Photoelectron Spectroscopy of Molecular Anions

Photoelectron spectroscopy of molecular anions is a powerful technique for interrogating the electronic structure and dynamics of negatively charged molecules. By photodetaching an excess electron with a tunable laser and analysing the kinetic energy and angular distribution of the emitted electron, researchers obtain direct information on electron binding energies, vibrational levels and non-valence excited states. Cryogenic cooling and high-resolution imaging have enabled the observation of subtle vibrational features that are inaccessible to conventional spectroscopy. In particular, resonant photoelectron spectroscopy via dipole- or quadrupole-bound excited states reveals highly non-Franck-Condon transitions, furnishing detailed maps of potential energy surfaces and insights into intramolecular mode coupling. This approach has been used to characterise fleeting transition-state regions in chemical reactions, to probe the dynamics of photodetachment in biologically relevant chromophores and to elucidate electron transfer mechanisms in atmospheric and astrochemical contexts. The ability to resolve mode-selective autodetachment and internal conversion pathways provides a unique window on how excess electrons influence molecular geometry, reactivity and energy redistribution.

Research from Nature Portfolio

Recent studies have combined high-resolution photoelectron spectroscopy with advanced quantum-dynamical calculations to capture transition-state resonances along complex reaction coordinates. By exploiting a dipole-bound state in FNH₃⁻, researchers have unveiled a series of high-lying Feshbach resonances and bound states that trace the activated complex of the F + NH₃ → HF + NH₂ reaction. In another advance, time-resolved photoelectron imaging of the pyruvate anion under UVA excitation has revealed unexpected decarboxylation pathways, yielding CO₂, CO and a methide anion that fragments into methyl radical and a free electron. These findings emphasise the role of non-valence states in steering photochemical outcomes and highlight the potential impact on atmospheric processes.

Photoelectron Spectroscopy of Molecular Anions publication trend

The graph below shows the total number of articles in photoelectron spectroscopy of molecular anions across all publications each year (not limited to Nature Index journals).

Technical terms

Photoelectron spectroscopy: Experimental method in which a photon detaches an electron from an anion and the energy distribution of the ejected electron is analysed to probe electronic and vibrational structure.

Molecular anion: A negatively charged molecule carrying one or more excess electrons bound to its neutral core.

Photodetachment: The process of removing an electron from an anion by absorption of a photon.

Dipole-bound state: A diffuse non-valence state in which an excess electron is weakly bound by the dipole moment of a polar molecule.

Feshbach resonance: A temporary anionic state in which an excess electron is trapped in a vibrationally excited level embedded in the continuum, often enhancing photodetachment signals.

Franck-Condon factor: A measure of the overlap between vibrational wavefunctions of initial and final electronic states, governing the intensity of vibronic transitions.

References

1. Probing the activated complex of the F + NH3 reaction via a dipole-bound state. Nature Communications (2024).
2. Photochemistry of the pyruvate anion produces CO2, CO, CH3–, CH3, and a low energy electron. Nature Communications (2022).
3. Excited-state chemistry of the nitromethane anion mediated by the dipole-bound states revealed by photofragment action spectroscopy. Chemical Science (2023).
4. High-resolution photoelectron imaging and resonant photoelectron spectroscopy via noncovalently bound excited states of cryogenically cooled anions. Chemical Science (2019).
5. Dynamic role of the correlation effect revealed in the exceptionally slow autodetachment rates of the vibrational Feshbach resonances in the dipole-bound state. Chemical Science (2022).