Observation of a Symmetry-Forbidden Excited Quadrupole-Bound State

Article abstract of DOI:10.1021/jacs.0c10552

We report the observation of a symmetry-forbidden excited quadrupole-bound state (QBS) in the tetracyanobenzene anion (TCNB-) using both photoelectron and photodetachment spectroscopies of cryogenically-cooled anions. The electron affinity of TCNB is accu

Full text of DOI:10.1021/jacs.0c10552

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Observation of a Symmetry-Forbidden Excited Quadrupole-Bound
State
Yuan Liu, Guo-Zhu Zhu, Dao-Fu Yuan, Chen-Hui Qian, Yue-Rou Zhang, Brenda M. Rubenstein,
and Lai-Sheng Wang*
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ABSTRACT: We report the observation of a symmetry-forbidden excited quadru-
pole-bound state (QBS) in the tetracyanobenzene anion (TCNB−) using both
photoelectron and photodetachment spectroscopies of cryogenically-cooled anions.
The electron affinity of TCNB is accurately measured as 2.4695 eV. Photodetachment
spectroscopy of TCNB− reveals selected symmetry-allowed vibronic transitions to the
QBS, but the ground vibrational state was not observed because the transition from
the ground state of TCNB− (Au symmetry) to the QBS (Ag symmetry) is triply
forbidden by the electric and magnetic dipoles and the electric quadrupole. The
binding energy of the QBS is found to be 0.2206 eV, which is unusually large due to
strong correlation and polarization effects. A centrifugal barrier is observed for near-
threshold autodetachment, as well as relaxations from the QBS vibronic levels to the ground and a valence excited state of TCNB−.
The current study shows a rare example where symmetry selection rules, rather than the Franck−Condon principle, govern vibronic
transitions to a nonvalence state in an anion.
strong electron correlation effects.28 Since the first suggestion
INTRODUCTION
■
of the rhombic (BeO)2 as a quadrupole-bound anion,29 the
−
Fermi and Teller first derived a critical value of 1.625 D for a
rhombic alkali halide dimers30,31 and several organic
molecules28 with vanishing dipole moments were proposed
to be targets for quadrupole-bound anions, and the rhombic
point dipole to bind an electron.1 This value was later
rederived by several investigators, as outlined in an interesting
account by Turner.2 For a realistic molecular system to support
a diffuse dipole-bound state (DBS), the critical dipole moment
has been determined empirically to be ∼2.5 D.3−8 Dipole-
bound anions have been formed by Rydberg electron transfer
(RET)3−6,9 for polar neutral molecules that do not form
valence anions. Stable valence-bound anions with polar neutral
cores can support dipole-bound excited states just below the
electron detachment threshold.10−12 DBSs as a special class of
electronically excited states of valence-bound anions have
allowed the observations of vibrational mode-specific autode-
tachment and the realization of resonant photoelectron
spectroscopy (rPES),13−16 which yields much richer spectro-
scopic information than conventional anion PES for relatively
complicated molecular systems. Dipole-bound states in simpler
molecular anions including diatomic species have been
observed recently.17−20 The autodetachment lifetimes from
different vibrational levels of the excited DBS of the phenoxide
anion have been directly measured using a pump−probe
technique in a very recent study.21
(MgO)2 was studied by PES.32 The CS2 anion formed via
RET was initially suggested to be quadrupole-bound.33,34
Quadrupole-bound anions were observed for the formamide
dimer and para-dinitrobenzene35,36 and trans-succinoni-
trile.37,38 Recently, RET in combination with PES has allowed
the binding energies of two quadrupole-bound anions to be
convincingly determined.39,40 Similar to excited DBSs, the
neutral core of a valence-bound anion with a vanishing dipole
moment but a large quadrupole moment can support an
excited QBS below the electron detachment threshold. An
excited QBS has been observed in the 4-cyanophenoxide
anion41 because neutral 4-cyanophenoxy has a very small
dipole moment of 0.3 D but a large quadrupole moment.
However, excited QBS supported by a quadrupolar field
without any residual dipole moment has not been observed.
