Ultrafast valence to non-valence excited state dynamics in a common anionic chromophore

Abstract

Non-valence states in neutral molecules (Rydberg states) have well-established roles and importance in photochemistry, however, considerably less is known about the role of non-valence states in photo-induced processes in anions. Here, femtosecond time-resolved photoelectron imaging is used to show that photoexcitation of the S1(ππ*) state of the methyl ester of deprotonated para-coumaric acid – a model chromophore for photoactive yellow protein (PYP) – leads to a bifurcation of the excited state wavepacket. One part remains on the S1(ππ*) state forming a twisted intermediate, whilst a second part leads to the formation of a non-valence (dipole-bound) state. Both populations eventually decay independently by vibrational autodetachment. Valence-to-non-valence internal conversion has hitherto not been observed in the intramolecular photophysics of an isolated anion, raising questions into how common such processes might be, given that many anionic chromophores have bright valence states near the detachment threshold.

Geometric and electronic structure probed along the isomerisation coordinate of a photoactive yellow protein chromophore

“…Additionally, Zewail and coworkers observed an oscillation in the high kinetic energy window, similar to that observed in ε 1 , but not the out-of-phase oscillation at lower energy (ε 2 ), probably because of contamination by autodetachment 16 . Dynamics involving isomerisation were also observed in a recent study on a closely related PYP chromophore anion in which the ketone is replaced by an ester group 31 . These dynamics were in competition with internal conversion to a non-valence state of the anion.…”

Electron scattering cross sections from nitrobenzene in the energy range 0.4–1000 eV: the role of dipole interactions in measurements and calculations

“…54 and references therein). Interconnections between valence states of anions and dipole-bound states have also been identified in the opposite direction (bifurcation of the excited state wavepacket leading to the formation of a non-valence state) for a common anionic chromophore, 55 and the stability of such orbitals in the presence of perturbing molecules has also been studied by the same group. 56 Very recently, Anstöter et al 57 showed that the observed emission peaks from the vibrational structure of the nitrobenzene anion can be correlated with an autodetachment mechanism via dipole-bound state formation.…”

Mode-Specific Vibrational Autodetachment Following Excitation of Electronic Resonances by Electrons and Photons

“…Although the initial geometry of the resonance M −Ã is different than in electron attachment, the same two types of excitation or emission are usually considered. A structured PE signal with low and constant ε f has also been seen over a range of hv in a number of targets, and this structure has been related to autodetachment from nonvalence states [10][11][12]. Here, we probe the electron detachment from electronic resonances in nitrobenzene (NB) using both 2D EEL and 2D PE spectroscopy in an attempt to gain insight into this structured low-energy electron emission channel.…”

“…There are two mechanisms by which the DBS can be populated. The first is by internal conversion through a conical intersection from a valence resonance [10][11][12]. For the second, a fraction of the population of the resonance internally converts to reform the vibrationally hot ground state of the anion, which could then populate the DBS.…”

“…The autodetachment process described here can be, in principle, viewed as a reverse mechanism: The electron leaving the hot molecule via a nonvalence state deexcites specific modes. (ii) Mode-specific vibrational autodetachment has been observed from nonvalence states populated by internal conversion from resonances; however, this emission occurs on a picosecond timescale and applies to resonances excited near the threshold [10][11][12]. (iii) Signatures of IR modes have also been seen in the direct photodetachment from nonvalence states.…”