Anna Giardini Guidoni

Short-range interactions within molecular complexes formed in supersonic beams: structural effects and chiral discrimination

One- and two-color, mass-selected R2PI spectra of the S1<--S0 transitions in the bare chiral chromophore R-(+)-1-phenyl-1-propanol (R) and its complexes with a variety of alcoholic solvent molecules (solv), namely methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, S-(+)-2-butanol, R-(-)-2-butanol, 1-pentanol, S-(+)-2-pentanol, R-(-)-2-pentanol, and 3-pentanol, were recorded after a supersonic molecular beam expansion. Spectral analysis, coupled with theoretical calculations, indicate that several hydrogen-bonded [R.solv] conformers are present in the beam. The R2PI excitation spectra of [R.solv] are characterized by significant shifts of their band origin relative to that of bare R. The extent and direction of these spectral shifts depend on the structure and configuration of solv and are attributed to different short-range interactions in the ground and excited [R.solv] complexes. Measurement of the binding energies of [R.solv] in their neutral and ionic states points to a subtle balance between attractive (electrostatic and dispersive) and repulsive (steric) forces, which control the spectral features of the complexes and allow enantiomeric discrimination of chiral solv molecules.

Luminescence from pigments and resins for oil paintings induced by laser excitation

Abstract The present work reports results of an extensive study of laser induced luminescence by tripled Nd:YAG laser ( λ =355 nm) of a few most common painting materials, namely, natural and synthetic pigments and resins. The luminescence spectra have been analyzed by an Optical Multichannel Analyzer (OMA III). Luminescence time decay has been measured by a Streak camera or by the OMA III. Pigments and resins show characteristic emission spectra with bands peaking in the visible. The decay ranges from less than 1 ns up to 700 μ s for pigments and for resins. The mechanism of excitation and relaxation leading to luminescence is discussed for the various materials. Oil colour specimens have been irradiated by a UV KrF laser ( λ =248 nm). Luminescence photographs have been detected by an intensified charge coupled device (ICCD) camera at different time delays.

Gas-dynamic effects in the laser-pulse sputtering of AlN: is there evidence for phase explosion?

Abstract The overall light intensity (fluorescence) of the sputtered atoms, ions, and molecules has been measured for polycrystalline AlN which was bombarded with 248 nm laser pulses in the presence of a background pressure of N 2 . AlN is unusual in that, in spite of a ∼6 eV band gap, it is easily rendered highly absorptive of 248, 308, or 694 nm laser pulses. In fact, since it is well established in other work that ∼1.5 J/cm 2 (308 nm) brings AlN to the melting temperature, ∼3050 K, we will assume that the fluence used here (∼20 J/cm 2 ) was more than enough to bring the target surface first to a temperature sufficient for normal vaporization but finally to the vicinity of the thermodynamic critical temperature, T tc . As a result a significant quantity of particles can be assumed to have been expelled by phase explosion. The tentativeness in the argument rests in the problem that some part of the incident fluence beyond ∼1.5 J/cm 2 will have been consumed in laser–plume interaction. Nevertheless there is evidence in work by Pedraza et al. [A.J. Pedraza, J-Y. Zhang, H. Esrom, Mater. Res. Soc. Symp. Proc. 285 (1993) 209] that both AlN and Al respond linearly to the fluence up to at least 6 J/cm 2 . It was found that the assumed phase-exploded particles decelerated rapidly, possibly due to their encounter with the normally vaporized particles, or possibly due to an electric field arising from positive charging of the target surface. The fluorescence maximum (which can be safely assumed to be also a density maximum) was then nearly stationary, a situation which characterized the lowest background pressures of N 2 (≤3.5 Pa). At higher pressures (≥3.5 Pa) a second fluorescence maximum appeared nearer the contact front and was found to move. Following the suggestion of Horwitz 1 we take this feature as being an artifact of electrons near the contact front diffusing (or scattering) backwards and causing fluorescence which is unrelated to the particle density. From the velocity of the contact front one obtains explicit information on the mean kinetic energies ( E 4 ) of the particles in the plume (1.5–2 eV). Another estimate of E 4 follows from the initial expansion observed from 0–200 ns (1.5–3 eV). Such energies suggest, independently of the fact that the fluence was high (∼20 J/cm 2 ), that a temperature near T tc was reached and that phase explosion may have occurred. We finally note that, however tentative is the claim for phase explosion, it is certain that a close relative of phase explosion, due to subsurface heating, was not involved. This is because the numerical demonstrations of subsurface heating have been flawed.

5. Plume Formation and Characterization in Laser-Surface Interactions

Publisher Summary This chapter presents a study on plume formation and characterization in laser-surface interactions. Plumes show well-defined gas-dynamic effects, the existence of which requires that a basic distinction be made in the matter of the mechanisms of laser sputtering. The primary action of the laser on a solid surface leads to the release of particles by primary mechanisms. These include electronic processes, normal vaporization, normal boiling, phase explosion (or explosive boiling or vapor explosion), and subsurface heating. Electronic processes can, in general, be expected with all insulators. The chapter presents a discussion on experimental evidence that with A1N, electronic processes appear to initiate the laser interaction, but provided that either incubation pulses are supplied or the fluence is sufficiently high; thermal processes, such as normal vaporization, then dominate. The chapter uses the term “normal vaporization” to indicate vaporization in the sense of particle emission from the extreme outer surface, with a particle flux given by the Hertz–Knudsen equation. Electronic sputtering is not a unique process, but rather a group of processes having the common feature of involving some form of excitation or ionization. Laser-pulse sputtering differs from ion sputtering in that thermal-spike processes can be very important.

GaN thin film fabrication by reaction of laser evaporated Ga and GaAs in NH3 atmosphere

Abstract The III group element nitrides (AlN, GaN, InN) have been prepared by laser ablation of the metals and simultaneous exposure to NH3. This study reports the growth of polycrystalline thin films of GaN on Si(100) by Nd:YAG (λ=532 nm) laser evaporation of bare Ga and GaAs in a NH3 atmosphere. The key problems are the gas phase solvation mechanism leading to Ga(NH3)n cluster formation and to direct nitridation of the target to yield GaN. Time of flight mass spectrometry has been used to monitor ablation plume components. The films were analyzed by conventional techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and IR spectroscopy.

Laser studies of chiral molecules

Laser ablated chiral molecules and their clusters are studied by mass resolved R2PI technique. Attractive (electrostatic and polarization) and repulsive (steric) interaction influence both the shift and the binding energy of the complexes. In monofuctional systems, the homochiral complexes are found to be more stable than the heterochiral ones. The formation of stable L-Tyrosine-Aluminum (Al-Tyr) cluster in a supersonic beam expansion of a laser ablated Al-Tyr target has been observed.

Laser spectroscopy applied to a conformational study of metals clustered with biological and inorganic systems

We present a novel optical system (patent pending), which makes homogeneous the spatial energy distribution of a light beam, independently of the local intensity fluctuations strength. It can also change the spot dimensions of the homogenized beam over a large range of values without replacement of any optical element and without changing the length of the radiation path.

Diagnostics of laser evaporated Al in NH 3 atmosphere [AlN deposition]

The authors report results of the preparation of AlN films obtained by diagnostics of the reaction of laser evaporated Al with NH/sub 3/. The films fabricated by this method have been characterized by Auger, EDX, scanning electron microscopy (SEM), and X-ray diffraction spectra.

LASER DEPOSITION OF THIN FILMS AND CHARACTERIZATION OF SUPERCONDUCTING BiSrCaCuO AND YBaCuO

An investigation on “in situ” laser induced deposition of superconductor Bi 2 Sr 2 CaCu 2 O x (BSCCO) and YBa 2 Cu 3 O x (YBCO) material is here reported. The laser irradiation of the solid sample was performed by a frequency doubled Nd-YAG laser. The thin films deposited on SrTiO 3 substrate were annealed in a high temperature oven. Resistance measurements showed the presence of superconductive transition above 80 K for BSCCO. XPS analysis of “as deposited” and annealed films appears to indicate that in the annealed sample the oxygen and copper valence are restored. Laser ionization mass analysis studies, performed on BSCCO sample showed the presence of cluster ions.