Such a highly-symmetric QBS may reveal additional
−
−
Received: October 9, 2020
Nonpolar neutral molecules can bind an electron via their
quadrupolar potential (−1/r3) and form quadrupole-bound
anions.22−27 Critical quadrupole moments were studied for a
quadrupole-bound state (QBS),24−27 but ab initio calculations
indicated that a critical quadrupole moment does not exist
because of the complexity of the molecular interactions and
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Figure 1. Photodetachment spectrum of TCNB− obtained by monitoring the electron yield as a function of photon energy. Resonant peaks are
labeled as 1−19 corresponding to vibrational levels of the QBS. Note the 0−0 transition from the anion ground state (Au) to the QBS (Ag) is E1,
M1, and E2 triply forbidden. It is deduced to be at an excitation energy of 18 139 cm−1 indicated by the dashed arrow and labeled as 0 (see text).
The detachment threshold at 19 918 cm−1 is indicated by a short arrow. The molecular structure of TCNB (D2h) and the associated coordinates are
given in the inset. The HOMO of TCNB− and the QBS (plotted with an isovalue of 0.0004) are also given in the inset. Note the σ-like symmetry
and the diffuseness of the QBS orbital.
information about the role that symmetry plays in these diffuse
electronic states near the detachment threshold.
(Figure 1b), and they can be assigned (Table S2) using the
calculated harmonic frequencies for TCNB (Table S3). A
Franck−Condon (FC) simulation for the ground state
transition is compared with the spectrum in Figure S1a. The
structural changes between the anion and neutral ground states
are shown in Figure S1c.
In this article, we report the observation of such an excited
QBS in the 1,2,4,5-tetracyanobenzene anion (TCNB−)
cryogenically cooled to its vibrational ground state. The
neutral TCNB molecule containing four polar −CN groups42
is a good electron acceptor and can form a stable valence-
bound anion.43,44 Owing to its high symmetry (D2h, see inset
of Figure 1), TCNB has no dipole or octupole moments but a
large quadrupole moment with only nonzero diagonal terms
(Qxx = 19.3, Qyy = −29.5, Qzz = 10.2 D·Å, see Table S1 for
details), making the TCNB− anion an ideal system to search
for an excited QBS. Theoretical calculations have indeed
suggested such a weakly-bound QBS with Ag symmetry below
the detachment threshold (see the inset of Figure 1 for the
QBS orbital and Supporting Information for details of the
calculations). Furthermore, unlike other highly symmetric
systems such as C60 whose anion is distorted due to the Jahn−
Teller effect,45 the TCNB− anion has the same D2h symmetry
as neutral TCNB, making symmetry selection rules of vibronic
transitions strictly obeyed.
Photodetachment Spectroscopy. To search for the QBS
of TCNB−, we measured the photodetachment spectrum by
monitoring the total electron yield as a function of photon
energy across the detachment threshold (Figure 1). The arrow
at 19 918 cm−1 indicates the detachment threshold of TCNB−,
as mentioned above from peak 000 in the PE spectrum (Figure
S1b). Eleven below-threshold vibrational peaks (labeled 1−11)
and eight above-threshold peaks, i.e., vibrational Feshbach
resonances (labeled 12−19) are identified (see Table S4 for
their wavelengths and excitation energies). The rising baseline
above the detachment threshold is due to nonresonant one-
photon detachment processes, while the resonant peaks should
correspond to the vibrational levels of the expected QBS. Since
no more peaks were observed below peak 1, it was tempting to
assume that peak 1 should represent the ground vibrational
level of the QBS. All other peaks should then be due to higher
vibrational levels of the QBS, and they should be assigned by
comparing their relative shifts to peak 1 with the theoretical
frequencies of the neutral TCNB (Table S3). Under this
assumption, however, none of the observed peaks could be
assigned to any vibrational levels of TCNB, suggesting that
peak 1 might not represent the ground vibrational level of the
QBS; i.e., the true vibrational ground state of the QBS was not
observed in the photodetachment spectrum of Figure 1.
