Gas-Phase Synthesis and Characterization of the Methyl-2,2-dicyanoacetate Anion Using Photoelectron Imaging and Dipole-Bound State Autodetachment

Probing Dipole-Bound States Using Photodetachment Spectroscopy and Resonant Photoelectron Imaging of Cryogenically Cooled Anions

Molecular anions with polar neutral cores can support highly diffuse dipole-bound states below their detachment thresholds due to the long-range charge-dipole interaction. Such nonvalence states constitute a special class of excited electronic states for anions and were observed in early photodetachment experiments to measure the electron affinities of organic radicals. Recent experimental advances, in particular, the ability to create cold anions using a cryogenically cooled Paul trap, have allowed the investigation of dipole-bound excited states at a new level. For the first time, the zero-point level of dipole-bound excited states can be observed via resonant two-photon detachment, and resonant photoelectron spectroscopy can be performed via the above-threshold vibrational levels (Feshbach resonances) of the dipole-bound states. This Perspective describes recent progress in the investigation of dipole-bound states in the authors' lab using an electrospray photoelectron spectroscopy apparatus equipped with a cryogenically cooled Paul trap and high-resolution photoelectron imaging.

Photoelectron Imaging Study of the Diplatinum Iodide Dianions [Pt2I6]2- and [Pt2I8]2

Photoelectron spectroscopy has been used to study the electronic structure, photodetachment, and photodissociation of the stable diplatinum iodide dianions [Pt₂I₆]^2– and [Pt₂I₈]^2–. Photoelectron spectra over a range of photon energies show the characteristic absence of low kinetic energy photoelectrons expected for dianions as a result of the repulsive Coulomb barrier (RCB). Vertical detachment energies of ∼1.6 and ∼1.9 eV and minimum RCBs of ∼1.2 and ∼1.3 eV are reported for [Pt₂I₆]^2– and [Pt₂I₈]^2–, respectively. Both of the diplatinum halides exhibit three direct detachment channels with distinct anisotropies, analogous to the previously reported spectra for PtI₂^– and PtI^–, suggesting a platinum-centered molecular core that dominates the photodetachment. Additionally, evidence for two-photon photodissociation and subsequent photodetachment channels producing I^– are observed for both dianions. Finally, an unexplained feature is observed at photon energies around 3 eV, whose origin is considered. Our work highlights the complex electronic structure of the heavy platinum-halide dianions that are characterized by a dense manifold of electronic states.

A Hückel Model for the Excited-State Dynamics of a Protein Chromophore Developed Using Photoelectron Imaging

Chemistry can be described as the movement of nuclei within molecules and the concomitant instantaneous change in electronic structure. This idea underpins the central chemical concepts of potential energy surfaces and reaction coordinates. To experimentally capture such chemical change therefore requires methods that can probe both the nuclear and electronic structure simultaneously and on the time scale of atomic motion. In this Account, we show how time-resolved photoelectron imaging can do exactly this and how it can be used to build a detailed and intuitive understanding of the electronic structure and excited-state dynamics of chromophores. The chromophore of the photoactive yellow protein (PYP) is used as a case study. This chromophore contains a para-substituted phenolate anion, where the substituent, R, can be viewed as an acrolein derivative. It is shown that the measured photoelectron angular distribution can be directly related to the electronic structure of the para-substituted phenolate anion. By incrementally considering differing R groups, it is also shown that these photoelectron angular distributions are exquisitely sensitive to the conformational flexibility of R and that when R contains a π-system the excited states of the chromophore can be viewed as a linear combination of the π* molecular orbitals on the phenolate (π_Ph*) and the R substituent (π_R*). Such Hückel treatment shows that the S₁ state of the PYP chromophore has predominantly π_R* character and that it is essentially the same as the chromophore of the green fluorescent protein (GFP). The S₁ excited-state dynamics of the PYP chromophore probed by time-resolved photoelectron imaging clearly reveals both structural (nuclear) dynamics through the energy spectrum and electronic dynamics through the photoelectron angular distributions. Both motions can be accurately assigned using quantum chemical calculations, and these are consistent with the intuitive Hückel treatment presented. The photoactive protein chromophores considered here are examples of where a chemists' intuitive Hückel view for ground-state chemistry appears to be transferable to the prediction of photochemical excited-state reactivity. While elegant and insightful, such models have limitations, including nonadiabatic dynamics, which is present in a related PYP chromophore, where a fraction of the S₁ state population forms a nonvalence (dipole-bound) state of the anion.

