One-photon Two-electron Research Articles

Catalytic reduction of proton, oxygen and carbon dioxide with cobalt macrocyclic complexes

The conversion of solar energy into chemical energy by the reduction of small molecules provides a promising solution for the effective energy storage and transport. In this manuscript, we have highlighted our recent researches on the catalysis of cobalt-macrocycle complexes for the reduction of O2, proton and CO2. We have successfully clarified the reaction mechanisms of catalytic O2 reduction with cobalt phthalocyanine (Co[Formula: see text](Pc)) and cobalt chlorin (Co[Formula: see text](Ch)) based on detailed kinetic study under homogeneous conditions. The presence of proton-accepting moieties on these macrocyclic ligands enhances the electron-accepting ability, leading to the efficient catalytic two-electron reduction of O2 to produce hydrogen peroxide (H2O[Formula: see text] with high stability and less overpotential in acidic solutions. When Co[Formula: see text](Ch) is adsorbed on multi-walled carbon nanotubes (MWCNTs) and employed as an electrocatalyst, CO2 was successfully reduced to form CO with a Faradaic efficiency of 89% at an applied potential of -1.1 V vs. NHE in an aqueous solution. Finally, photocatalytic H2 evolution was attained from ascorbic acid with Co[Formula: see text](Ch) as a catalyst and [Ru(bpy)3][Formula: see text] (bpy [Formula: see text] 2,2[Formula: see text]-bipyridine) as a photocatalyst via a one-photon two-electron process.

Laser-Induced Dynamics of Peroxodicopper(II) Complexes Vary with the Ligand Architecture. One-Photon Two-Electron O2 Ejection and Formation of Mixed-Valent Cu(I)Cu(II)-Superoxide Intermediates.

Photoexcitation of end-on trans-μ-1,2-peroxodicopper(II) complex [(tmpa)2Cu(II)2(O2)](2+) (1) (λmax = 525 and 600 nm) and side-on μ-η(2):η(2)-peroxodicopper(II) complexes [(N5)Cu(II)2(O2)](2+) (2) and [(N3)Cu(II)2(O2)](2+) (3) at -80 °C in acetone led to one-photon two-electron peroxide-to-dioxygen oxidation chemistry (O2(2-) + hν → O2 + 2e(-)). Interestingly, light excitation of 2 and 3 (having side-on μ-η(2):η(2)-peroxo ligation) led to release of dioxygen, while photoexcitation of 1 (having an end-on trans-1,2-peroxo geometry) did not, even though spectroscopic studies revealed that both reactions proceeded through previously unknown mixed-valent superoxide species: [Cu(II)(O2(•-))Cu(I)](2+) (λmax = 685-740 nm). For 1, this intermediate underwent further fast intramolecular electron transfer to yield an "O2-caged" dicopper(I) adduct, Cu(I)2-O2, and a barrierless stepwise back electron transfer to regenerate 1 occurred. Femtosecond laser excitation of 2 and 3 under the same conditions still led to [Cu(II)(O2(•-))Cu(I)](2+) intermediates that, instead, underwent O2 release with a quantum yield of 0.14 ± 0.1 for 3. Such remarkable differences in reaction pathways likely result from the well-known ligand-derived stability of 2 and 3 vs 1 indicated by ligand-Cu(II/I) redox potentials; (N5)Cu(I) and (N3)Cu(I) complexes are far more stable than (tmpa)Cu(I) species. The fast Cu(I)2/O2 rebinding kinetics was also measured after photoexcitation of 2 and 3, with the results closely tracking those known for the dicopper proteins hemocyanin and tyrosinase, for which the synthetic dicopper(I) precursors [(N5)Cu(I)2](2+) and [(N3)Cu(I)2](2+) and their dioxygen adducts serve as models. The biological relevance of the present findings is discussed, including the potential impact on the solar water splitting process.

Mechanism of a one-photon two-electron process in photocatalytic hydrogen evolution from ascorbic acid with a cobalt chlorin complex.

A one-photon two-electron process was made possible in photocatalytic H2 evolution from ascorbic acid with a cobalt(II) chlorin complex [Co(II)(Ch)] via electron transfer from ascorbate to the excited state of [Ru(bpy)3](2+) followed by electron transfer from [Ru(bpy)3](+) to Co(II)(Ch) with proton to give the hydride complex, which reacts with proton to produce H2. [Co(III)(Ch)](+) was reduced by ascorbate to reproduce Co(II)(Ch).