It should be noted that the QBS has Ag symmetry and the
TCNB− anion has Au symmetry (see the inset of Figure 1 and
Table S5). A symmetry analysis reveals that the transition from
the Au to Ag state is triply forbidden by the electric dipole (E1),
magnetic dipole (M1), and electric quadrupole (E2) and is
only allowed by the electric octupole (E3). Given such a high-
order forbidden transition, we would not expect that the 0−0
RESULTS AND DISCUSSION
■
Photoelectron Spectroscopy. The experiment was
performed using an electrospray-PES apparatus46 equipped
with a cryogenically cooled Paul trap47 and a high-resolution
photoelectron (PE) imaging system48 (see Supporting
Information for more details). We first measured the
nonresonant PE spectra of TCNB− at two photon energies,
0
as shown in Figure S1. The peak 00 labeled in Figure S1b
corresponds to the 0−0 detachment transition from the
vibrational ground state of TCNB− to that of neutral TCNB.
The binding energy of peak 000 is 2.4695 0.0025 eV (19 918
20 cm−1) representing the adiabatic electron affinity (EA) of
TCNB, which was not accurately known before.43,44
Numerous well-resolved vibrational peaks (labeled as A−D
and a−e) are observed in the higher resolution spectrum
B
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Figure 2. R2PD PE spectra of TCNB− for selected below-threshold QBS vibrational levels (a−f) and resonant PE images and spectra for selected
above-threshold QBS vibrational levels (g−j). The peak numbers and excitation wavelengths are labeled. The energy shift relative to the 0−0
transition corresponding to each below-threshold vibrational peak is indicated in the parentheses in the left column. In the right column, peaks
labeled in boldface with capital letters indicate autodetachment-enhanced vibrational levels in the final neutral TCNB. The assignments of the
above-threshold QBS vibrational levels and their symmetries (in parentheses) are also given. The double arrow below the PE image represents the
laser polarization direction.
transition from TCNB− to the QBS to be observable.
However, selected higher vibrational levels of the QBS with
suitable vibronic symmetries may be allowed (vide infra),
corresponding to the observed peaks (1−19). We should note
that similar vibronic selection rules are well known in Rydberg
spectroscopy and vibronic transitions of neutral polyatomic
molecules.49−51
deduce a very precise excitation energy to the ground
vibrational level of the QBS (18 139 8 cm−1) or a binding
energy of the QBS as 1779 8 cm−1 (0.2206 0.0010 eV)
below the detachment threshold.
Assignments of the QBS Vibrational Levels and Their
Symmetry Requirements. The deduced ground vibrational
level of the QBS is indicated by the dashed arrow and the
putative label 0 in Figure 1. With the determination of the
vibrational ground state of the QBS, all the observed 19
vibrational peaks in the photodetachment spectrum in Figure 1
should be readily assigned using the computed frequencies of
TCNB (Table S3) and the proper vibronic symmetries.
Symmetry analyses for the excited vibrational levels of the QBS
suggest that only transitions to vibrational (vibronic) states of
the QBS with B1g, B2g, or B3g symmetries are dipole-allowed
(see the Supporting Information). Therefore, all the vibrational
peaks observed in Figure 1 should correspond to vibrational
levels with B1g, B2g, or B3g symmetries. For example, peak 1 is
456 cm−1 above the QBS ground state, which is in excellent
agreement with the calculated frequency of mode v13′ (456
cm−1, Table S3) of the QBS with B2g symmetry. Peaks 2 and 3
are assigned to the v18′ (B1g) and v20′ (B3g) modes, respectively,
again in excellent agreement with the computed frequencies
and the expected vibronic symmetries (Tables S3 and S4).
Similarly, other peaks can be assigned to vibrational levels of
the QBS with B1g, B2g, or B3g symmetries, as summarized in
Table S4, which shows that all the higher energy peaks (4−19)
correspond to combinational vibrational levels. In most cases,
more than one assignment was possible due to the large
number of low frequency modes and the increased vibrational
density of states. It should be pointed out that the symmetry
constraint has greatly simplified the photodetachment
spectrum of TCNB− in Figure 1. For example, we observed
46 DBS vibrational peaks in the photodetachment spectrum of
Resonant Two-Photon Photoelectron Spectroscopy.
