Journal of the American Chemical Society
Article
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much higher vibrational density of states than C4H3N2O2 , but
within 1800 cm−1 above the ground state of the QBS, only 11
vibrational peaks were observed.
accurately measure the vibrational frequencies of the enhanced
modes (see Table S2). Note that the highly accurate ν13
frequency is used to deduce the precise binding energy of the
QBS as discussed above, because peak 1 in the photodetach-
ment spectrum (Figure 1) is due to the same ν13 mode in the
QBS. A broad PE feature is also observed in Figure 2i (and
Figure 2j to a lesser extent), similar to that in Figure 2e and
Figure 2f, suggesting that IC-IVR is competing with
autodetachment in the above-threshold vibrational levels of
the QBS. An extreme case is observed in Figure 2g, where no
vibrational autodetachment was observed and IC-IVR seemed
to be completely dominating. Since the excited QBS
vibrational level (peak 12) is just above the detachment
threshold only by 138 cm−1 (Figure 1), this observation
suggests the existence of a potential barrier, which suppresses
the near-threshold autodetachment. As described in the
interaction [l(l + 1)/r2] and the attractive quadrupolar
potential (−1/r3) can indeed produce a potential barrier for
the outgoing electron with a nonzero angular momentum (l >
0).
Intramolecular Vibrational Energy Redistribution
and Radiative Processes from the QBS. The 0.22 eV
R2PD PE peak is much weaker in Figure 2d−f. In all the R2PD
spectra (Figure S2), broad and continuous signals were
observed in the high binding energy side, implying other
relaxation processes following the first photon absorption. Two
types of signals were observed. The feature in Figure 2e and
Figure 2f rises with increasing binding energies (lowering
kinetic energies) and is peaked at the highest binding energy.
This behavior suggests that the second photon detaches an
electron from the vibrational manifold of TCNB− in its ground
electronic state, due to internal conversion (IC) from the QBS
and subsequent intramolecular vibrational energy redistrib-
utions (IVRs).55,56 In Figure 2a−d, the broad feature has an
onset around 1.55 eV, indicating electron detachment from an
electronically excited state of TCNB−.52 Interestingly, the
observed onset binding energy is in good agreement with the
theoretical prediction of a valence excited state (VES) at a
binding energy of 1.55 eV with B1u symmetry (Table S5). The
sharp onset of this feature suggests that the population of the
VES should come from a radiative process from the QBS,
following the first photon absorption.52 It appears that the IC-
IVR and the radiative process are competing and depend on
the specific vibrational levels of the QBS. In Figure 2e and
Figure 2f, the IC-IVR dominates, whereas in Figure 2a−d the
radiative process is more favored. Figure S2 shows that most of
the below-threshold vibrational levels of the QBS favor the
radiative process.
Figure 3 displays a schematic energy level diagram showing
autodetachment from three excited QBS vibrational levels to
vibrational levels of the neutral TCNB (corresponding to
Figure 2h−j) and the suppression of autodetachment by a
centrifugal barrier (Figure 2g). Different relaxation mecha-
nisms from the QBS to the anion ground state and a VES are
also indicated. A complete energy level diagram showing all the
Resonant Photoelectron Spectroscopy and Vibra-
tional Autodetachment from the QBS. By tuning the
detachment laser to the vibrational Feshbach resonances (12−
19), eight resonantly-enhanced PE images and spectra were
obtained (Figure S3), four of which are presented in Figure
2g−j. As discussed previously,13−16,41 two processes contribute
to the rPES signals: (1) the nonresonant direct detachment
represented by the above-threshold baseline in Figure 1 and
(2) the much stronger vibrational autodetachment that gave
rise to the resonant peaks (12−19). Figure S3 indicates that
the nonresonant PE signals were very weak, only observable in
process. First, the TCNB− anion is excited to an above-
threshold vibrational level of the QBS. Then, vibronic coupling
transfers the vibrational energy to the quadrupole-bound
electron to detach it into the continuum. The vibrational
autodetachment enhances the intensities of certain vibrational
peaks, resulting in highly non-Franck−Condon PE spectra. If
there is little geometry change from the QBS to neutral TCNB,
the autodetachment is expected to obey the Δv = −1
propensity rule under the harmonic approximation,57,58 as
has been shown for both DBS and QBS previously.13−16,41
Violations of the Δv = −1 propensity rule have been observed
due to anharmonic effects. Note that the QBS vibrational levels
must also have the proper symmetries (B1g, B2g, or B3g). For
example, Figure 2j is assigned as an excitation to the 13′126′2
vibrational levelof the QBS (B2g symmetry), followed by the
transfer of the two quanta vibrational energies in 26′2 to the
quadrupole-bound electron to result in the enhancement of the
131 final state in the PE spectra. Figure 2h and Figure 2i and
the rest of the resonant PE spectra in Figure S3 can be
understood similarly. The resonant PE spectra allow us to
Figure 3. A schematic energy level diagram showing the autodetach-
ment from the QBS vibrational levels of TCNB− to the neutral
vibrational manifold of TCNB (left). The possible radiative decay to
the VES and relaxation to the vibrational manifold of the anion
ground state via IVR (right), as well as the centrifugal barrier in the
near above-threshold region (middle), are also shown. See Figure S5
for a more complete diagram.
D
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX