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.

Anion Resonances and Photoelectron Spectroscopy of the Tetracenyl Anion.

The photoelectron spectroscopy of the tetracenyl anion using slow electron velocity-map imaging (SEVI) of cryogenically cooled ions is presented. The total photodetachment yield as a function of photon energy is used to reveal a rich manifold of anion excited states above the detachment threshold. The lowest energy anionic resonance has a sufficiently long lifetime to yield a vibrationally resolved absorption spectrum that can be directly compared with theoretical predictions. Excitation of this state mostly results in electron detachment via thermionic emission. The total photodetachment yield spectrum is used to select photon wavelengths that minimize the indirect detachment signal to allow acquisition of vibrationally resolved photoelectron spectra that can inform on the neutral tetracenyl radical. Assignment of spectral features corresponding to the ground and first excited state of the neutral 12-tetracenyl isomer is made with the aid of Franck-Condon simulations. This yields adiabatic electron affinity and term energies that differ significantly from the previously reported values. Weak features corresponding to the ground state of the minor 2-teracenyl and 1-tetracenyl isomers are also identified, which allows for the experimental determination of their electron affinities for the first time.

Vibrationally resolved photoelectron spectroscopy of oligothiophene radical anions.

Vibrationally resolved photoelectron spectroscopy of terthiophene, quaterthiophene, and quinquethiophene radical anions is presented. The increased spectral resolution afforded by the combination of slow photoelectron velocity-map imaging and ion cooling in a cryogenic ion trap allows the characterization of vibronic structures within the S0 and T1 states. Analysis of the spectra, aided by electronic structure calculations and Franck-Condon simulations, revealed evidence for significant contributions from kinetically trapped higher energy conformers in the anion-to-triplet transitions. Unlike the lowest energy structures, where all the thiophene linkers are in the trans configuration, these higher energy conformers contain at least one cis linker. We also found that the adiabatic Franck-Condon simulations drastically underestimated the intensities of some vibronic features in the singlet ground state spectra due to large geometry changes upon photodetachment and anharmonic couplings in the singlet state.