The below-threshold vibronic levels (1−11) were observed
due to resonant two-photon detachment (R2PD). By tuning
the laser wavelength to these bound vibronic levels, R2PD PE
spectra could be obtained, as shown in Figure 2 for six cases
(left column) and Figure S2 for all 11 vibronic levels. The
binding energies of the spectra were obtained by subtracting
the electron kinetic energy from the photon energies used. The
spectra in Figure 2a−c display a low binding energy peak at
0.22 eV, which should represent the energy difference between
the detachment threshold and the intermediate QBS vibronic
levels. However, the binding energy of this peak is constant,
regardless of the vibronic levels. This observation suggests an
adiabatic detachment process by the second photon, where the
vibrational states in the QBS and the final neutral are not
changed due to their similar geometries. In other words, if the
second photon detaches the electron from the v = n vibrational
level of the QBS, the final neutral state would also be in the v =
n vibrational level. Similar adiabatic detachment processes have
been reported recently for R2PD processes via DBS.52,53 This
is an important observation, suggesting that the 0.22 eV
binding energy should correspond to that of the ground
vibrational level of the QBS. The 0.22 eV binding energy of the
QBS agrees well with the computed value of 0.16 eV (Table
S5). As will be shown below, peak 1 in Figure 1 is due to the
ν13 mode, which is observed in the PE spectrum as the most
intense vibrational peak (peak C in Figure S1b). The frequency
of the ν13 mode is measured more accurately as 456 cm−1
(Table S2) from the near-threshold resonant PE spectrum
presented in Figure 2j (vide infra). This result allows us to
−
deprotonated uracil anion (C4H3N2O2 ) with 11 atoms in an
energy range only 1800 cm−1 above the DBS ground state.54
The TCNB− (C10H2N4 ) anion with 16 atoms should have a
−
C
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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
Supporting Information and Figure S4, the centrifugal
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
Figure S3e−h. The vibrational autodetachment is a two-step
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
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observed QBS vibrational levels can be found in Figure S5. By
combining the photodetachment spectroscopy and the
resonant and nonresonant PE spectra, we obtained the
fundamental vibrational frequencies for nine vibrational
modes of TCNB, as summarized in Table S6.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c10552.
Experimental and computational details, group theoreti-
Evidence of Electron Correlation and Polarization
Effects in the QBS. The observed binding energy (0.2206
eV) of the QBS in TCNB− is quite large in comparison to that
of the recently reported quadrupole-bound anions,39,40
suggesting the importance of electron correlation and polar-
ization in the electron binding in the QBS. The term QBS is
largely from consideration of the one-particle model potential
using multiple expansion of the electrostatic interactions. It has
been recognized that electron correlation plays a central role in
the electron binding in all noncovalence states.8,23,28,38,59−61 In
the case of QBS, such multielectron effects are even more
critical.28,38 For TCNB, the large polarizability of the aromatic
π system is also expected to be important for the electron
binding. Thus, TCNB provides an interesting system to
investigate the different effects to the binding of the
nonvalence state, in addition to the symmetry selection rules.
We computed the binding energy of the QBS at the Hartree−
Fock level using the aug-cc-pvdz+4s3p2d basis set and
obtained a value of 0.007 eV, indicating the importance of
the quadrupolar interaction. We expect that a basis set with
more diffuse functions should yield an even higher binding
energy for the QBS. We should point out that a nonvalence
state was predicted for hexafluorobenzene (C6F6) with a large
binding energy and it was viewed as a correlation-bound
state.62 However, the magnitude of the quadrupole moment of
C6F6 (Qxx = −3.2, Qyy = −3.2, Qzz = 6.4) is significantly smaller
than that of TCNB− (Table S1). Hence, we think the
nonvalence state in TCNB− should be viewed as a QBS with
strong correlation and polarization effects. It should be noted
that recently a similar anion, tetracyanoethylene (TCNE−), has
been studied theoretically and predicted to have a nonvalence
excited state with a binding energy of 0.07 eV.61 The smaller
binding energy for the QBS in TCNE− is consistent with its
smaller size and lower polarizability in comparison to that of
TCNB.