Valence shell direct double photodetachment in polyanions

A valence shell study of electrosprayed insulin protein polyanion photodetachment was carried out on a vacuum ultra-violet synchrotron radiation beamline coupled to a radiofrequency ion trap, for both close- and open-shell species. A two-electron photodetachment is observed, which arises from two different mechanisms that are disentangled: a sequential multi-photon absorption and a direct one-photon two-electron process. The threshold for the direct double-electron ejection is measured at 11.4 eV and corresponds to electronic excitation in the valence shell, which makes it the first observation of direct double photodetachment in the valence shell. The results are discussed in the light of previous knowledge from multiple photoionization and ab initio calculations on model polyanions. Double photodetachment appears to be a relaxation mechanism that leads to oxidized anions of striking stability, a feature of high relevance in radiobiology.

One-photon two-electron transitions for metastable helium

We have obtained very accurate excitation energies for helium corresponding to the transitions 2p23Pe ↦nln’l’ 3Do\((n, n' \geqslant 3)\) using highly correlated Hylleraas-type wave functions. The doubly excited 2p23Pe metastable bound state energy is obtained using Ritz variational principle. The upper bound of 2p23Pe energy is –0.7105001556783 a.u., which is close to the best results –0.71050015567833 [R. Hilger et al., Chem. Phys. Lett. 262, 400 (1996)]. We employ complex-coordinate rotation method to obtain 3Do resonances. The 3Do resonance energies and widths along with one-photon two-electron transition energies are reported.

Bulk- and surface-state two-electron photoemission from Cu(1 1 1)

We study theoretically the one-photon two-electron emission from the Cu(1 1 1) surface. Tuning the energy to either the surface state or to the d-band energy regions this sample provides a paradigm example of the two-particle emission from a (quasi) two-dimensional and a three-dimensional system, respectively. The independent electron–gas behavior of the surface-state electrons results in a characteristic angular pattern. We inspect the dependence of this angular shape on the strength of the electron–electron interaction. Furthermore, it is shown that the two-particle energy correlation behaves differently for bulk and surface states.

Mechanisms of superconductivity studied by two-particle emission

It is shown theoretically that the one-photon two-electron emission spectroscopy of superconducting surfaces provides novel information on the mechanisms of superconductivity. In particular, measured angular correlations of the photo-excited (Cooper) pairs can be related to the Cooper pair wave function in momentum space. We outline how the use of circularly polarized photons gives access to phase properties of the superconducting gap function. Furthermore, we discuss the influence of the topology of the sample and the influence of an external magnetic field for the case of superconducting rings pierced by a magnetic flux.

Spectroscopy of electron correlations in superconductors

The one-photon two-electron emission process is suggested as a spectroscopic tool for studying the features of Cooper pairs in superconducting materials with a pairing mechanism. It is shown that the correlated angle distributions of the photo-excited electron pair carry detailed information on the Cooper pair wave function in the momentum space. Utilising circularly polarised photons offers the possibility of investigating aspects pertinent to the phase of the superconducting gap function. Phase information are accessed via measuring the circular dichroism in the two-electron angular and energy distribution, i.e. the difference in the recorded two-electron photoemission spectra for left and right circularly polarised photon.

Local Ionization in 2-Phenylethyl-N,N-dimethylamine: Charge Transfer and Dissociation Directly after Ionization

The 2-phenylethyl-N,N-dimethylamine (PENNA) radical cation offers two functional groups for a positive charge to reside, the benzyl ring and the substituted amine group. Previously published HeI photoelectron spectra (PES) and our B3LYP data of the cation ground state agree that the amine site has the lowest ionization energy (IE). In this work we present evidence that by resonant laser multiphoton ionization the electron is removed from the phenyl site. B3LYP calculations of the neutral molecule predict that by far the most stable structure is the nonsymmetric unfolded anti conformer and that no other conformer should be significantly populated. This is confirmed by the S0−S1 resonant two-photon ionization (R2PI) spectrum in which one conformer is found to be predominant. The presence of the vibrational fingerprint of the phenyl chromophore and the absolute energetic position of the R2PI spectrum clearly show the local character of the first photoexcitation. Surprisingly, the R2PI mass spectra taken via S1 resonances show strong fragmentation. The parent-to-fragment-ion ratio is about 1:10 and mostly independent of laser intensity. The metastable character of the decay excludes a fragmentation caused by cation photoabsorption. The strong dissociation directly after ionization is explained by (i) a local ionization at the phenyl chromophore, (ii) a fast charge transfer (CT) to the lower-energetic amine site, and (iii) a subsequent metastable dissociation. A first detailed analysis of the ionization process indicates that intersite R2PI ionization between local states is a one-photon two-electron process which is expected to be improbable and that a failure of local ionization only happens in cases of mixed intermediate S1 states or mixed cation states. PENNA with two possible charge sites spaced by a −CH2−CH2− bridge is an ideal model system to study the dynamics of a downhill charge transfer after local ionization to the first excited cation state as presented in a forthcoming paper.

