Stefano Turchini

Valence photoionization dynamics in circular dichroism of chiral free molecules: The methyl-oxirane

The dynamical behavior of circular dichroism for valence photoionization processes in pure enantiomers of randomly oriented methyl-oxirane molecules has been studied by circularly polarized synchrotron radiation. Experimental results of the dichroism coefficient obtained for valence photoionization processes as a function of photon energy have been compared with theoretical values predicted by state-of-the-art ab initio density-functional theory. The circular dichroism measured at low electron kinetic energies was as large as 11%. Trends in the experimental dynamical behavior of the dichroism coefficients D(i)(omega) have been observed. Agreement between experimental and theoretical results permits unambiguous identification of the enantiomer and of the individual orbitals.

Laboratory Studies of Molecular Growth in the Titan Ionosphere

Experimental simulations of the initial steps of the ion-molecule reactions occurring in the ionosphere of Titan were performed at the synchrotron source Elettra in Italy. The measurements consisted of irradiating gas mixtures with a monochromatic photon beam, from the methane ionization threshold at 12.6 eV, up to and beyond the molecular nitrogen dissociative ionization threshold at 24.3 eV. Three gas mixtures of increasing complexity were used: N(2)/CH(4) (0.96/0.04), N(2)/CH(4)/C(2)H(2) (0.96/0.04/0.001), and N(2)/CH(4)/C(2)H(2)/C(2)H(4) (0.96/0.04/0.001/0.001). The resulting ions were detected with a high-resolution (1 T) FT-ICR mass spectrometer as a function of time and VUV photon energy. In order to interpret the experimental results, a Titan ionospheric model was adapted to the laboratory conditions. This model had previously allowed the identification of the ions detected in the Titan upper atmosphere by the ion neutral mass spectrometer (INMS) onboard the Cassini spacecraft. Comparison between observed and modeled ion densities validates the kinetic model (reactions, rate constants, product branching ratios) for the primary steps of molecular growth. It also reveals differences that we attribute to an intense surface chemistry. This result implies that heterogeneous chemistry on aerosols might efficiently produce HCN and NH(3) in the Titan upper atmosphere.

Double perovskite Sr2FeMoO6 films : Growth, structure, and magnetic behavior

The structural and magnetic properties of Sr2FeMoO6 thin films prepared by pulsed laser deposition were studied as a function of the laser energy density used to ablate a noncommercial stoichiometric target. Films deposited at a laser fluence as low as 1.6J∕cm2 exhibit a high degree of double perovskite lattice ordering, a ferromagnetic-paramagnetic transition around 355K, and a saturation magnetization of ∼3.4μB∕f.u. On the contrary, films deposited at φ values >3J∕cm2 show a vertically elongated lattice unit cell and the lack of long range ferromagnetic order with a severe decrease of the saturation magnetization (Ms∼1μB∕f.u.). The structural and magnetic properties observed in the latter samples are attributed to lattice disorder and to secondary phases resulting from the ablation process performed at high laser energy density. In these samples the presence of Fe ions not arranged in the Sr2FeMoO6 is confirmed by x-ray absorption measurement at the Fe L2,3 edge.

Chirality Transfer from a Single Chiral Molecule to 2D Superstructures in Alaninol on the Cu(100) Surface

The formation of 2D chiral monolayers obtained by self-assembly of chiral molecules on surfaces has been widely reported in the literature. Control of chirality transfer from a single molecule to surface superstructures is a challenging and important aspect for tailoring the properties of 2D nanostructures. However, despite the wealth of investigations performed in recent years, how chiral transfer takes place on a large scale still remains an open question. In this paper we report a coupling of scanning tunneling microscopy and low energy electron diffraction measurements with an original theoretical approach, combining molecular dynamics and essential dynamics with density functional theory, to investigate self-assembled chiral structures formed when alaninol adsorbs on Cu(100). The peculiarity of this system is related to the formation of tetrameric molecular structures which constitute the building blocks of the self-assembled chiral monolayer. Such characteristics make alaninol/Cu(100) a good candidate to reveal chiral expression changes. We find that the deposition of alaninol enantiomers results in the formation of isolated tetramers that are aligned along the directions of the substrate at low coverage or when geometrical confinement prevents long-range order. Conversely, a rotation of 14° with respect to the Cu(100) unit vectors is observed when small clusters of tetramers are formed. An insight to the process leading to a 2D globally chiral surface has been obtained by monitoring molecular assemblies as they grow from the early stages of adsorption, suggesting that the distinctive orientation of the self-assembled monolayer originates from a balance of cooperating forces which start acting only when tetramers pack together to form small clusters.