cal analysis, and additional tables and figures (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Lai-Sheng Wang − Department of Chemistry, Brown
University, Providence, Rhode Island 02912, United States;
orcid.org/0000-0003-1816-5738; Email: Lai-
Sheng_Wang@brown.edu
Authors
Yuan Liu − Department of Chemistry, Brown University,
Providence, Rhode Island 02912, United States
Guo-Zhu Zhu − Department of Chemistry, Brown University,
Providence, Rhode Island 02912, United States
Dao-Fu Yuan − Department of Chemistry, Brown University,
Providence, Rhode Island 02912, United States;
orcid.org/0000-0001-8461-6889
Chen-Hui Qian − Department of Chemistry, Brown
University, Providence, Rhode Island 02912, United States
Yue-Rou Zhang − Department of Chemistry, Brown
University, Providence, Rhode Island 02912, United States
Brenda M. Rubenstein − Department of Chemistry, Brown
University, Providence, Rhode Island 02912, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c10552
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. Department of Energy,
Office of Basic Energy Sciences, Chemical Sciences, Geo-
sciences, and Biosciences Division under Grant
DESC0018679. We thank Professors Richard Stratt and
Peter Weber for stimulating discussions. Y.L. was supported
by the Brown Presidential Fellowship. The calculation was
done using computational resources and services provided by
the Center for Computation and Visualization at Brown
University.
CONCLUSIONS
■
We report the observation of an excited quadrupole-bound
state in the negative ion of TCNB, which forms a stable
valence anion with an electron binding energy of 2.4695 eV.
The QBS is 0.2206 eV below the detachment threshold of
TCNB−, but the transition from the ground vibrational state of
TCNB− to that of the QBS is E1, M1, and E2 triply forbidden
due to the high symmetry of the system (D2h). Only transitions
to selected vibrational levels of the QBS with B1g, B2g, or B3g
symmetries are dipole-allowed, yielding a relatively simply
photodetachment spectrum. A centrifugal barrier is found to
suppress autodetachment for the near-threshold vibrational
Feshbach resonance. Both internal conversion and relaxation
to a low-lying valence excited state were observed from the
QBS. The current work reports a rare case where symmetry-
selection rules are observed to be critical for vibronic
transitions from a valence to a nonvalence state in a molecular
anion.
REFERENCES
■
(1) Fermi, E.; Teller, E. The capture of negative mesotrons in
matter. Phys. Rev. 1947, 72, 399−408.
(2) Turner, J. E. Minimum dipole moment required to bind an
electronmolecular theorists rediscover phenomenon mentioned in
Fermi-Teller paper twenty years earlier. Am. J. Phys. 1977, 45, 758−
766.
(3) Desfrancois, C.; Abdoul-Carime, H.; Schermann, J. P. Ground-
state dipole-bound anions. Int. J. Mod. Phys. B 1996, 10, 1339.
(4) Desfrancois, C.; Abdoulcarime, H.; Khelifa, N.; Schermann, J. P.
From 1/r to 1/r2 potentials: Electron exchange between Rydberg
atoms and polar molecules. Phys. Rev. Lett. 1994, 73, 2436−2439.
(5) Hammer, N. I.; Diri, K.; Jordan, K. D.; Desfrancois, C.;
Compton, R. N. Dipole-bound anions of carbonyl, nitrile, and
sulfoxide containing molecules. J. Chem. Phys. 2003, 119, 3650−3660.
(6) Hammer, N. I.; Hinde, R. J.; Compton, R. N.; Diri, K.; Jordan, K.
D.; Radisic, D.; Stokes, S. T.; Bowen, K. H. Dipole-bound anions of
E
https://dx.doi.org/10.1021/jacs.0c10552
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
highly polar molecules: Ethylene carbonate and vinylene carbonate. J.
Chem. Phys. 2004, 120, 685−690.
(28) Sommerfeld, T.; Dreux, K. M.; Joshi, R. Excess electrons bound
to molecular systems with a vanishing dipole but large molecular
quadrupole. J. Phys. Chem. A 2014, 118, 7320−7329.
(7) Jordan, K. D.; Wang, F. Theory of dipole-bound anions. Annu.
Rev. Phys. Chem. 2003, 54, 367−396.
(29) Jordan, K. D.; Liebman, J. F. Binding of an electron to a
molecular quadrupole: (BeO)2. Chem. Phys. Lett. 1979, 62, 143−147.
(30) Gutsev, G. L.; Jena, P.; Bartlett, R. J. Search for “quadrupole-
bound” anions. I. J. Chem. Phys. 1999, 111, 504−511.
