Anion Resonances and Photoelectron Spectroscopy of the Tetracenyl Anion

Probing Dipole-Bound States Using Photodetachment Spectroscopy and Resonant Photoelectron Imaging of Cryogenically Cooled Anions

Molecular anions with polar neutral cores can support highly diffuse dipole-bound states below their detachment thresholds due to the long-range charge-dipole interaction. Such nonvalence states constitute a special class of excited electronic states for anions and were observed in early photodetachment experiments to measure the electron affinities of organic radicals. Recent experimental advances, in particular, the ability to create cold anions using a cryogenically cooled Paul trap, have allowed the investigation of dipole-bound excited states at a new level. For the first time, the zero-point level of dipole-bound excited states can be observed via resonant two-photon detachment, and resonant photoelectron spectroscopy can be performed via the above-threshold vibrational levels (Feshbach resonances) of the dipole-bound states. This Perspective describes recent progress in the investigation of dipole-bound states in the authors' lab using an electrospray photoelectron spectroscopy apparatus equipped with a cryogenically cooled Paul trap and high-resolution photoelectron imaging.

Predicted Negative Ion Photoelectron Spectra of 1-, 2-, and 9-Cyanoanthracene Radical Anions and Computed Thermochemical Values of the Three Cyanoanthracene Isomers

In this work, the negative ion photoelectron spectra of 1-, 2-, and 9-cyanoanthracene (anthracenecarbonitrile, ACN) radical anions, obtained via the calculation of Franck-Condon (FC) factors based on a harmonic oscillator model, are reported. The FC calculations utilize harmonic vibrational frequencies and normal mode vectors derived from density functional theory using the B3LYP/6-311++G (2d,2p) basis set. The removal of an electron from the doublet anion allows for the computation of the negative ion photoelectron spectra that represents the neutral ground singlet state (S_o) and the lowest triplet state (T₁) in each of the three ACN molecules. The respective adiabatic electron affinity (EA) values for the S_o state in 1-, 2-, and 9-ACN isomers are calculated to be 1.353, 1.360, and 1.423 eV. The calculated EA of the 9-cyanoanthracene singlet isomer is in close agreement with the previously reported experimental value of 1.27 ± 0.1 eV. Calculations show that the T₁ states in 1-, 2-, and 9-ACN are located 1.656, 1.663, and 1.599 eV above the S_o state. The calculated T₁ negative ion spectra exhibit intense vibrational origins and weak FC activity beyond the origins, indicating little change in geometry following electron detachment from the doublet anionic state. Upon deprotonation, the EA values of the radical isomers increase by ∼400-700 meV, resulting in neutral deprotonated radicals with EAs between 1.740 and 2.220 eV. The calculated site-specific gas-phase acidity values of ACN isomers indicate that ACN molecules are more acidic than benzonitrile.

Spectroscopy of Radicals, Clusters, and Transition States Using Slow Electron Velocity-Map Imaging of Cryogenically Cooled Anions

Slow electron velocity-map imaging of cryogenically cooled anions (cryo-SEVI) is a high-resolution variant of anion photoelectron spectroscopy that has been applied with considerable success over the years to the study of radicals, size-selected clusters, and transition states for unimolecular and bimolecular reactions. Cryo-SEVI retains the versatility of conventional anion photoelectron spectroscopy while offering sub-meV resolution, thereby enabling the resolution of vibrational structure in the photoelectron spectra of complex anions. This Feature Article describes recent experiments in our laboratory using cryo-SEVI, including a new research direction in which anions are vibrationally pre-excited with an infrared laser pulse prior to photodetachment.

Spectroscopy and Theoretical Modeling of Tetracene Anion Resonances

The positions and widths of the optically allowed electronic states of the tetracene radical anion located above the detachment threshold energy (i.e anion resonances) are mapped out using total photodetachment yield spectroscopy of cryogenically cooled ions. The presence of these states is detected via the sharp increase in the photodetachment yield compared to that of the monotonic nonresonant direct photodetachment background. The resolution of the resulting spectrum is limited by the ∼5 cm^-1 line width of the tunable laser and thus provides a stringent benchmark for computations of the energies and autodetachment lifetimes of these resonance states. The experimental results are compared to high-level electronic structure computations and line width modeling using the orbital stabilization method. These theoretical results are found to be in near quantitative agreement with the experimental data, highlighting their capability to accurately describe the energies and lifetimes of anion resonances for relatively large molecules.