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

Abstract

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 [H.T. Liu et al., Angew. Chem. Int. Ed. 53 (2014) 2464], 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 lowfrequency 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.

Observation of Excited Quadrupole-Bound States in Cold Anions

“…All the major vibrational peaks and their relative intensities agree with each other in the two spectra, except that the photodetachment spectrum has higher resolution. This resemblance provides strong evidence for the weakly bound nature of the excess electron in the QBS, which has little effect to the neutral core, just as that observed for DBSs [20][21][22][23]. Hence, the photodetachment spectrum of the QBS can be used to yield more accurate vibrational information for the neutral radical.…”

“…The latter involves two steps, i.e., resonant excitation to a vibrational level of the QBS, followed by transfer of vibrational energies to the weakly bound electron to induce detachment. The vibrationally induced autodetachment from DBSs generally obeys the Δν ¼ −1 propensity rule under the harmonic-oscillator approximation [45,46] and yields highly non-Franck-Condon PE spectra [20][21][22][23]. This propensity rule is expected to hold true for vibrational autodetachment from the QBS due to the even more weakly bound nature of the QBS electron.…”

“…Excited DBSs of valence-bound anions were first observed as resonances in photodetachment cross sections [16] and studied via rotational and vibrational autodetachment [17][18][19]. Vibrationally induced autodetachment from selected vibrational levels of an excited DBS, yielding highly non-Franck-Condon resonant PE spectra, has also been investigated recently [20][21][22][23].…”

Tautomer‐Specific Resonant Photoelectron Imaging of Deprotonated Cytosine Anions

“…Thea ssignments of all the resonances are based on the calculated vibrational frequencies (Table S2) and the vibrational autodetachment in the resonant photoelectron spectra (Figures 3a-h), which yield tautomer-specific information. We have previously shown that vibrational frequencies in the DBS are the same as those in the neutral states, [33][34][35][42][43][44] that is,t he diffuse dipole-bound electron has little effect on the neutral cores.…”

Channel branching ratios in CH2CN− photodetachment: Rotational structure and vibrational energy redistribution in autodetachment

“…The data represent an action spectrum for spectral feature B, and hence this behavior indicates coupling between ν 3 and other modes (a breakdown of the normal mode approximation) either in the dipole-bound state itself or as the result of inelastic "rescattering" of the autodetaching electron off the neutral core (the latter explanation has been invoked to explain non-mode specific autodetachment in photoexcitation of uracil and 2-hydroxypyrimidine anions). 44,45 Feature B in the direct detachment spectrum includes contributions from the 6 0 1 , 5 1 1 , 5 0 1 , 9 0 2 , and 6 0 2 bands (Table I). The two step excitationautodetachment sequence v = 0 → ν (v = n) → ν (v = m) produces electrons with kinetic energies equivalent to those of ν 0 m direct detachment channels.…”