Resonant photoelectron imaging of deprotonated uracil anion via vibrational levels of a dipole-bound excited state

• Forty-four resonant photoelectron imaging and spectra of N1[U-H]− are reported.
• The autodetachment is largely governed by the Δv = −1 vibrational propensity rule.
• The breakdown of the propensity rule is observed due to electron rescattering.
• The breakdown of the propensity rule is observed due to the anharmonic effect.

We report both non-resonant and resonant high-resolution photoelectron imaging of cryogenically-cooled deprotonated uracil anions, N1[U-H]−, via vibrational levels of a dipole-bound excited state. Photodetachment spectroscopy of N1[U-H]− was reported previously (Liu et al., 2014), in which forty-six vibrational autodetachment resonances due to the excited dipole-bound state were observed. By tuning the detachment laser to the vibrational levels of the dipole-bound state, we obtained high-resolution resonant photoelectron spectra, which are highly non-Franck–Condon. The resonant photoelectron spectra reveal many Franck–Condon inactive vibrational modes, significantly expanding the capability of photoelectron spectroscopy. A total of twenty one fundamental vibrational frequencies for the N1[U-H] radical are obtained, including all eight low-frequency out-of-plane modes, which are forbidden in non-resonant photoelectron spectroscopy. Furthermore, the breakdown of the Δv = −1 propensity rule is observed for autodetachment from many vibrational levels of the dipole-bound state, due to anharmonic effects. In particular, we have observed intramolecular electron rescattering in a number of resonant photoelectron spectra, leading to excitations of low-frequency vibrational modes. Further theoretical study may be warranted, in light of the extensive experimental data and new observations, to provide further insight into the autodetachment dynamics and vibronic coupling in dipole-bound states, as well as electron molecule interactions.