Ionization of aqueous cations: photoelectron spectroscopy and ab initio calculations of protonated imidazole

Photoelectron spectroscopy and ab initio calculations employing a nonequilibrium polarizable continuum model were employed for determining the vertical ionization potential of aqueous protonated imidazole. The experimental value of 8.96 eV is in in excellent agreement with calculations, which also perform quantitatively for ionization of aqueous alkali cations as benchmark species. The present results show that protonation of imidazole increases its vertical ionization potential up in water by 0.7 eV, which is significantly larger than the resolution of the experiment or the error of the calculation. This combined experimental and computational approach may open the possibility for quantitatively analyzing the protonation state of histidine, of which imidazole is the titratable side chain group, in aqueous peptides and proteins.     

Excited state dynamics and fragmentation channels of the protonated dipeptide H2N-Leu-Trp-COOH

The excited state dynamics of the isolated and protonated peptide H(2)N-Leu-Trp-COOH are analyzed by fs pump-probe spectroscopy. The peptides are brought into the gas phase by electrospray ionization, and fs pump-probe excitation is detected by fragment ion formation. The pump laser addressed the excited pipi* state of the indole chromophore of the amino acid tryptophan. The subsequent excited state dynamics agreed with a biexponential decay with time constants of 500 fs and 10 ps. This is considerably shorter than the lifetime of neutral tryptophan in solution and in proteins, but similar to isolated, protonated tryptophan. Several models are discussed to explain the experimental results but the detailed quenching mechanism remains unresolved.     

X‐Treme beamline at SLS: X‐ray magnetic circular and linear dichroism at high field and low temperature

X‐Treme is a soft X‐ray beamline recently built in the Swiss Light Source at the Paul Scherrer Institut in collaboration with École Polytechnique Fédérale de Lausanne. The beamline is dedicated to polarization‐dependent X‐ray absorption spectroscopy at high magnetic fields and low temperature. The source is an elliptically polarizing undulator. The end‐station has a superconducting 7 T–2 T vector magnet, with sample temperature down to 2 K and is equipped with an in situ sample preparation system for surface science. The beamline commissioning measurements, which show a resolving power of 8000 and a maximum flux at the sample of 4.7 × 10¹² photons s^?1, are presented. Scientific examples showing X‐ray magnetic circular and X‐ray magnetic linear dichroism measurements are also presented.     

On the origin of surface states in a correlated local-moment film

The electronic quasiparticle structure of a ferromagnetic local moment film is investigated within the framework of the s-f model. For the special case of a single electron in an otherwise empty energy band being exchange coupled to a fully ordered localised spin system the problem can be solved exactly and, for the spin- electron, some marked correlation effects can be found. We extend our model to incorporate the influence of the surface on the electronic structure. Therefore we modify the hopping integrals in the vicinity of the surface. This leads to the existence of surface states, both for the spin- and the spin- spectral density of states. The interplay between the modification of the hopping integrals and the existence of surface states and correlation effects is discussed in detail. Received: 22 September 1997 / Revised: 12 December 1997 / Accepted: 15 December 1997     

Methode der Spektralmomente für das Hubbard-Modell eines schmalenS-Bandes

An improved approximation for the narrows-band model is proposed. We assume a “two-peak”-ansatz for the spectral density and fit the relative weights and the positions of the poles by equating moments. The one-particle energies, the general form of which has also been obtained by Roth and Tahir Kheli and Jarrett, are the same as the Hubbard-energies except for the appearance of an additional energy termB(k, — σ) which leads to a spin-dependent band shift.B(k, — σ) contains higher correlation functions which are also calculated by using spectral densities and spectral moments. The theory is applied to a simple cubic lattice in the strongly correlated limit.     

“Influence of valence instabilities on the Curie-temperature of ferromagnetic 4f-systems”

We inspect the stability of ferromagnetism in 4f-systems, which have an insulating, ferromagnetic ground state in the integral valent phase and fluctuate between a magnetic and a nonmagnetic configuration in the intermediate valent phase. It turns out that for large gaps between 4f-level and lower edge of the empty conduction band ferromagnetism is created by a certain interplay between the s-f hybridization, which allows real or virtual electron transitions between f-level and conduction band, and the s-f exchange interaction, which mediates an indirect coupling between the localized 4f-moments. When the gap is reduced by external pressure or by alloying with proper impurities the local f-moment becomes steadily quenched while simultaneously the coupling between the more and more reduced moments is drastically enhanced. These two competitive effects lead to a distinct maximum of the Curie-temperature just in the intermediate valence phase.