A. Giardini

Gas-phase enantioselectivity

Abstract Determination of the intrinsic noncovalent interactions governing chiral recognition in diastereomeric complexes constitutes the basis for understanding information transfer between molecules in living systems as well as in synthetic supramolecular structures. The most important experimental methodologies so far employed for this task are illustrated in the present review. Emphasis is put on the principles and the applications of techniques, such as radiolysis, Fourier transform ion cyclotron resonance (FTICR) and collision-induced dissociation (CID) mass spectrometry, and resonance-enhanced multiphoton ionization time-of-flight (REMPI-TOF) spectroscopy, that allow measurement of the relative stability of diastereomeric ion/molecule and molecule/molecule complexes and quantification of the short-range forces controlling their enantioselective evolution to products.

LIBS as a diagnostic tool during the laser cleaning of copper based alloys: experimental results

In spite of difficulties in quantitative LIBS analysis on copper based alloys, the very low invasiveness of the technique strongly sustains attempts to use it with cultural heritage materials, including ancient bronzes. Analytical results obtained with calibration curves and a calibration free model are compared here on a set of ancient roman coins. An attempt is presented to monitor the laser ablation process on bronze coins and artificially aged standards during the cleaning. The double pulse technique showed that LIBS analytical results could benefit from synchronization between the UV laser sources used, respectively, for cleaning (266 nm) and for LIBS analysis (335 nm).

PULSED LASER REACTIVE ABLATION OF AL IN AN AMMONIA ATMOSPHERE : PHOTOIONIZATION THRESHOLDS AND STRUCTURES OF AL-NH3 CLUSTERS

Abstract A photoionization study of the aluminum ammonia clusters produced by the interaction of the laser ablated aluminum vapour with gaseous NH 3 is reported. In the near-threshold photoionization spectrum of Al(NH 3 ) the observed structures are assigned to vibrational frequencies of the ionized cluster. Ionization potentials of Al(NH 3 ) n clusters with 2≤ n ≤13 have been measured. The cluster ionization potentials are shifted to lower energies compared with the bare Al atom and they decrease almost monotonically with increasing cluster size. This is consistent with a model in which the metal valence electron is delocalized in a Rydberg-like surface state. For small clusters the difference between the binding energy of the ion and the neutral complexes is compared with energetics calculated by a density functional method.

Energetics of monohydrated chiral R(+)-1-phenyl-1-propanol: supersonic beam experiments and density functional calculations

Abstract Isolated hydrated clusters of chiral R(+)-1-phenyl-1-propanol have been studied in a supersonic molecular beam by means of one- and two-color resonant two-photon ionization mass-resolved spectroscopy. Excitation and threshold ionization spectra have been determined for monohydrated clusters together with the electronic ground state binding energy. Cluster spectra have been analyzed with the aid of ab initio molecular orbital calculations conducted at the B3LYP/6-31G ∗∗ level of the theory. The analysis gives information on the cluster structures and the binding energies.

Chiral discrimination of monofunctional alcohols and amines in the gas phase

Abstract A methodology has been developed for enantiodiscriminating chiral monoalcohols and monoamines by mass spectrometry. The approach is based on the generation of supersonically expanded complexes of these molecules with suitable chromophores, i.e. R-(+)-1-phenyl-ethanol (ER) or R-(+)-1-phenyl-1-propanol (PR). The jet-cooled diastereomeric complexes, otherwise elusive at room temperature, have been ionized by one-color resonant two-photon absorption (R2PI) and their fragmentation pattern analyzed by time-of-flight (TOF) spectrometry. Enantiodifferentiation of the chiral monoalcohols and monoamines is based on: (1) the different spectral shifts of the band origin of their molecular complexes relative to that of the bare chromophore (Δ) and (2) the different mass spectral fragmentation patterns of the jet-cooled diastereomeric adducts. Detection of stable aggregates of methane, n-butane, and other simple molecules with the selected chromophores suggests that the R2PI/TOF method can be a potential tool for enantiodifferentiating chiral hydrocarbons in the gas phase.

Laser ablation of highly oriented CdSe thin films and CdSeCdTe multilayers on silicon substrates

Abstract The laser ablation technique has been successfully used in our laboratory for the deposition of CdSe and CdTe CdSe multilayers on Si(100) and Si(111) substrates. X-ray analysis showed that CdSe Si(111) and CdSe Si(100) films were grown in the hexagonal phase. Their orientation changed from (100) to (002) by only varying the substrate temperature from 200°C to 400°C. The same hexagonal growth was obtained in multilayers of CdSe and CdTe on Si(111). Photoluminescence spectra of CdSe films were studied as a function of the excitonic features and results on the extrinsic luminescence are reported.

Sensors based on pulsed laser deposition of multilayers of metal oxides

Abstract Pulsed laser ablation has been utilized in our laboratory to deposit thin films of semiconducting oxides such as indium oxide and tin oxide. Deposition of these thin films has been carried out by a frequency doubled Nd-YAG laser (λ=532 nm) on silicon (100) substrates. A comparison has been performed, among indium oxide, tin oxide, and multilayers of indium and tin oxides, to evaluate their use as NO gas sensors. The influence of physical parameters, such as substrate temperature and laser fluence, on the velocity response of the films and on their resistance variation, has been investigated. The deposited films have been characterized by X-ray diffraction (XRD) and electric resistance measurements.

Molecular and supramolecular chirality: R2PI spectroscopy as a tool for the gas-phase recognition of chiral systems of biological interest.

In life sciences, diastereomeric chiral molecule/chiral receptor complexes are held together by a different combination of intermolecular forces and are therefore endowed with different stability and reactivity. Determination of these forces, which are normally affected in the condensed phase by solvent and supramolecular interactions, can be accomplished through the generation of diastereomeric complexes in the isolated state and their spectroscopic investigation. This review presents a detailed discussion of the mass resolved Resonant Two Photon Ionization (R2PI-TOF) technique in supersonic beams and introduces an overview of various other technologies currently available for the spectroscopic study of gas phase chiral molecules and supramolecular systems. It reports case studies primarily from our recent work using R2PI-TOF methodology for chiral recognition in clusters containing molecules of biological interest. The measurement of absorption spectra, ionization and fragmentation thresholds of diastereomeric clusters by this technique allow the determination of the nature of the intrinsic interactions, which control their formation and which affect their stability and reactivity.

Microsolvation effects on the π*←π electronic transitions in simple aromatic chromophores: The role of the Slater-type Gaussian orbitals in the complete active space self-consistent field approach

One of the most studied feature of aromatic molecular clusters in the gas phase is the shift of the pi*<--pi electronic transitions between the bare aromatic chromophore and its clusters. In the present ab initio complete active space self-consistent field study the coupling of the basis set superposition error and counterpoise procedure and a combination of Pople- and Slater-type Gaussian basis sets has been proven to reproduce quantitatively the gas-phase experimental shifts. The quantal results are here analyzed with respect to the electrostatic and polarization forces and electron density differences, and connected with the sign of the shifts of the electronic transitions.

Chiral discrimination of 2,3-butanediols by laser spectroscopy

The resonance enhanced two-photon ionization time-of-flight (R2PI-TOF) excitation spectra of supersonically expanded complexes of isomeric 2,3-butanediols with a suitable chromophore, i.e. R-(+)-1-phenyl-1-propanol, represent powerful means for structurally discriminating the diol moiety and for investigating the nature of the intra- and intermolecular interactions involved in the complexes.