Papers by Raffaele Velotta
Applied Surface Science, 1996
We report on TOF mass spectrometric analysis of plasmas produced by laser ablation on Al targets.... more We report on TOF mass spectrometric analysis of plasmas produced by laser ablation on Al targets. We obtained mass spectra of ionized species in the UV (A = 355 nm) and in the visible (h = 532 nm) and at high laser fluences (I-80 J cm-'). The total ion yield, the relative abundance of different charged species and the laser fluence threshold for the appearance of specific ions as a function of energy density were measured.
Physical Chemistry Chemical Physics, 2013
A numerical study is reported concerning the first and second singlet excited-states of 5-benzylu... more A numerical study is reported concerning the first and second singlet excited-states of 5-benzyluracil using the multireference self-consistent field (state-averaged CASSCF) method. The vertical excitation energies of low-lying excited-states were characterized using the SA-CASSCF method, as well as using higher-level methods, such as CASPT2, MRCI and EOM-CCSD. The local minima and conical intersections found on the potential energy surfaces (PESs) were characterized in terms of molecular geometry and natural population analysis. Different relaxation pathways on the PESs are identified and discussed by comparing with the similar pathways found for the individual monomers of uracil and benzene. The molecule can be thought of as a model system for the study of crosslink reaction between DNA and proteins induced by UV light. state) that could compete with the crosslink. The system being amenable to simulation by the use of modern computational methods, we set out to determine the molecular structure, the energy of electronic excited states and the potential energy surfaces (PESs) of states potentially involved in the interaction of 5BU with a UV femtosecond laser pulse. In particular, this will be done for the ground state and the first two singlet excited electronic states of 5BU using calculations based on geometry optimization at the multireference self-consistent field (state-averaged CASSCF) level of theory. Special care should be taken on the search of noticeable points on the investigated PESs, i.e. local minima and conical intersections that can lead to ultrafast non-radiative decay processes. In the proximity of such points the dynamics of the process would involve more electronic states with consequent cross-talk and quantum interference between different decay paths. In addition, since the saturated methylene bridge hinders the electronic communication between components of 5BU via the p systems, the description of the excited states of the uracil base is expected to contribute to the growing knowledge of the photophysical properties of substituted nucleic acid bases. 10
Rapid Communications in Mass Spectrometry, 2013
The aim of this study was to demonstrate, and to characterize by high-resolution mass spectrometr... more The aim of this study was to demonstrate, and to characterize by high-resolution mass spectrometry that it is possible to preferentially induce covalent cross-links in peptides by using high-energy femtosecond ultraviolet (UV) laser pulses. The cross-link is readily formed only when aromatic amino acids are present in the peptide sequence. Three peptides, xenopsin, angiotensin I, and interleukin, individually or in combination, were exposed to high-energy femtosecond UV laser pulses, either alone or in the presence of spin trapping molecules, the reaction products being characterized by high resolution mass spectrometry. High-resolution mass spectrometry and spin trapping strategies showed that cross-linking occurs readily, proceeds via a radical mechanism, and is the highly dominant reaction, proceeding without causing significant photo-damage in the investigated range of experimental parameters. High-energy femtosecond UV laser pulses can be used to induce covalent cross-links between aromatic amino acids in peptides, overcoming photo-oxidation processes, that predominate as the mean laser pulse intensity approaches illumination conditions achievable with conventional UV light sources.
The Journal of Physical Chemistry B, 2014
The photophysical and photochemical properties of 5benzyluracil and 5,6-benzyluracil, the latter ... more The photophysical and photochemical properties of 5benzyluracil and 5,6-benzyluracil, the latter produced by photocyclization of the former through irradiation with femtosecond UV laser pulses, are investigated both experimentally and theoretically. The absorption spectra of the two molecules are compared, and the principal electronic transitions involved are discussed, with particular emphasis on the perturbation induced on the two chromophore species (uracil and benzene) by their proximity. The photoproduct formation for different irradiation times was verified with high-performance liquid chromatography and nuclear magnetic resonance measurements. The steady-state fluorescence demonstrates that the fluorescence is a distinctive physical observable for detection and selective identification of 5-and 5,6benzyluracil. The principal electronic decay paths of the two molecules, obtained through TDDFT calculations, explain the features observed in the emission spectra and the photoreactivity of 5-benzyluracil. The order of magnitude of the lifetime of the excited states is derived with steady-state fluorescence anisotropy measurements and rationalized with the help of the computational findings. Finally, the spectroscopic data collected are used to derive the photocyclization and fluorescence quantum yields. In obtaining a global picture of the photophysical and photochemical properties of the two molecules, our findings demonstrates that the use of 5-benzyluracil as a model system to study the proximity relations of a DNA base with a close-lying aromatic amino acid is valid at a local level since the main characteristics of the decay processes from the excited states of the uracil/thymine molecules remain almost unchanged in 5-benzyluracil, the main perturbation arising from the presence of the close-lying aromatic group.
