Photoelectron spectroscopy of anthracene and fluoranthene radical anions

Abstract

We report the slow electron velocity map imaging spectroscopy of cryogenically cooled anthracene and fluoranthene radical anions, two similarly sized polycyclic aromatic hydrocarbon molecules. The results allow us to examine the lowest energy singlet and triplet states in the neutral molecules on equal footing from the anionic ground state. The analysis of the experimental spectra is aided by harmonic calculations and Franck-Condon simulations, which generally show good agreement with experimental values and spectra. The electron affinity of fluoranthene is measured to be 0.757(2) eV, which is larger than that of anthracene at 0.532(3) eV. The lowest energy triplet state in anthracene is observed at 1.872(3) eV above the singlet ground state, while that of fluoranthene is observed at 2.321(2) eV above its singlet ground state. Comparisons of experimental and calculated spectra show that in addition to the Franck-Condon active modes, there is a clear presence of vibrational modes that gain intensity via vibronic coupling in both the singlet and triplet states in both molecules. In addition, the triplet state generally exhibits increased vibronic coupling compared to the singlet state, with the fluoranthene triplet state exhibiting evidence of distortion from C symmetry.

Low energy electron impact resonances of anthracene probed by 2D photoelectron imaging of its radical anion

“…presented a very high resolution slow-electron PE imaging of C14H10 -. 23 The PE spectra by Song et al at wavelengths of 532, 610 and 700 nm contained evidence of indirect detachment features that were red-shifted relative to the direct detachment peak accessing the neutral ground state. 22 The spectral range over which this indirect band appeared was consistent with the vertical excitation energy reported for a * resonance, from which autodetachment can occur.…”

“…33 Dessent used anthracene as a test molecule in a DFT benchmark study of the ground state vibrational properties of the neutral and anion, 29 and Kreger et al showed good agreement between their DFT calculations and vibrational structure in the high-resolution PE spectra. 23 Malloci et al…”

“…The adiabatic detachment energy (ADE) of C14H10has been determined by Garand and co-workers using high-resolution SEVI measurements to be 0.532(3) eV and the adiabatic detachment energy to reach the first triplet state was found to be 2.404(3) eV. 23 These are in agreement with the onset of the DD1 channel at 0.53 eV and the DD2 channel at 2.40 eV and allows us to assign DD1 to X 2 B3u + hv → X 1 Ag + eand DD2 to X 2 B3u + hv → 3 B2u + e -. Channel DD3 corresponds to the excitation of the neutral to its first singlet excited state in the photodetachment: X 2 B3u + hv → 1 B2u + e -.…”

“…15 Experimentally, anion resonances can be probed either by electron scattering experiments such as electron transmission spectroscopy, 16,17 or by optical (photoelectron) spectroscopy of the corresponding anion. [18][19][20][21][22][23] In the case of optical spectroscopy of the anion:…”

High-resolution photoelectron imaging and resonant photoelectron spectroscopy via noncovalently bound excited states of cryogenically cooled anions

“…9c), peaks a (27 1 ), b (26 1 ), c 27 2, and e (25 1 ), which are symmetry-forbidden in the non-resonant spectra. 5 Vibronic coupling or Herzberg-Teller coupling 4,63,72,89 has been previously invoked to explain the observations of FCinactive vibrational modes or anomalous vibrational intensities in non-resonant PES. While we cannot rule out the effects of vibronic coupling for the appearance of the low-frequency FCinactive and symmetry-forbidden bending modes in the resonant PE spectra shown in Fig.…”