(8) Qian, C. H.; Zhu, G. Z.; Wang, L. S. Probing the critical dipole
moment to support excited dipole-bound states in valence-bound
anions. J. Phys. Chem. Lett. 2019, 10, 6472−6477.
(31) Anusiewicz, I.; Skurski, P.; Simons, J. First evidence of rhombic
(9) Stockdale, J. A.; Davis, F. J.; Compton, R. N.; Klots, C. E.
Production of negative ions from CH3X molecules (CH3NO2,
CH3CN, CH3I, CH3Br) by electron impact and by collisions with
atoms in excited Rydberg states. J. Chem. Phys. 1974, 60, 4279−4285.
(10) Zimmerman, A. H.; Brauman, J. I. Resonances in electron
photodetachment cross sections. J. Chem. Phys. 1977, 66, 5823−5825.
(11) Lykke, K. R.; Mead, R. D.; Lineberger, W. C. Observation of
dipole-bound states of negative ions. Phys. Rev. Lett. 1984, 52, 2221−
2224.
−
(NaCl)2 . Ab initio reexamination of the sodium chloride dimer
anion. J. Phys. Chem. A 2002, 106, 10636−10644.
(32) Gutowski, M.; Skurski, P.; Li, X.; Wang, L. S. (MgO)n− (n = 1-
5) clusters: Multipole-bound anions and photodetachment spectros-
copy. Phys. Rev. Lett. 2000, 85, 3145−3148.
(33) Desfrancois, C.; Khelifa, N.; Schermann, J. P.; Kraft, T.; Ruf, M.
W.; Hotop, H. Energy exchanges following very low energy electron
attachment to neat CS2 and CS2-containing clusters. Z. Phys. D: At.,
Mol. Clusters 1993, 27, 365−369.
(34) Compton, R. N.; Dunning, F. B.; Nordlander, P. On the
binding of electrons to CS2: Possible role of quadrupole-bound states.
Chem. Phys. Lett. 1996, 253, 8−12.
(35) Desfrancois, C.; Periquet, V.; Carles, S.; Schermann, J. P.;
Adamowicz, L. Neutral and negatively-charged formamide, N-
methylformamide and dimethylformamide clusters. Chem. Phys.
1998, 239, 475−483.
(36) Desfrancois, C.; Periquet, V.; Lyapustina, S. A.; Lippa, T. P.;
Robinson, D. W.; Bowen, K. H.; Nonaka, H.; Compton, R. N.
Electron binding to valence and multipole states of molecules:
Nitrobenzene, para- and meta-dinitrobenzenes. J. Chem. Phys. 1999,
111, 4569−4576.
(12) Mead, R. D.; Lykke, K. R.; Lineberger, W. C.; Marks, J.;
Brauman, J. I. Spectroscopy and dynamics of the dipole-bound state
of acetaldehyde enolate. J. Chem. Phys. 1984, 81, 4883−4892.
(13) Liu, H. T.; Ning, C. G.; Huang, D. L.; Dau, P. D.; Wang, L. S.
Observation of mode-specific vibrational autodetachment from
dipole-bound states of cold anions. Angew. Chem., Int. Ed. 2013, 52,
8976−8979.
(14) Huang, D. L.; Liu, H. T.; Ning, C. G.; Wang, L. S. Vibrational
state-selective autodetachment photoelectron spectroscopy from
dipole-bound states of cold 2-hydroxyphenoxide: o-HO(C6H4)O−. J.
Chem. Phys. 2015, 142, 124309.
(15) Zhu, G. Z.; Huang, D. L.; Wang, L. S. Conformation-selective
resonant photoelectron imaging from dipole-bound states of cold 3-
hydroxyphenoxide. J. Chem. Phys. 2017, 147, 013910.
(16) Zhu, G. Z.; Wang, L. S. High-resolution photoelectron imaging
and resonant photoelectron spectroscopy via noncovalently bound
excited states of cryogenically cooled anions. Chem. Sci. 2019, 10,
9409−9423.
(17) Dao, D. B.; Mabbs, R. The effect of the dipole bound state on
AgF− vibrationally resolved photodetachment cross sections and
photoelectron angular distributions. J. Chem. Phys. 2014, 141, 154304.
(18) Lyle, J.; Wedig, O.; Gulania, S.; Krylov, A. I.; Mabbs, R.