A numerical study is reported concerning the first and second singlet excited-states of 5-benzylu... more A numerical study is reported concerning the first and second singlet excited-states of 5-benzyluracil using the multireference self-consistent field (state-averaged CASSCF) method. The vertical excitation energies of low-lying excited-states were characterized using the SA-CASSCF method, as well as using higher-level methods, such as CASPT2, MRCI and EOM-CCSD. The local minima and conical intersections found on the potential energy surfaces (PESs) were characterized in terms of molecular geometry and natural population analysis. Different relaxation pathways on the PESs are identified and discussed by comparing with the similar pathways found for the individual monomers of uracil and benzene. The molecule can be thought of as a model system for the study of crosslink reaction between DNA and proteins induced by UV light. state) that could compete with the crosslink. The system being amenable to simulation by the use of modern computational methods, we set out to determine the molecular structure, the energy of electronic excited states and the potential energy surfaces (PESs) of states potentially involved in the interaction of 5BU with a UV femtosecond laser pulse. In particular, this will be done for the ground state and the first two singlet excited electronic states of 5BU using calculations based on geometry optimization at the multireference self-consistent field (state-averaged CASSCF) level of theory. Special care should be taken on the search of noticeable points on the investigated PESs, i.e. local minima and conical intersections that can lead to ultrafast non-radiative decay processes. In the proximity of such points the dynamics of the process would involve more electronic states with consequent cross-talk and quantum interference between different decay paths. In addition, since the saturated methylene bridge hinders the electronic communication between components of 5BU via the p systems, the description of the excited states of the uracil base is expected to contribute to the growing knowledge of the photophysical properties of substituted nucleic acid bases. 10
Chemical Physics Letters, 2005
ABSTRACT
International Journal of Mass Spectrometry, 2007
Methyl tert-butyl ether (MTBE) is the most widely used additive for gasoline as it is able to inc... more Methyl tert-butyl ether (MTBE) is the most widely used additive for gasoline as it is able to increase the oxygen content and to improve the efficiency in the internal combustion engine, controlling the autoignition.
Chemical Physics Letters, 2005
Corrigendum to ÔDissociative electron impact ionization of methyl tert-butyl ether: Total ionizat... more Corrigendum to ÔDissociative electron impact ionization of methyl tert-butyl ether: Total ionization cross-section and kinetic energy distributionsÕ [Chem. Phys. Lett. 400 (2004) 191-195] In on p. 194, there is an error in the value of thermal energy of the ion at m/z = 27 (0.045 eV). The corrected value is 0.45 eV. The conclusions remain unchanged.
Laser Chemistry, 1988
We present a very simple theoretical model aimed at the analysis of non-linear relaxation process... more We present a very simple theoretical model aimed at the analysis of non-linear relaxation processes in molecular gases in the presence of partial molecular dissociation induced by vibrational-vibrational exchange between highly excited molecules. The model has a phenomenological character, since it analyzes the behavior of a system of anharmonic diatomic molecules, which is a very rough approximation of a polyatomic molecule such as SF6. Nonetheless, it provides an interesting key for the interpretation of a number of peculiar features characterizing our experimental observation, with which a comparison is made. In particular, the model takes realistic account of the influence of dissociation processes on the relaxation time.
Journal of Modern Optics, 2005
The strong-field response of molecules exhibits interference effects due to the geometric and ele... more The strong-field response of molecules exhibits interference effects due to the geometric and electronic structure of the molecules and provides a basis for ultrafast imaging of molecular structure. This is demonstrated for high-order harmonic generation and high-order above-threshold ionization in aligned molecules by numerical solution of the time-dependent Schrödinger equation. Experimental and theoretical results for high-order harmonic generation with aligned CO2 molecules show that the harmonics exhibit an orientation dependence that is explained by the valence orbital symmetry. A detailed discussion of phase-matching effects due to the presence of different molecular orientations in an ensemble of imperfectly aligned molecules is presented.
