Tonia M. Di Palma

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.

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.

Tunable Single-Photon Ionization TOF Mass Spectrometry Using Laser-Produced Plasma as the Table-Top VUV Light Source

Here we report on a laser plasma-based tunable VUV photoionization time-of-flight (TOF) mass spectrometer conceived mainly to study complex gaseous mixtures. Ionizing photons at tunable vacuum UV (VUV) wavelengths are generated by a gas-target laser-produced plasma, spectrally dispersed in the range 100–160 nm and efficiently focused onto a sample molecular beam. As a test case, we studied the exhaust gas of a four-stroke moped, a typical example of a complex gaseous mixture. Due to the VUV “soft” ionization, the mass spectra are less congested and more easily interpretable. Substituted benzene derivatives are found to give the most intense signals. Several aliphatic hydrocarbons are also detected. The use of tunable VUV radiation allowed the investigation of the contribution of isomers in the mass spectrum from the onset and shape of the photoionization efficiency spectra. Semiquantitative analysis was performed using known literature data detailing the photoionization cross sections. Our findings suggest that using combined data on the mass/photoionization efficiency spectra may be very helpful for a comprehensive analysis of complex gaseous mixtures.

Laser-plasma-based vacuum-ultraviolet light source for tunable single-photon ionization

Vacuum-UV radiation from Xe jet-target laser-produced plasmas has been produced, spectrally dispersed, and efficiently focused onto a line-shaped interaction volume by adopting an embedded-in-the-chamber spectrograph geometry. Time-resolved 2D Rayleigh light scattering imaging has been carried out for visualizing the gas jet-laser plasma interaction and optimizing the emission intensity and the spectral resolution. We have calibrated the measured photon fluxes, reaching values higher than 10(13) photons/pulse cm(2) nm in the 100-200 nm wavelength range within the first 20 ns from the laser pulse onset. The vacuum-UV light source is predicted to enable sensitive and selective single-photon ionization for time-of-flight mass spectrometry and similar vacuum-UV spectroscopy applications.

Tautomerism and proton transfer in photoionized acetaldehyde and acetaldehyde–water clusters

Understanding the gas-phase chemistry of acetaldehyde can be challenging because the molecule can assume several tautomeric forms, namely keto, enol and carbene. The two last forms are the most stable ionic forms. Here, insight into the gas-phase cluster ion chemistry of homogeneous acetaldehyde and mixed water-acetaldehyde clusters is provided by mass spectrometry/vacuum ultraviolet photoionization combined with density functional theory calculations. (AA)nH(+) clusters (AA = acetaldehyde) and mixed (AA)nH3O(+) clusters were detected using tunable vacuum ultraviolet photoionization. Barrierless proton transfers were observed during the geometry optimization of the most stable dimer structures and helped to explain the cluster ion chemistry induced by photoionization, namely the formation of deprotonated tautomers and protonated keto tautomers. Water was found to catalyze the keto-enol and keto-carbene isomerizations and facilitate the proton transfer from the ionized enol or carbene part of the cluster to the neutral keto part, resulting in protonated keto structures. The production of protonated keto structures was identified to be the main fragmentation channel following ionization of the homogeneous acetaldehyde cluster and a channel for ionized mixed clusters as well. These findings are significant for a broad range of fields, including current atmospheric models, because acetaldehyde is one of the most prominent organic species in the troposphere and ions play a crucial role in aerosol formation.

Inception of Acetic Acid/Water Cluster Growth in Molecular Beams

The influence of carboxylic acids on water nucleation in the gas phase has been explored in the supersonic expansion of water vapour mixed with acetic acid (AcA) at various concentrations. The sodium-doping method has been used to detect clusters produced in supersonic expansions by using UV photoionisation. The mass spectra obtained at lower acid concentrations show well-detected Na(+) -AcA(H2O)n and Na(+)-AcA2 (H2O)n clusters up to 200 Da and, in the best cooling expansions, emerging Na(+)-AcAm (H2O)n signals at higher masses and unresolved signals that extend beyond m/e values >1000 Da. These signals, which increase with increasing acid content in water vapour, are an indication that the cluster growth taking place arises from mixed water-acid clusters. Theoretical calculations show that small acid-water clusters are stable and their formation is even thermodynamically favoured with respect to pure water clusters, especially at lower temperatures. These findings suggest that acetic acid may play a significant role as a pre-nucleation embryo in the formation of aerosols in wet environments.

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.

Preparation of the group III nitride thin films AlN, GaN, InN by direct and reactive pulsed laser ablation

The methods of preparation of the group III nitrides AlN, GaN, and InN by laser ablation (i.e. laser sputtering), is here reviewed including studies on their properties. The technique, concerns direct ablation of nitride solid targets by laser to produce a plume which is collected on a substrate. Alternatively nitride deposition is obtained as a result of laser ablation of the metal and subsequent reaction in an NH3 atmo- sphere. Optical multichannel emission spectroscopic analysis, and time of flight (TOF) mass spectrometry have been applied for in situ identification of deposition precursors in the plume moving from the target. Epitaxial AlN, GaN, and InN thin films on various substrates have been grown. X-ray diffraction, scanning electron microscopy, have been used to characterise thin films deposited by these methods.

Mass spectrometry of toluene clusters using a laser produced plasma for tunable single photon ionization

Here we report the first operation of a new apparatus of time of flight mass spectrometry using the tunable Vacuum-UV radiation emitted by a laser produced plasma as ionising source. Toluene cluster mass spectra acquired at different ionising wavelength are presented. Due to massive fragmentation the mass spectra change significantly by increasing the photon energy above the toluene ionisation potential. We found that the more reliable information on the cluster distribution can be gained by tuning the photon energy at value slightly higher than the toluene ionisation energy. The results demonstrate that the developed tunable vacuum-UV source can be successfully used as a soft ionisation method in the study of weakly bound complexes as well as of real complex gaseous mixture.