Infrared threshold absorption in strongly correlated two-band superconductors

Two-band models (one wide band and one narrow band close to the Fermi energy) and local-pair models (periodic Anderson model) of high-Tc superconductors give rise to the same equilibrium thermodynamics despite the different physical pictures underlying the models. The low-energy excitations in these models are, however, not of the same character, which leads to a qualitative difference in their non-equilibrium properties. In particular, this is true for the interband optical absorption at threshold. Using the Golden rule for absorption and a mean-field treatment of the superconducting state, we calculate the threshold absorption at zero temperature for three different cases: a two-band model without additional correlation besides the interband interaction between Cooper pairs; a two-band model with attractive correlation (negative-U) between electrons on the same site in the narrow band; and a local-pair model with the same correlation as in the previous model, but with a pair interaction due to the hybridization (in second order) of the wide band and the local states. In the two latter models (which are closely related), we assume that the local-pair correlation energy is much higher than Tc. In the uncorrelated case the absorption has a square-root singularity at threshold, which is due to the diverging density of states of the one-particle excitations. In the negative-U correlated cases no single-particle transitions are possible at threshold, and only direct two-photon two-particle processes are possible. Further, in the local-pair model the hybridization can act together with a photon, giving an additional one-photon two-electron threshold in the absorption.

Photodetachment of the H − ion

Detachment of electrons from the H − ion is investigated with an experimental technique whereby an H − beam moving at a relativistic velocity (2.5 × 10 10 cm/s) is intersected with a fixed frequency laser. The Doppler effect allows systematic variation of the center-of-mass (CM) photon energy over a wide range (factor of 10) by simply adjusting the angle between the ion and laser beams. The focused output from a pulsed, linearly polarized, CO 2 TEA laser operating at 10.6 μm, with peak intensities on the order of 10 GW/cm 2, was used to examine the multiphoton absorption process in H −2. The fourth harmonic (266 nm) of a Nd:YAG laser was used to investigate some of the doubly-excited state resonances in H −. In the multiphoton absorption work, electron detachment was observed at photon energies where as few as 2 and as many as 8 photons are required to get above the 1-electron detachment threshold (EDT) of H − (0.754 eV). Electron yield vs photon energy plots exhibit structure that is laser intensity dependent. Electron yield vs laser pulse energy data was obtained at a few selected CM wavelengths and laser pulse energies. In the single-photon UV laser work, numerous resonances within the H − photodetachment continuum corresponding to one-photon two-electron excitation processes were observed. The doubly-excited resonances appear to be of the Feshbach type. A simple, semi-empirical recursion formula predicts the resonance energy levels. The experimental techniques described here can be used to accurately determine accelerator beam and ion source parameters such as beam energy, energy-spread, and ion density spatial distribution.

Single-photon two-electron rearrangement transitions

The observation and identification of $K$ x rays resulting from one-photon two-electron rearrangement are made for Mg, Al, and Si following heavy-ion bombardment. Two types of rearrangement processes, which lead to $K$ x rays below the energy of the characteristic $K\ensuremath{\alpha}$ x ray, are delineated: the radiative Auger effect (RAE) and radiative electron rearrangement (RER). The RAE process previously described by \AA{}berg and Utriainen competes with the characteristic $K\ensuremath{\alpha}$ x ray. The observed energies and relative intensities $\frac{\mathrm{RAE}}{K\ensuremath{\alpha}}$ from heavy-ion bombardment agree with those from electron and photon excitation. The RER process which we have recently proposed is the emission of a photon from the ${(1s)}^{\ensuremath{-}1}{(2p)}^{\ensuremath{-}n}$ to ${(2s)}^{\ensuremath{-}2}{(2p)}^{\ensuremath{-}n+1}$ electron rearrangement for $n=1\ensuremath{-}6$. This model gives good agreement between measured and Hartree-Fock calculated RER energies. $K{L}^{n}$ RER and $K\ensuremath{\alpha}{L}^{n}$ satellites are competing decay branches of the ${(1s)}^{\ensuremath{-}1}{(2p)}^{\ensuremath{-}n}$ initial configuration. The $\frac{(K{L}^{n}\mathrm{RER})}{K\ensuremath{\alpha}{L}^{n}}$ branching ratios are measured. The RER transitions are shown to be allowed by configuration mixing of final states with the same spin and parity. The branching ratio is a product of the mixing coefficient and a ratio of fluorescence yields. The theoretical branching ratios $\frac{(K{L}^{1}\mathrm{RER})}{K\ensuremath{\alpha}{L}^{1}}$ and $\frac{(K{L}^{2}\mathrm{RER})}{K\ensuremath{\alpha}{L}^{2}}$ are calculated and compared to experiment.