Channel branching ratios in CH2CN− photodetachment: Rotational
structure and vibrational energy redistribution in autodetachment. J.
Chem. Phys. 2017, 147, 234309.
(19) Mascaritolo, K. J.; Dermer, A. M.; Green, M. L.; Gardner, A.
M.; Heaven, M. C. Photodetachment spectroscopy of the beryllium
oxide anion, BeO−. J. Chem. Phys. 2017, 146, 054301.
(20) Castellani, M. E.; Anstoter, C. S.; Verlet, J. R. R. On the
stability of a dipole-bound state in the presence of a molecule. Phys.
Chem. Chem. Phys. 2019, 21, 24286−24290.
(21) Kang, D. H.; An, S.; Kim, S. K. Real-time autodetachment
dynamics of vibrational Feshbach resonances in a dipole-bound state.
Phys. Rev. Lett. 2020, 125, 093001.
(22) Compton, R. N.; Hammer, N. I. In Advances in Gas-Phase Ion
Chemistry; Adams, N., Babcock, I., Eds.; Elsevier Science: New York,
2001; pp 257−305.
(23) Simons, J. Molecular anions. J. Phys. Chem. A 2008, 112, 6401−
6511.
(37) Desfrancois, C.; Bouteiller, Y.; Schermann, J. P.; Radisic, D.;
Stokes, S. T.; Bowen, K. H.; Hammer, N. I.; Compton, R. N. Long-
range electron binding to quadrupolar molecules. Phys. Rev. Lett.
2004, 92, 083003.
(38) Sommerfeld, T. Multipole-bound states of succinonitrile and
other dicarbonitriles. J. Chem. Phys. 2004, 121, 4097−4104.
(39) Liu, G.; Ciborowski, S. M.; Graham, J. D.; Buytendyk, A. M.;
Bowen, K. H. The ground state, quadrupole-bound anion of
succinonitrile revisited. J. Chem. Phys. 2019, 151, 101101.
(40) Liu, G.; Ciborowski, S. M.; Pitts, C. R.; Graham, J. D.;
Buytendyk, A. M.; Lectka, T.; Bowen, K. H. Observation of the
dipole-and quadrupole-bound anions of 1, 4-dicyanocyclohexane.
Phys. Chem. Chem. Phys. 2019, 21, 18310−18315.
(41) Zhu, G. Z.; Liu, Y.; Wang, L. S. Observation of excited
quadrupole-bound states in cold anions. Phys. Rev. Lett. 2017, 119,
023002.
(42) Campanelli, A. R.; Domenicano, A.; Ramondo, F.; Hargittai, I.
Molecular structure and benzene ring deformation of three
cyanobenzenes from gas-phase electron diffraction and quantum
chemical calculations. J. Phys. Chem. A 2008, 112, 10998−11088.
(43) Farragher, A. L.; Page, F. M. Experimental determination of
electron affinities. Part 11.Electron capture by some cyanocarbons
and related compounds. Trans. Faraday Soc. 1967, 63, 2369−2378.
(44) Nakashima, H.; Honda, Y.; Shida, T.; Nakatsuji, H. Electronic
excitation spectra of doublet anion radicals of cyanobenzene and
nitrobenzene derivatives: SAC-CI theoretical studies. Mol. Phys. 2015,
113, 1728−1739.
(24) Prasad, M. V. N. A.; Wallis, R. F.; Herman, R. Theory of the
binding energy of an electron in the field of a linear electric
quadrupole. Phys. Rev. B: Condens. Matter Mater. Phys. 1989, 40,
5924−5928.
(45) Huang, D. L.; Dau, P. D.; Liu, H. T.; Wang, L. S. High-
−
resolution photoelectron imaging of cold C60 anions and accurate
determination of the electron affinity of C60. J. Chem. Phys. 2014, 140,
224315.
(25) Abdoul-Carime, H.; Desfrancois, C. Electrons weakly bound to
molecules by dipolar, quadrupolar or polarization forces. Eur. Phys. J.
D 1998, 2, 149−156.
(46) Wang, L. S. Perspective: Electrospray photoelectron spectros-
copy: From multiply-charged anions to ultracold anions. J. Chem.
