The ground state, quadrupole-bound anion of succinonitrile revisited

On the performance of second-order approximate coupled-cluster singles and doubles methods for non-valence anions

We investigate the capability of several variants of the second-order approximate coupled-cluster singles and doubles (CC2) method to describe dipole-bound, quadrupole-bound, and correlation-bound molecular anions. The binding energy of anions formed by electron attachment to closed-shell molecules is computed using the electron attachment variant of CC2 (EA-CC2), whereas anions with a closed-shell ground state are treated with the standard CC2 method that preserves the number of particles. We find that EA-CC2 captures the binding energies of dipole-bound radical anions quite well, whereas results for other types of non-valence anions are less reliable. We also test the performance of semi-empirical spin-scaling factors for all types of non-valence anions and observe that the spin-scaled CC2 variants generally do not provide more accurate binding energies for dipole-bound anions, while the binding energies of quadrupole-bound and correlation-bound anions are improved. As exemplary applications of EA-CC2, we investigate the dipole-bound anions of the steroids cortisol, progesterone, and testosterone. In addition, we characterize electron attachment to sym-tetracyanonaphthalene, a molecule that supports five anionic states, two of which can be interpreted as hitherto unobserved π-type quadrupole-bound states.

How Accurately Can DFT Describe Non-valence Anions?

Weakly bound non-valence anions are molecular systems where the excess electron stabilizes in a very diffuse orbital whose size, shape, and binding energy (∼1-100 meV) are governed by the long-range electrostatic potential of the molecule. Its binding energy comes mainly from charge-dipole or charge-multipole interactions or dispersion forces. While highly correlated methods, like coupled cluster methods, are considered to be the state of the art for describing anionic systems, especially when the electron lies in a very diffuse orbital, we consider here the possibility to use DFT-based calculations. In such molecular anions, the outer electron experiences long-range exchange and correlation interactions. We show that DFT can describe long-range bound states provided that a correct asymptotic exchange and correlation potential is used, namely, that from a range-separated hybrid functional. This opens an alternative to the computationally demanding highly correlated method calculations. It is also suggested that the study of weakly bound anions could help in the construction of new DFT potentials to study systems where nonlocal effects are significant.

Observation of the ponderomotive effect in non-valence bound states of polyatomic molecular anions

The ponderomotive force on molecular systems has rarely been observed hitherto, despite potentially being extremely useful for the manipulation of the molecular properties. Here, the ponderomotive effect in the non-valence bound states has been experimentally demonstrated, for the first time to the best of our knowledge, giving great promise for the manipulation of polyatomic molecules by the dynamic Stark effect. Entire quantum levels of the dipole-bound state (DBS) and quadrupole-bound state (QBS) of the phenoxide (or 4-bromophenoxide) and 4-cyanophenoxide anions, respectively, show clear-cut ponderomotive blue-shifts in the presence of the spatiotemporally overlapped non-resonant picosecond control laser pulse. The quasi-free electron in the QBS is found to be more vulnerable to the external oscillating electromagnetic field compared to that in the DBS, suggesting that the non-valence orbital of the former is more diffusive and thus more polarizable compared to that of the latter.

Observation of a dipole-bound excited state in 4-ethynylphenoxide and comparison with the quadrupole-bound excited state in the isoelectronic 4-cyanophenoxide

Negative ions do not possess Rydberg states but can have Rydberg-like nonvalence excited states near the electron detachment threshold, including dipole-bound states (DBSs) and quadrupole-bound states (QBSs). While DBSs have been studied extensively, quadrupole-bound excited states have been more rarely observed. 4-cyanophenoxide (4CP^-) was the first anion observed to possess a quadrupole-bound exited state 20 cm^-1 below its detachment threshold. Here, we report the observation of a DBS in the isoelectronic 4-ethynylphenoxide anion (4EP^-), providing a rare opportunity to compare the behaviors of a dipole-bound and a quadrupole-bound excited state in a pair of very similar anions. Photodetachment spectroscopy (PDS) of cryogenically cooled 4EP^- reveals a DBS 76 cm^-1 below its detachment threshold. Photoelectron spectroscopy (PES) at 266 nm shows that the electronic structure of 4EP^- and 4CP^- is nearly identical. The observed vibrational features in both the PDS and PES, as well as autodetachment from the nonvalence excited states, are also found to be similar for both anions. However, resonant two-photon detachment (R2PD) from the bound vibrational ground state is observed to be very different for the DBS in 4EP^- and the QBS in 4CP^-. The R2PD spectra reveal that decays take place from both the DBS and QBS to the respective anion ground electronic states within the 5 ns detachment laser pulse due to internal conversion followed by intramolecular vibrational redistribution and relaxation, but the decay mechanisms appear to be very different. In the R2PD spectrum of 4EP^-, we observe strong threshold electron signals, which are due to detachment, by the second photon, of highly rotationally excited anions resulted from the decay of the DBS. On the other hand, in the R2PD spectrum of 4CP^-, we observe well-resolved vibrational peaks due to the three lowest-frequency vibrational modes of 4CP^-, which are populated from the decay of the QBS. The different behaviors of the R2PD spectra suggest unexpected differences between the relaxation mechanisms of the dipole-bound and quadrupole-bound excited states.

