The Dynamophore – Localization of Excited State Dynamics Studied by Time–Resolved Photoelectron Spectroscopy

Abstract

Abstract. Photoinduced dynamics tend to localize at a confined region of a molecule, called dynamophore. Here, we show examples from time-resolved photoelectron spectroscopy.

Hexamethylcyclopentadiene: time-resolved photoelectron spectroscopy and ab initio multiple spawning simulations

“…Furthermore, experimental time-resolved photoelectronphotoion coincidence spectra of butadiene clearly resolve a shift by 20 fs and a broadening by 30 fs of the photoelectron band at time zero, which seems comparable to our observations in CPDMe 6 . 16,33 Our observations of excited state dynamics in CPDMe 6 seem to be of more general validity for small polyenes. However, this clearly has to be checked thoroughly with different model systems by further combinations of experimental and theoretical investigations.…”

“…This effect was attributed to large amplitude motion and to localization of the dynamics to a dynamophore, a dynamical subunit of the molecule, such as a single carbon double bond. 11,12,16,17,33 Many dynamical simulations of small polyenes, on the other hand, show that population transfer from the initially excited to a lower lying electronic state is preceded by a time period during which the nuclear wavepacket needs to evolve along specific degrees of freedom in order to reach the conical intersection (CoIn) seam. 20,23 This period was referred to as an ''induction time''.…”

“…However, it was observed in TRPES studies of several systems, including CPD, that the excited state dynamics can be slowed by increasing substituent masses. 6,10,17,33,41 Hence, the dynamics would be expected to be considerably slower in the case of complete methyl substitution, as in hexamethylcyclopentadiene, CPDMe 6 . On the theoretical side, substitution of hydrogens by methyl groups is not desirable since it leads to a significant increase in computational demand.…”

Two-Photon Excited State Dynamics of Dark Valence, Rydberg, and Superexcited States in 1,3-Butadiene

“…These are due to Franck−Condon effects, in direct analogy with the valence state dynamics observed in the butadiene 1 2 B u state. 33,37 As a final note, we would like to mention that we did not observe any clear signs that we directly accessed the 'dark' 2 1 A g state. Possible reasons are a bad Franck−Condon overlap, the wrong spectral region, or a low two-photon oscillator strength.…”

“…This motion is experimentally evidenced by TRPES through a large spectral shift. 33,37 The time scale of the reaction is sub 100 fs, although a small fraction might remain in the excited state for an extended period. 36 Upon direct two-photon excitation to Rydberg states that populate the dark 2 1 A g state, one might expect similar spectral dynamics as the motion toward the S 1 −S 0 intersection and localization at the same C C-dynamophore 25,37 should obtain whether the S 1 state is populated via the bright 1B u state or the dark Rydberg states.…”