Autodetachment over Broad Photon Energy Ranges in the Anion Photoelectron Spectra of [O2-]- ( = Glyoxal, Methylglyoxal, or Biacetyl) Complex Anions

Complexes of anion-neutral pairs are prevalent in chemical and physical processes in the interstellar medium, the atmosphere, and biological systems, among others. However, bimolecular anionic species that cannot be described as simple ion-molecule complexes due to their competitive electron affinities have received less attention. In this study, the [O₂-M]^- (M = glyoxal, methylglyoxal, or biacetyl) anion photoelectron spectra obtained with several different photon energies are reported and interpreted in the context of ab initio calculations. The spectra do not resemble the photoelectron spectra of M^- or O₂^- "solvated" by a neutral partner. Rather, all spectra are dominated by near-threshold autodetachment from what are likely transient dipole bound states of the cis conformers of the complex anions. Very low Franck-Condon overlap between the neutral M·O₂ van der Waals clusters and the partial covalently bound complex anions results in low-intensity, broad direct detachment observed in the spectra. The [O₂-glyoxal]^- spectra measured with 2.88 and 3.495 eV photon energies additionally exhibit features at ∼0.5 eV electron kinetic energy, which is more difficult to explain, though there are numerous quasibound states of the anion that may be involved. Overall, these features point to the inadequacy of describing the complex anions as simple ion-molecule complexes.

Modeling the Photoelectron Angular Distributions of Molecular Anions: Roles of the Basis Set, Orbital Choice, and Geometry

A study investigating the effect of the basis set, orbital choice, and geometry on the modeling of photoelectron angular distributions (PADs) of molecular anions is presented. Experimental and modeled PADs for a number of molecular anions, including both closed- and open-shell systems, are considered. Guidelines are suggested for chemists who wish to design calculations to capture the correct chemical physics of the anisotropy of photodetachment, while balancing the computational cost associated with larger molecular anions.

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.

Autodetachment dynamics of 2-naphthoxide and implications for astrophysical anion abundance

Astrochemical modelling has proposed that 10% or more of interstellar carbon could be tied up as polycyclic aromatic hydrocarbon (PAH) molecules. Developing reliable models of the interstellar carbon lifecycle requires calibration data obtained through laboratory studies on relevant chemical and physical processes, including on the photo-induced and electron-induced dynamics of potential interstellar PAHs. Here, the excited state dynamics of the S1(ππ*) state of 2-naphthoxide are investigated using frequency-, angle-, and time-resolved photoelectron imaging. Frequency-resolved photoelectron spectra taken over the S1(ππ*) band reveal low electron kinetic energy structure consistent with an indirect, vibrational mode-specific electron detachment mechanism. Time-resolved photoelectron imaging using a pump photon energy tuned to the 0-0 transition of the S1(ππ*) band (hν = 2.70 eV) and a non-resonant probe photon provides the excited state autodetachment lifetime at τ = 130 ± 10 fs. There is no evidence for internal conversion to the ground electronic state or a dipole-bound state. These results imply that 2-naphthoxide has no resilience to photodestruction through the absorption of visible radiation resonant with the S1(ππ*) band, and that electron capture by the S1(ππ*) state, which is formally a shape resonance, is not a doorway state to a stable interstellar anion.

Influence of the N atom and its position on electron photodetachment of deprotonated indole and azaindole

Dipole bound state and its vibrational structure observed in deprotonated 7-azaindole by recording the signal of 7-azaindolyl stable neutral radical.