Fragmentation of Nickel(II) and Cobalt(II) Bis(acetylacetonate) Complexes Induced by Slow (<10 eV) Electrons

Metal acetylacetonate complexes have high potentiality in nanoscale fabrication processes (e.g., focus electron beam-induced deposition) thanks to the versatile character and ease of preparation compounds. In this work, we study and compare the physics and the physicochemistry induced by the interaction of low-energy (<10 eV) electrons with nickel(II) and cobalt(II) bis(acetylacetonate) complexes. The slow particles decompose the molecules via dissociative electron attachment. The nickel(II) and cobalt(II) bis(acetylacetonate) anions and the acetylacetonate negative fragments are the most dominant detected species. The experimental data are completed with density functional theory calculations to provide information on the electronic states of the molecules and the energetics for fragmentation. Finally, it is found that the interaction of low-energy electrons resulting in the decomposition of organometallic complexes in the gas phase is more efficient with the nickel(II) than with the cobalt(II) bis(acetylacetonate) complex. These results are found to be in a relative agreement with the surface experiments.

Mode-Specific Autodetachment Dynamics of an Excited Non-valence Quadrupole-Bound State

The autodetachment dynamics of vibrational Feshbach resonances of the quadrupole-bound state (QBS) for the first time has been investigated in real time for the first excited state of the 4-cyanophenoxide (4-CP) anion. Individual vibrational resonances of the cryogenically cooled 4-CP QBS have been unambiguously identified, and their autodetachment rates state-specifically measured using the picosecond time-resolved pump-probe technique employing the photoelectron velocity-map imaging method. The autodetachment lifetime (τ) is found to be strongly dependent on mode, giving τ values of ∼56, ∼27, and ≤2.8 ps for the 12'¹ (E_vib = 406 cm^-1), 12'² (E_vib = 806 cm^-1), and 21'¹ (E_vib = 220 cm^-1) modes, respectively. The striking mode-specific behavior of the QBS lifetime has been invoked by the physical model in which the loosely bound electron falls off by the dynamic wobbling of the three-dimensional quadrupole moment ellipsoid associated with the corresponding vibrational motion in the autodetachment process.

Observation of a Symmetry-Forbidden Excited Quadrupole-Bound State

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 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^- (A_u symmetry) to the QBS (A_g 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.

Decomposition of Bis(acetylacetonate)zinc(II) by Slow Electrons

The production of zinc-containing nanostructures has a large variety of applications. Using electron beam techniques to degrade organometallic molecules for that purpose is perhaps one of the most versatile methods. In this work, we investigate the scattering of low-energy (<12 eV) electrons with bis(acetylacetonate)zinc(II) molecules. We show that core excited and high-lying shape resonances are mainly responsible for the production of the precursor anions as well as the ligand negative fragments, which are observed exclusively at electron energies of >3 eV. The mechanisms for electron capture and then molecular dissociation are discussed in terms of density functional theory studies.

Observation of a π-Type Dipole-Bound State in Molecular Anions

We report the observation of a π-type dipole-bound state (π-DBS) in cryogenically cooled deprotonated 9-anthrol molecular anions (9AT^{-}) by resonant two-photon photoelectron imaging. A DBS is observed 191  cm^{-1} (0.0237 eV) below the detachment threshold, and the existence of the π-DBS is revealed by a distinct (s+d)-wave photoelectron angular distribution. The π-DBS is stabilized by the large anisotropic in-plane polarizability of 9AT. The population of the dipole-forbidden π-DBS is proposed to be via a nonadiabatic coupling with the dipole-allowed σ-type DBS mediated by molecular rotations.

Excess electrons bound to H2S trimer and tetramer clusters

We have prepared the hydrogen sulfide trimer and tetramer anions, (H2S)3− and (H2S)4−, measured their anion photoelectron spectra, and applied high-level quantum chemical calculations to interpret the results.