Physical Review A, 2002
We demonstrate high-order harmonic generation in high-density vapors of laser-aligned molecules. ... more We demonstrate high-order harmonic generation in high-density vapors of laser-aligned molecules. Ensembles of aligned CS2 and N2 are formed in the focus of a 300 ps duration laser pulse with sufficient density (~1017 molecules cm-3) to enable efficient high-order harmonic generation by a second, 70-fs, high-intensity laser pulse. We are able to modulate and significantly enhance the harmonic intensity in
Physical Review A, 2004
High-order harmonic generation has been explored theoretically and experimentally in CO2 , a line... more High-order harmonic generation has been explored theoretically and experimentally in CO2 , a linear molecule characterized by doubly antisymmetric highest-occupied molecular orbitals. For the first time to our knowledge, the high-harmonic yield is studied as a function of the angle between the molecular axis and the polarization direction. A minimum yield is found at 0° for all harmonics, which constitutes
Journal of Modern Optics, 2003
We review recent work examining electron dynamics and highorder harmonic generation (HHG) in mole... more We review recent work examining electron dynamics and highorder harmonic generation (HHG) in molecules exposed to high intensity (10 13 -10 15 Wcm À2 ) ultrashort pulse laser fields. The use of adiabatic laser alignment techniques, using relatively long duration (300 ps) laser pulses of moderately high intensity (10 11 -10 12 Wcm À2 ) enabled us to perform the first experiments investigating the role of the angle between the laser polarization vector and the molecular axis. We have shown that alignment of a sample can enhance the harmonic yield by a small but significant factor. HHG in model H 2 and H þ 2 molecules was treated theoretically by numerical integration of the Schro¨dinger equation. The unique influence of the dipole phase in molecular HHG is clearly demonstrated and is shown to offer a consistent explanation for our experimental observations. The electron wavepacket dynamics exhibit strong interference effects that result in the reduction or enhancement of harmonic yield at certain alignment angles. Experiments to further investigate these effects and to increase the modulation of the harmonics by molecular alignment are discussed.
Sensors, 2011
A monolithic silica gel matrix with entrapped glucose oxidase (GOD) was constructed as a bioactiv... more A monolithic silica gel matrix with entrapped glucose oxidase (GOD) was constructed as a bioactive element in an optical biosensor for glucose determination. Intrinsic fluorescence of free and immobilised GOD was investigated in the visible range in presence of different glucose concentrations by time-resolved spectroscopy with time-correlated single-photon counting detector. A three-exponential model was used for analysing the fluorescence transients. Fractional intensities and mean lifetime were shown to be sensitive to the enzymatic reaction and were used for obtaining calibration curve for glucose concentration determination. The sensing system proposed achieved high resolution (up to 0.17 mM ) glucose determination with a detection range from 0.4 mM to 5 mM .
Examples are given which prove the ICARUS detector quality through relevant physics measurements.... more Examples are given which prove the ICARUS detector quality through relevant physics measurements. We study the µ decay energy spectrum from a sample of stopping µ events acquired during a also at IFSI del CNR, sezione presso LNF.
Visible and near-infrared dyes are largely used in diagnostics and sensing. For this reason, it i... more Visible and near-infrared dyes are largely used in diagnostics and sensing. For this reason, it is very important to study their time-resolved fluorescence in presence or in absence of proper scattering medium in order to simulate the optical characteristics of biological tissues. Moreover, if one- or two-photon excitation processes are available also visible dyes will be employed taking advantages from using exciting sources in the diagnostic window (red and near IR) of the electromagnetic spectrum, where the photons are rarely absorbed and more often scattered. Visible and near IR fluorescent samples (Indocyanine Green and Rhodamine 6G) in absence and in presence of scattering agents (different Intralipid concentrations) and one- and two- photon time-resolved experiments have been performed. As expected, the presence of scattering agents modified time-resolved spectra and the related lifetime components. The experimental results have been used to preliminarly test different theoretical approaches describing the propagation of fluorescence signals in scattering media.
Applied Physics Letters, 2000
We have investigated the emission of positive ions from metallic targets irradiated with intense,... more We have investigated the emission of positive ions from metallic targets irradiated with intense, ultrashort laser pulses (~120 fs) at 780 nm, in both S and P polarized states. The measured energy spectra show the presence of a nonthermal, high-energy (several keV) ion component accompanying low-energy ions (tens of eV) produced by a thermal mechanism. The yield of the high-energy component shows a strong dependence on both laser fluence and light polarization. For the low-energy component a higher ablation efficiency was observed for P polarization, and ascribed to a more effective absorption mechanism active during the laser-target interaction.
In this work we will show that such techniques can be implemented with an ultrabroadband optical ... more In this work we will show that such techniques can be implemented with an ultrabroadband optical parametric amplifier operating in the mid-IR and generating multi-cycle pulses, thus avoiding the use of a few-cycle Ti:Sapphire laser source and, thanks to the favorable cutoff scaling law for mid-IR drivers, paving the way to the production of IAPs well above the 100-eV photon energy.
Physical Review A, 2004
The dynamics of a two-dimensional H 2 + molecule interacting with a strong laser pulse beyond the... more The dynamics of a two-dimensional H 2 + molecule interacting with a strong laser pulse beyond the usual approximation of fixed nuclei is presented. The motion of the nuclei is studied by using the classical Newton laws while the electron is described with a full quantal treatment. The axis of the molecule, initially not aligned to the laser field, performs a long-period pendular motion around the laser polarization axis. Vibrational degrees of freedom are seen to be excited. The radiation emitted by the oscillating charges presents variations that are synchronous to the pendular motion. The possibility of monitoring the motion of the molecule through the emitted radiation is discussed.
Journal of Applied Physics, 1994
ABSTRACT
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Papers by Raffaele Velotta