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Quinones feature prominently as electron acceptors in nature. Their electron-transfer reactions are often highly exergonic, for which Marcus theory predicts reduced electron-transfer rates because of a free-energy barrier that occurs in the inverted region. However, the electron-transfer kinetics that involve quinones can appear barrierless. Here, we consider the intrinsic properties of the para-benzoquinone radical anion, which serves as the prototypical electron-transfer reaction product involving a quinone-based acceptor. Using time-resolved photoelectron spectroscopy and ab initio calculations, we show that excitation at 400 and 480 nm yields excited states that are unbound with respect to electron loss. These excited states are shown to decay on a sub-40 fs timescale through a series of conical intersections with lower-lying excited states, ultimately to form the ground anionic state and avoid autodetachment. From an isolated electron-acceptor perspective, this ultrafast stabilization mechanism accounts for the ability of para-benzoquinone to capture and retain electrons his work was funded by the Engineering and Physical Sciences Research Council (EP/D073472/1), the Spanish Ministerio de Ciencia e Innovacion (MICINN) (CTQ2011-26573 and UNGI08-4E-003 from the European Fund for Regional Development) and the Catalan Agencia de Gestio d'Ajuts Universitaris i de Recerca (SGR0528)

A photoelectron spectroscopy study of the anionic model chromophore of the green fluorescent protein is presented. From the photoelectron spectra taken at 3.496 eV, 4.62 eV, and 6.15 eV the vertical and adiabatic detachment energies are determined to be 2.8 ± 0.1 eV and 2.6 ± 0.2 eV, respectively. The vertical detachment energy is higher than the S1 ← S0 absorption maximum (2.57 eV) and indicates that the S1 state is bound with respect to electron detachment in the Franck–Condon region. The photoelectron spectrum taken at 6.15 eV, together with TD-DFT calculations, are used to assign a number of excited states in the neutral radical that correspond to electron loss from occupied orbitals in the anion. The photoelectron spectrum at 2.58 eV shows evidence for electrons formed by thermionic emission, suggesting that internal conversion is the dominant relaxation pathway following S1 ← S0 excitation.

We present a velocity-map imaging (VMI) setup for photoelectron imaging that utilizes low electric extraction fields. This avoids any complications that could arise from electrostatic interactions between the extraction field and the molecular properties that are probed and has a minimal effect on the trajectory of ions in ion beam experiments. By using an attractive potential supplied to the detector, and keeping the electrodes at ground (zero) potential, we show that fringe fields between the VMI arrangement and the vacuum chamber can be eliminated, which is important in experiments on ions.

The first time-resolved photoelectron imaging study of a polyanion is presented. Using the alignment induced through resonance excitation, the photoelectron angular distributions can be qualitatively understood in terms of the position of localized excess charges on the molecular skeleton, which influence the photoemission dynamics. Pump–probe experiments are used to demonstrate that the photoelectron angular distribution is also sensitive to molecular dynamics. This is shown here for the rotational dynamics of a polyanion, in which the photoelectron anisotropy tracks the rotational coherence as it dephases. The methodology can in principle be applied to general molecular dynamics in large polyanions, providing a new route to studying ultrafast structural dynamics in complex gas-phase systems.

The nature of the repulsive Coulomb barrier in isolated molecular polyanions is studied by means of the photodetachment dynamics of the S(1) excited state of the fluorescein dianion which is bound solely by the repulsive Coulomb barrier. Photoelectron spectra reveal a feature at a constant electron kinetic energy, regardless of the excitation energy. This is explained by using an adiabatic tunneling picture for electron loss through successive repulsive Coulomb barriers correlating to vibrationally excited states. This physical picture is supported by time-resolved photoelectron spectra, showing that the tunneling lifetime is also invariant with excitation energy.

The spectroscopy and dynamics of near-threshold excited states of the isolated chloranil radical anion are investigated using photoelectron imaging. The photoelectron images taken at 480 nm clearly indicate resonance-enhanced photodetachment via a bound electronic excited state. Time-resolved photoelectron imaging reveals that the excited state rapidly decays on a timescale of 130 fs via internal conversion. The ultrafast relaxation dynamics of excited states near threshold are pertinent to common electron acceptor molecules based on the quinone moiety and may serve as doorway states that enable efficient electron transfer in the highly exergonic inverted regime, despite the presence of large free energy barriers.

Time-resolved photoelectron imaging of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion is presented. Photoelectron angular distributions (PADs) are qualitatively analyzed in terms of the simple s-p model that is based on symmetry arguments. The internal conversion dynamics from the first excited state (1(2)B(3u)) to the ground state ((2)B(2g)) may be observed through temporal changes in the PADs of the spectrally overlapping photoelectron features arising from photodetachment of the ground state and the excited state. A formulism for extracting the population dynamics from the β(2) anisotropy parameter of overlapping spectroscopic features is presented. This is used to extract the lifetime of the first excited state, which is in good agreement with that observed in the time-resolved photoelectron spectra.

The photoelectron spectrum of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion has been measured at 3.1 eV. Additionally, the ultrafast relaxation dynamics of the first excited state (1(2)B(3u)) of TCNQ(-) have been studied using time-resolved photoelectron spectroscopy, which reveals that it undergoes internal conversion back to the ground state ((2)B(2g)) with an associated lifetime of 650 fs and shows evidence of coherent nuclear motion.