Stefano Simonucci

Tunable Band Gap in Hydrogenated Quasi-Free-Standing Graphene

We show by angle-resolved photoemission spectroscopy that a tunable gap in quasi-free-standing monolayer graphene on Au can be induced by hydrogenation. The size of the gap can be controlled via hydrogen loading and reaches approximately 1.0 eV for a hydrogen coverage of 8%. The local rehybridization from sp(2) to sp(3) in the chemical bonding is observed by X-ray photoelectron spectroscopy and X-ray absorption and allows for a determination of the amount of chemisorbed hydrogen. The hydrogen induced gap formation is completely reversible by annealing without damaging the graphene. Calculations of the hydrogen loading dependent core level binding energies and the spectral function of graphene are in excellent agreement with photoemission experiments. Hydrogenation of graphene gives access to tunable electronic and optical properties and thereby provides a model system to study hydrogen storage in carbon materials.

A mixed basis set of plane waves and Hermite Gaussian functions. Analytic expressions of prototype integrals

SummaryAnalytic expressions of prototype integrals for electronic-energy calculations involving plane waves and Hermite-Gaussian functions of every order are explicitly derived. A few notes on the use of such a mixed basis set in solid-state and molecular physics are given.RiassuntoSi presentano le espressioni analitiche d’integrali che compaiono nel calcolo dell’energia elettronica e coinvolgono contemporaneamente onde piane e funzioni gaussiane modulate da polinomi di Hermite di ogni grado. Si fanno alcune osservazioni sull’uso di un tale insieme misto di base in problemi di fisica molecolare e dello stato solido.

Non-adiabatic ab initio molecular dynamics of supersonic beam epitaxy of silicon carbide at room temperature

In this work, we investigate the processes leading to the room-temperature growth of silicon carbide thin films by supersonic molecular beam epitaxy technique. We present experimental data showing that the collision of fullerene on a silicon surface induces strong chemical-physical perturbations and, for sufficient velocity, disruption of molecular bonds, and cage breaking with formation of nanostructures with different stoichiometric character. We show that in these out-of-equilibrium conditions, it is necessary to go beyond the standard implementations of density functional theory, as ab initio methods based on the Born-Oppenheimer approximation fail to capture the excited-state dynamics. In particular, we analyse the Si-C(60) collision within the non-adiabatic nuclear dynamics framework, where stochastic hops occur between adiabatic surfaces calculated with time-dependent density functional theory. This theoretical description of the C(60) impact on the Si surface is in good agreement with our experimental findings.

Theoretical analysis of core-ionization and autoionization spectra of the CO molecule

Abstract A general approach, recently proposed for treating multichannel scattering processes, is summarized. Applications to core-ionization and autoionization spectra of the CO molecule are presented and the results compared with experimental spectra. The presence of asymmetric (Fano) profiles in the autoionization spectrum of the CO is discussed.

Correlation effects in Auger cascade studied by angle resolved Coincidence Electron Spectroscopy: the 1s->3p excitation in neon

Abstract In this work, the angular correlation of the two electrons produced in the cascade of Ne 1s −1 3p→Ne + 2s −1 2p −1 ( 1 P) 3p+e A ( E =778.4 eV)→Ne ++ 2p −2 ( 1 D, 3 P)+e C ( E =22.3, 25.7 eV) has been measured for the first time in a coincidence experiment. These measurements have been complemented by the non-coincidence angular distributions and energy spectra of both resonant Auger and second step Coster–Kronig electrons. Some results of an ab-initio calculation of the first step decay are also reported.

Theoretical analysis of line shapes in resonant Auger electron spectra

General expressions for predicting line shapes in resonant Auger electron spectra are derived. These formulas take into account explicitly the interference effects due to the presence of several resonances embedded in the continua of different final states having finite lifetimes and include also the effects due to the finite bandwidth of the incident radiation and the finite resolving power of the electron spectrometer. The characteristic features of the Auger resonant Raman effect are derived analytically. The theory is applied to the analysis of high resolution resonant Auger spectra recently measured at the VUV beam line at Elettra, Sincrotrone Trieste.

On the use of a Hamiltonian with projected potential for the calculation of scattering wave functions: Method and general properties

SummaryThe theoretical framework of a method that utilizes a projected potential operator to construct scattering wave functions is presented. Theorems and spectral properties of a Hamiltonian with the potential energy operator represented in terms ofL2(ℛ3) are derived. The computational advantages offered by the method for calculating spectroscopic quantities, like resonance energies, decay probabilities and photoionization cross-sections, are discussed.

The Lippmann-Schwinger equation for obtaining the continuum orbital in Auger problems: Application to the KLL spectrum of atomic neon

SummaryA new method is proposed for obtaining the continuum orbital representing the emitted electron in Auger problems through the use of a Lippmann-Schwinger equation. As a test of its accuracy, a comparison is made with previous calculations of the Auger spectrum of atomic Ne.

Tetrapeptide unfolding dynamics followed by core-level spectroscopy: a first-principles approach.

In this work we demonstrate that core level analysis is a powerful tool for disentangling the dynamics of a model polypeptide undergoing conformational changes in solution and disulphide bond formation. In particular, we present computer simulations within both initial and final state approximations of 1s sulphur core level shifts (S1s CLS) of the CYFC (cysteine-phenylalanine-tyrosine-cysteine) tetrapeptide for different folding configurations. Using increasing levels of accuracy, from Hartree-Fock and density functional theory to configuration interaction via a multiscale algorithm capable of reducing drastically the computational cost of electronic structure calculations, we find that distinct peptide arrangements present S1s CLS sizeably different (in excess of 0.5 eV) with respect to the reference disulfide bridge state. This approach, leading to experimentally detectable signals, may represent an alternative to other established spectroscopic techniques.

A scattering view of the Bogoliubov-de Gennes equations

We advocate the use of the T -matrix of the pair potential to study the properties of ultracold Fermi gases in the mean-field approximation. Our approach does not require renormalization procedures even in the limit of contact interaction, and it provides a rigorous definition of the range of the potential. We also rewrite the Bogoliubov-de Gennes equation for the pairing function as a function of the T-matrix, and use it to investigate finite-range effects on the main thermodynamic observables in a gas of 6Li atoms at unitarity, calculating the pair potential with ab initio quantum chemical methods.