly decays to the lower-lying ππ* state via the n3p y state.…”

Machine learning and excited-state molecular dynamics

“…In order to get a more comprehensive understanding, theoretical simulations can complement experimental findings and can provide explanations for observed reactions [9]. For instance, simulated UV spectra can be used to unveil the states relevant for photodamage and -stability of molecules [28][29][30][31][32][33][34][35][36] and the temporal evolution of molecules can be studied via nonadiabatic molecular dynamics (NAMD) simulations [37][38][39][40][41][42][43][44][45][46]. The latter gives access to different reaction channels, branching ratios, and excited-state lifetimes and will be the main topic of discussion here.…”

Vacuum Ultraviolet Excited State Dynamics of the Smallest Ketone: Acetone

“…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.…”

“…Prior ultrafast dynamical studies of acetone in the VUV region used either a fixed wavelength harmonic of the driving laser , or a multiphoton pump scheme. ,, The latter potentially involves intermediate resonances with lower-lying valence states, such as the A 2 (nπ*) state . Here we used a tunable femtosecond VUV source to selectively excite the A 1 (n3d yz ) Rydberg state of acetone via a single-photon transition at 153.3 nm (ℏω pump = 8.09 eV).…”