Conformational Effects in Photoelectron Circular Dichroism of Alaninol

A photoelectron circular dichroism (CD) study of the valence states of 2-amino-1-propanol (alaninol) in the gas phase is presented. The aim of the investigation is to reveal conformer population effects in the valence-state photoelectron spectrum. The experimental dispersion of the dichroic D parameter of valence states as a function of the photon excitation energy is compared with its theoretical value calculated by employing a multicentric basis set of B-spline functions and a Kohn-Sham Hamiltonian. The theoretical values are in very good agreement with the experimental data when the conformer population distribution is taken into account. Moreover, thanks to a comparison between experiment and theory, a clear assignment of the molecular orbital character and conformer geometry is given to the features of the photoelectron spectrum. This work indicates in a detailed experimental analysis that CD in photoelectron spectroscopy is an effective technique to disentangle the conformer assignment in photoelectron spectra.

Vibrational state dependence of β and D asymmetry parameters: The case of the highest occupied molecular orbital photoelectron spectrum of methyl-oxirane

The beta angular asymmetry and D dichroic asymmetry parameters of the methyl-oxirane highest occupied molecular orbital (HOMO) band have been experimentally investigated with vibrational resolution using synchrotron radiation. A theoretical calculation of the Franck-Condon factors between vibrational ground state and different ionic vibrational states, in the Born-Oppenheimer harmonic approximation, has been performed in order to gain information on the vibrational states mainly involved in the HOMO photoelectron band. The general good agreement between theoretical and experimental results allows a reliable assignment of the major features. The experimental determination of beta and D shows their dependence on the different final vibrational states. This paper reports, for the first time, experimental evidence of the dependence of the dichroic D parameter on the vibrational excitation of the ion.

Conformational Sensitivity in Photoelectron Circular Dichroism of 3-Methylcyclopentanone

A study of (R)-3-methylcyclopentanone [(R)-3-MCP] by photoelectron spectroscopy and photoelectron circular dichroism (PECD) is presented. The synchrotron radiation gas-phase photoelectron spectra of (R)-3-MCP were measured and are discussed on the basis of different theoretical methodologies. The experimental dichroism of (R)-3-MCP for selected deconvoluted valence states and for the carbonyl carbon 1s core state are reported and reproduced well by calculated dispersions generated by considering the contributions of two different conformers. The theoretical dichroic parameters are calculated by employing a multicentre basis set of B-spline functions and a Kohn-Sham Hamiltonian. Temperature-dependent PECD studies of the HOMO state and the carbonyl carbon 1s core level allowed the separation of the contributions of each conformer by photoelectron dichroism. This new approach clearly shows how the PECD methodology is sensitive to conformational and structural changes of unoriented (R)-3-MCP in the gas phase, opening up new perspectives in the characterisation of chiral molecular systems.

An experimental and computational study of the valence photoelectron spectra of halogenated pyrimidines

The electronic structures of pyrimidine and a selection of its halogen-substituted derivatives have been investigated using ultraviolet photoelectron spectroscopy and ab initio quantum chemical methods. Assignments are proposed for all of the features in the PES spectra by comparison with the vertical ionization energies of the molecular orbitals calculated using the partial third-order quasiparticle approximation as applied to electron propagator theory and a corrected density functional method based on the B3LYP functional. The shifts of the outermost five molecular orbitals of the pyrimidine ring structure in the halogen-substituted derivatives with respect to the binding energies of the equivalent orbitals in the parent pyrimidine molecule are discussed as a function of the identity and ring position of the halogen atom.

Microscopic investigation of the strain distribution in InGaAs/GaAs quantum well structures grown by molecular beam epitaxy

Extended X-ray absorption fine structure in the glancing angle geometry has been used to study the strain accommodation in quantum well structures of In x Ga 1-x As/GaAs (x<0.25). The results show that a number of Ga-As bond lengths are stretched. Indeed, two Ga-As bonds distances coexist: 2.45±0.01 A and 2.64±0.02 A, which correspond to the Ga-As and In-As bond distances in the binary compounds GaAs and InAs, respectively. This result is independent of the In molar fraction in the strained alloy layers

Glancing‐angle extended x‐ray absorption fine structure study of strained InGaAs/GaAs heterostructures

The structural properties of strained InGaAs grown by molecular beam epitaxy on GaAs(100) substrates, have been studied by glancing‐angle extended x‐ray absorption fine structure (EXAFS). The very low incidence angle of the x‐ray beam on the sample makes it possible to collect the signal coming from a thin quasi‐surface layer allowing the study of a single strained sample built up by only 6 ML of InGaAs. The EXAFS results show that a slight deformation of the first shell Ga–As distance occurs and that the strain is accommodated also by bond‐bending mechanism as deduced by the second and third coordination shells analysis. The lattice expands in the growth direction in agreement within the limits predicted by the elastic theory.