Gian Paolo Brivio

Experimental and theoretical Raman results in trans polyacetylene

Abstract We present here the experimental and theoretical calculations of Resonant Raman scattering from trans (CH) x . The new experimental data obtained with U.V. exciting wavelengths are interpreted in terms of the theoretical model based on the bimodal statistical distribution of long and short chains which constitute a given trans (CH) x sample.

Non-adiabatic processes in adsorption/desorption phenomena

Abstract The coupling between electronic and nuclear motions, which is neglected in the adiabatic approximation, allows energy flow between the two sorts of motion. In chemisorption systems, such energy flow is a non-adiabatic effect which has several consequences; among them are the broadening of the IR vibration line of a chemisorbed atom, and the possibility of a gas-phase atom losing enough energy to stick on to the substrate. In this paper model calculations are reported which indicate that IR linewidths and sticking coefficients due to non-adiabatic effects can be important.

Resonant Raman scattering of partially isomerized and doped polyacetylene: An application of the conjugation length distribution model☆

Abstract We present experimental data and theoretical calculations of Resonant Raman Scattering spectra from doped and partially isomerized transpolyacetylene (CH)x and (CD)x. The experimental data are explained by using the model based on the hypothesis that the sample properties can be interpreted in terms of a bimodal distribution of long and short conjugation length segments. A good agreement is found between the experimental data and the theoretical evaluations and we can prove that in the case of (CHBr0.17)x, the Resonant Raman Scattering spectra are completely determined by the unperturbed conjugation length segments contribution.

A classical analysis of photodesorption by resonant IR-laser adsorbate coupling

Abstract Photodesorption induced by IR-laser resonant excitation of an internal vibrational mode of physisorbed molecules is investigated in terms of a simple model, which only considers a non-dissipative energy transfer between the excited intramolecular vibration and the molecule-surface van der Waals bond. Classical simulations are performed for CH 3 F adsorbed on NaCl and Poincare maps are used to clarify details of the microscopic process responsible for desorption and to identify conditions where the non-exponential time decay fo the adsorbate coverage is expected. Photodesorption rates extracted from the classical simulations differ significantly from those obtained using a quantum-mechanical perturbation approach.

Ab initio calculation of core-valence-valence Auger spectra in closed shell systems

We propose an ab initio method to evaluate the core-valence-valence (CVV) Auger spectrum of systems with filled valence bands. The method is based on the Cini-Sawatzky theory, and aims at estimating the parameters by first-principles calculations in the framework of density-functional theory (DFT). Photoemission energies and the interaction energy for the two holes in the final state are evaluated by performing DFT simulations for the system with varied population of electronic levels. Transition matrix elements are taken from atomic results. The approach takes into account the non-sphericity of the density of states of the emitting atom, spin-orbit interaction in core and valence, and non quadratic terms in the total energy expansion with respect to fractional occupation numbers. It is tested on two benchmark systems, Zn and Cu metals, leading in both cases to L23M45M45 Auger peaks within 2 eV from the experimental ones. Detailed analysis is presented on the relative weight of the various contributions considered in our method, providing the basis for future development. Especially problematic is the evaluation of the hole-hole interaction for systems with broad valence bands: our method underestimates its value in Cu, while we obtain excellent results for this quantity in Zn.

High resolution NEXAFS of perylene and PTCDI: a surface science approach to molecular orbital analysis

We made use of synchrotron radiation to perform near edge X-ray absorption fine structure spectroscopy, NEXAFS, at the carbon K-edge of perylene and perylene-tetracarboxylic-diimide, PTCDI. Reference spectra measured for isolated molecules in the gas phase are compared with polarization dependent NEXAFS spectra measured on highly oriented thin films in order to study the symmetry of the molecular orbitals. The molecular overlayers are grown onto the rutile TiO2(110) surface for which the large anisotropic corrugation effectively drives the molecular orientation, while its dielectric nature prevents the rehybridization of the molecular orbitals. We employed density functional theory, DFT, calculations to disentangle the contribution of specific carbon atoms to the molecular density of states. Numerical simulations correctly predict the observed NEXAFS azimuthal dichroism of the σ* resonances above the ionization threshold, from which we determine the full geometric orientation of the overlayer molecules. A discrepancy observed for the spectral contribution of the imide carbon atom to the calculated unoccupied molecular orbitals has been explained in terms of initial state effects, as determined by Hartree-Fock corrections and in full agreement with the corresponding shift of the C 1s core level measured by X-ray photoelectron spectroscopy, XPS.

Conductance of a photochromic molecular switch with graphene leads

We report a full self-consistent ab initio calculation of the conductance of a diarylethene-based molecular switch with two graphene electrodes. Our result show the contributions of the resonant states of the molecule, of the electrode density of states, and of graphene unique features such as edge states. The conductivities are found to be significantly different for the two photochromic isomers at zero and finite applied bias. Further we point out the possibility of causing the switching by the application of a large potential difference between the two electrodes. Organic electronics has rapidly grown into a fundamental field whose potential is still to be fully developed and exploited. Research focuses on novel functional organic materials and existing applications already comprise, among others, nanoscale electronic devices such as thin film transistors and diodes, solar cells, integrated circuits on flexible substrates, carbon nanotube field effect transistors and molecular switches. A promising class of organic molecular-scale photoswitching units is provided by photochromic molecules which are able to switch between two chemical structures when irradiated by light of appropriate wavelengths. Such states correspond to photochemically interconvertible isomers [1].

Lattice-gas model of diffusion of NH3 on Re(0001)

A lattice-gas model for the chemical diffusion of NH3 on Re(0001) is presented. The system is modelled as a triangular lattice gas with nearest-neighbour repulsive lateral interactions. The average jump rate and the static structure factor are calculated within the cluster variation method framework by using a cluster of seven sites. Then the chemical diffusion coefficient is evaluated in the Darken approximation. This results in the jump rate and diffusion coefficient strongly increasing with coverage in agreement with experimental measurements.

Mechanically controllable bi-stable states in a highly conductive single pyrazine molecular junction

We report the fabrication of a highly conductive single pyrazine molecular junction with Pt leads. Mechanically controllable break-junction measurements at low temperatures show two distinct high and low conductance states. These conductance values are two orders of magnitude larger than those of a conventional single molecular junction with anchoring groups because of direct binding of the π conjugated molecule to a metal electrode with large density of states at the Fermi energy. Inelastic electron tunneling spectroscopy combined with density functional theory calculations highlights the vibration modes of the system for the two regimes. Theory allows us to assign the high and low conductance states of the molecular junction to two configurations in which the pyrazine axis is tilted and parallel with respect to the junction axis, respectively. Finally, we show that the pyrazine junction can be reversibly switched between the two bi-stable conductance states by mechanically stretching and relaxing the junction.

Photoinduced infrared activity in trans(CH)x: Inter[retation in terms of a lattice dynamical model

Abstract A lattice dynamical calculation for the photoinduced infrared vibrational modes is reported. By using the perturbed Green function method we have been able to compute the new frequencies and the integrated intensities of the photoinduced infrared modes at ω1H⋍500 cm−1, ω2H⋍1260 cm−1 and ω3H⋍1370 cm−1 in trans(CH)x and at ω1D⋍400 cm−1 and ω2D⋍1045 cm−1 in trans(CD)x. Most of the features of the photoinduced infrared spectra can be explained in terms of long conjugation length segment properties. Only the higher frequency tail of the band shape peaked at ω1H and ω1D can be related to the pertubbation determined by a photogenerated charge trapped in short segments. The calculated frequencies and integrated intensities are in good agreement with the experimental results.