Phys. 2015, 143, 040901.
(47) Wang, X. B.; Wang, L. S. Development of a low-temperature
photoelectron spectroscopy instrument using an electrospray ion
source and a cryogenically controlled ion trap. Rev. Sci. Instrum. 2008,
79, 073108.
(26) Ferron, A.; Serra, P.; Kais, S. Finite-size scaling for critical
conditions for stable quadrupole-bound anions. J. Chem. Phys. 2004,
120, 8412−8419.
(27) Garrett, W. R. Quadrupole-bound anions: Efficacy of positive
versus negative quadrupole moments. J. Chem. Phys. 2012, 136,
054116.
(48) Leon, I.; Yang, Z.; Liu, H. T.; Wang, L. S. The design and
construction of a high-resolution velocity-map imaging apparatus for
F
https://dx.doi.org/10.1021/jacs.0c10552
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
photoelectron spectroscopy studies of size-selected clusters. Rev. Sci.
Instrum. 2014, 85, 083106.
(49) Vitenberg, R.; Katz, B.; Scharf, B. The assignment of the
benzene first-ionization Rydberg spectrum via J-T splittings involving
linearly inactive modes. Chem. Phys. Lett. 1980, 71, 187−191.
(50) Signorell, R.; Merkt, F. General symmetry selection rules for the
photoionization of polyatomic molecules. Mol. Phys. 1997, 92, 793−
804.
(51) Philis, J. G.; Mondal, T.; Mahapatra, S. Vibronic structure in
the low lying Rydberg states of hexafluoro-benzene and 1,3,5-
trifluorobenzene detected by two-photon spectroscopy. Chem. Phys.
Lett. 2010, 495, 187−191.
(52) Zhu, G. Z.; Cheung, L. F.; Liu, Y.; Qian, C. H.; Wang, L. S.
Resonant two-photon photoelectron imaging and intersystem crossing
from excited dipole-bound states of cold anions. J. Phys. Chem. Lett.
2019, 10, 4339−4344.
(53) Yuan, D. F.; Liu, Y.; Qian, C. H.; Kocheril, G. S.; Zhang, Y. R.;
Rubenstein, B. M.; Wang, L. S. Polarization of valence orbitals by the
intramolecular electric field from a diffuse dipole-bound electron. J.
Phys. Chem. Lett. 2020, 11, 7914−7919.
(54) Liu, H. T.; Ning, C. G.; Huang, D. L.; Wang, L. S. Vibrational
spectroscopy of the dehydrogenated uracil radical by autodetachment
of dipole-bound excited states of cold anions. Angew. Chem., Int. Ed.
2014, 53, 2464−2468.
(55) Stannard, P. R.; Gelbart, W. M. Intramolecular vibrational
energy redistribution. J. Phys. Chem. 1981, 85, 3592−3599.
(56) Yuan, D. F.; Liu, Y.; Qian, C. H.; Zhang, Y. R.; Rubenstein, B.
M.; Wang, L. S. Observation of a π-type dipole-bound state in
molecular anions. Phys. Rev. Lett. 2020, 125, 073003.
(57) Berry, R. S. Ionization of molecules at low energies. J. Chem.
Phys. 1966, 45, 1228−1245.
(58) Simons, J. Propensity rules for vibration-induced electron
detachment of anions. J. Am. Chem. Soc. 1981, 103, 3971−3976.
(59) Gutowski, M.; Jordan, K. D.; Skurski, P. Electronic structure of
dipole-bound anions. J. Phys. Chem. A 1998, 102, 2624−2633.
(60) Voora, V. K.; Cederbaum, L. S.; Jordan, K. D. Existence of a
correlation bound s-type anion state of C60. J. Phys. Chem. Lett. 2013,
4, 849−853.
(61) Voora, V. K.; Kairalapova, A.; Sommerfeld, T.; Jordan, K. D.
Theoretical approaches for treating non-valence correlation-bound
anions. J. Chem. Phys. 2017, 147, 214114.
(62) Voora, V. K.; Jordan, K. D. Nonvalence correlation-bound state
of C6F6: doorways to low-energy electron cpature. J. Phys. Chem. A
2014, 118, 7201−7205.
G
https://dx.doi.org/10.1021/jacs.0c10552
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX