Influence of the repulsive Coulomb barrier on photoelectron spectra and angular distributions in a resonantly excited dianion

Beyond Duality: Rationalizing Repulsive Coulomb Barriers in Host-Guest Cyclodextrin-Dodecaborate Complexes

The repulsive Coulomb barrier (RCB), an intrinsic potential energy barrier along electron detachment or charge-separation coordinates in multiply charged anions (MCAs), provides dynamic stability to MCAs whose electronic and thermodynamic stabilities are largely dictated by strong internal Coulomb repulsions. Spectroscopic and theoretical characterizations of the RCB have been focused on isolated MCAs. In this work, we extend the RCB investigation beyond the previous scope by including noncovalent host-guest cyclodextrin-closo-dodecaborate dianionic complexes χCD·B₁₂X₁₂^2- (χ = α, β, γ; X = H, F-I). Photodechment photoelectron spectroscopy reveals the existence of two distinctly different RCBs, derived from detaching electrons from the guest dianions (RCB₁) or ionizing the host neutrals (RCB₂), respectively, with the latter being substantially smaller than the former. Theoretical calculations support the duality of RCBs in these complexes and further exhibit highly anisotropic nature of the RCBs.

Kasha's Rule and Koopmans' Correlations for Electron Tunnelling through Repulsive Coulomb Barriers in a Polyanion

The long-range electronic structure of polyanions is defined by the repulsive Coulomb barrier (RCB). Excited states can decay by resonant electron tunnelling through RCBs, but such decay has not been observed for electronically excited states other than the first excited state, suggesting a Kasha-type rule for resonant electron tunnelling. Using action spectroscopy, photoelectron imaging, and computational chemistry, we show that the fluorescein dianion, Fl^2-, partially decays through electron tunnelling from the S₂ excited state, thus demonstrating anti-Kasha behavior, and that resonant electron tunnelling adheres to Koopmans' correlations, thus disentangling different channels.

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.

Photoelectron spectroscopy of the protoporphyrin IX dianion

Two-dimensional photoelectron spectroscopy using nanosecond and femtosecond lasers has been used to study the protopophyrin IX dianion at photon energies between 1.8-4.1 eV. The photoelectron spectra indicated the presence of two direct detachment channels, tunnelling through the repulsive Coulomb barrier (RCB) and thermionic emission from monoanions. A direct detachment feature suggested a near 0 eV electron affinity, which may be attributable to the repulsive through space interaction of the unshielded carboxylate groups. The minimum height of the repulsive Coulomb barrier (RCB) was found to be between 1.4-1.9 eV. Adiabatic tunnelling through the RCB was seen to occur on a timescale faster than rotational dephasing of the molecule. The observation of thermionic emission below the RCB in the nanosecond spectra originated from monoanions, which were produced via photon-cycling of the dianion.

Ultrafast Dynamics of the Isolated Adenosine-5'-triphosphate Dianion Probed by Time-Resolved Photoelectron Imaging

The excited state dynamics of the doubly deprotonated dianion of adenosine-5'-triphosphate, [ATP-H₂]^2-, has been spectroscopically explored by time-resolved photoelectron spectroscopy following excitation at 4.66 eV. Time-resolved photoelectron spectra show that two competing processes occur for the initially populated ¹ππ* state. The first is rapid electron emission by tunneling through a repulsive Coulomb barrier as the ¹ππ* state is a resonance. The second is nuclear motion on the ¹ππ* state surface leading to an intermediate that no longer tunnels and subsequently decays by internal conversion to the ground electronic state. The spectral signatures of the features are similar to those observed for other adenine-derivatives, suggesting that this nucleobase is quite insensitive to the nearby negative charges localized on the phosphates, except of course for the appearance of the additional electron tunneling channel, which is open in the dianion.

An instrument combining an electrospray ionization source and a velocity-map imaging spectrometer for studying delayed electron emission of polyanions

An instrument combining an electrospray ionization source and a velocity-map imaging (VMI) spectrometer has been developed in order to study the delayed electron emission of molecular anions and especially of polyanions. It operates at a high repetition rate (kHz) in order to increase the acquisition speed. The VMI spectrometer has been upgraded for nanosecond time resolution by gating the voltages applied on the position-sensitive detector. Kinetic energy release distribution of thermionic emission (without any contribution from direct detachment) can be recorded for well-defined delays after the nanosecond laser excitation. The capability of the instrument is demonstrated by recording photodetachment spectra of the benchmark C60(-) anion and C84(2-) dianion.

Anion resonances of para-benzoquinone probed by frequency-resolved photoelectron imaging

The resonant attachment of a free electron to a closed shell neutral molecule and the interplay between the following electron detachment and electronic relaxation channels represents a fundamental but common process throughout chemical and biochemical systems. The new methodology of anion frequency-resolved photoelectron imaging is detailed and used to map out molecular excited state dynamics of gas-phase para-benzoquinone, which is the electron accepting moiety in many biological electron-transfer chains. Three-dimensional spectra of excitation energy, electron kinetic energy, and electron ejection anisotropy reveal clear fingerprints of excited and intermediate state dynamics. The results show that many of the excited states are strongly coupled, providing a route to forming the ground state radical anion, despite the fact that the electron is formally unbound in the excited states. The relation of our method to electron impact attachment studies and the key advantages, including the extension to time-resolved dynamics and to larger molecular systems, are discussed.

Excited states of multiply-charged anions probed by photoelectron imaging: riding the repulsive Coulomb barrier

Recent progress towards understanding the repulsive Coulomb barrier in multiply-charged anion using photoelectron spectroscopy is discussed.