Photoelectron spectroscopy of cryogenically cooled NiO2-via slow photoelectron velocity-map imaging

The overlooked role of excited anion states in NiO2- photodetachment

Photodetachment spectra of anionic species provide significant insights into the energies and nature of ground and excited states of both the anion and resultant neutral molecules. Direct detachment of the excess electron to the continuum may occur via formally allowed or forbidden transitions (perhaps as the result of intensity borrowing through vibronic coupling). However, alternate indirect pathways are also possible and often overlooked. Here, we report a two-dimensional photoelectron spectral study, combined with correlated electronic structure calculations, to elucidate the nature of photodetachment from NiO2-. The spectra are comprised of allowed and forbidden transitions, in excellent agreement with previously reported slow electron velocity mapped imaging spectra of the same system, which were interpreted in terms of direct detachment. In the current work, the contributions of indirect processes are revealed. Measured oscillations in the branching ratios of the spectral channels clearly indicate non-direct detachment processes, and the electronic structure calculations suggest that excited states of the appropriate symmetry and degeneracy lie slightly above the neutral ground state. Taken together, the results suggest that the origin of the observed forbidden transitions is the result of anion excited states mediating the electron detachment process.

Dipole-Bound State, Photodetachment Spectroscopy, and Resonant Photoelectron Imaging of Cryogenically-Cooled 2-Cyanopyrrolide

Valence-bound anions with polar neutral cores can have diffuse dipole-bound excited states just below the electron detachment threshold. Because of the similarity in geometry and vibrational frequencies between the dipole-bound states (DBSs) and the corresponding neutrals, DBSs have been exploited as intermediate states to conduct resonant photoelectron spectroscopy (PES), resulting in highly non-Franck-Condon photoelectron spectra via vibrational autodetachment and providing much richer vibrational information than conventional PES. Here, we report a photodetachment and high-resolution photoelectron imaging study of the 2-cyanopyrrolide anion, cooled in a cryogenic ion trap. The electron affinity of the 2-cyanopyrrolyl radical is measured to be 3.0981 ± 0.0006 eV (24 988 ± 5 cm^-1). A DBS is observed for 2-cyanopyrrolide at 240 cm^-1 below its detachment threshold using photodetachment spectroscopy. Twenty-three above-threshold vibrational resonances (Feshbach resonances) of the DBS are observed. Resonant PES is conducted at each Feshbach resonance, yielding a wealth of vibrational information about the 2-cyanopyrrolyl radical. Resonant two-photon PES confirms the s-like dipole-bound orbital and reveals a relatively long lifetime of the bound zero-point level of the DBS. Fundamental frequencies for 19 vibrational modes (out of a total of 24) are obtained for the cyanopyrrolyl radical, including six out-of-plane modes. The current work provides important spectroscopic information about 2-cyanopyrrolyl, which should be valuable for the study of this radical in combustion or astronomical environments.