Reprints of recent selected publications

An Intermolecular potential for Nitrogen from a multi-property analysis
 Molecular Physics, 93, 485, (1998).
D.Cappelletti, F.Vecchiocattivi, F.Pirani, E.L.Heck and A.S.Dickinson

New measurements are reported for the velocity dependence of the integral
cross-section for N2-N2 scattering showing the glory oscillations.
These data are analysed along with recent high precision second-virial
coefficient data and traditional transport property measurements to yield an
improved N2-N2 potential energy surface by adjusting certain
parameters in the fit to this surface by van der Avoird et al. The new
potential gives improved agreement with the integral cross section, virial and transport data.
Transport and relaxation cross-sections sensitive to
anisotropy are very close to those obtained previously using the van der Avoird et al.
surface. New comparisons with rotational relaxation and NMR relaxation data
are reported.

Production and magnetic analysis of beams of S atoms and SO radicals.
A collisional study of their interactions with hydrogen molecules
International Journal of Mass spectrometry and Ion Processes, 179/180, 67 (1998).
Vincenzo Aquilanti, Daniela Ascenzi, Elisabetta Braca, David Cappelletti, and Fernando Pirani

An intense and stable continuous beam of S atoms and SO radicals has been produced with a microwave discharge source operating in the torr range in mixtures of SO2 with various gases. The beam emerging from the plasma source is velocity analyzed by a mechanical velocity selector and detected by a quadrupole mass filter. Stern-Gerlach magnetic analysis indicates that both species are generated mainly in their electronic ground state and in the case of sulfur atoms with fine-structure levels populated according to their degeneracies. Total integral cross section measurements are reported, as a function of velocity in the range 1.0-2.5 km s-1, for the scattering of S(3PJ) atoms and SO (3Sigma) radicals by D2 molecules. The analysis of experimental data (cross sections and their velocity dependence, which exhibits glory interference patterns) allows a characterization of the spherically average component of the interaction potential for the investigated systems. A discussion of the effect of the Sigma-Pi splitting, spin-orbit and electrostatic (including quadrupole-quadrupole) interaction on the dependence of the hydrogen molecule orientation is given for the S-H2 potential energy surface.

Scattering of Aligned Molecules. The Potential Energy Surfaces for the Kr-O2 and Xe-O2 Systems.
Journal of Chemical Physics, 109, 3898 (1998)
Vincenzo Aquilanti, Daniela Ascenzi, David Cappelletti, Miguel de Castro, and Fernando Pirani

Total integral cross sections for scattering of oxygen molecules on krypton and xenon atoms are measured in the thermal energy range, as a function of the collision energy and under a controlled alignment of the rotational angular momentum. The cooling and control of the alignment have been achieved using supersonic expansion in beams of oxygen molecules seeded in various mixtures of lighter carriers  [Nature, 371, 399 (1994)]. The analysis of the experimental results, based upon close coupling, adiabatic and semiclassical calculations of the cross sections, provides an accurate characterization in the intermediate and long intermolecular distance range of the full anisotropic potential energy surfaces for O2-Kr and O2-Xe systems. It also sheds light on some important aspects of the collision dynamics of the aligned molecules.

Quantum Interference Scattering of Aligned Molecules: Bonding in O4 and role of spin coupling.
Physical Review Letters, 82, 69 (1999)
Vincenzo Aquilanti, Daniela Ascenzi, Massimiliano Bartolomei, David Cappelletti, Simonetta Cavalli,
Miguel de Castro Vitores, Fernando Pirani

Molecular beam experiments on collisions between oxygen molecules were performed at low energy and high angular resolution to permit observation of a quantum-mechanical interference effect (the "glory"), and to accurately probe intermolecular interactions. A novel technique [V. Aquilanti et al. Nature, 371, 399 (1994)] for cooling oxygen to the lowest vibro-rotational state and for aligning the rotational angular momentum, allows the control of the relative orientation of the colliding molecules [V.Aquilanti et al., Phys. Rev. Lett., 74, 2929 (1995)]. Analysis of scattering cross section data yields for the O2-O2 bond an energy of 1.65 +- 0.08 kJ mol-1 for the most stable configuration which is established to be for the two molecules parallel at a distance of 0.356 + -0.007 nm. Dependence on relative orientations for the ground singlet state surface and for the excited triplet and quintet states is also obtained. These results indicate that most of the bonding in the dimer comes from electrostatic (van der Waals) forces but chemical (spin-spin) contributions in this open-shell--open-shell system are not negligible. This first complete characterization of the interaction basic to the theory of weak chemical bonds, is also relevant for the spectroscopy and the dynamics of atmospheric gases, and is of interest for modeling magnetism in solid O2 and in O2 clusters.

A Quantum Mechanical View of Molecular Alignment and Cooling in Seeded Supersonic Expansions.
Journal of Chemical Physics, 111, 2620 (1999).
Vincenzo Aquilanti, Daniela Ascenzi, Miguel de Castro Vitores, Fernando Pirani, David Cappelletti

Experimental investigations on the collisional alignment of the rotational angular momentum, occurring in supersonic
seeded beams and in drift tubes, have recently documented a strong dependence
of the observed effects on the final molecular velocity.
The present investigation aims at elucidating the possible mechanisms at the molecular collision level.
Quantum state-to-state differential scattering cross sections, calculated for the prototype system
O2-He, for an interaction potential previously obtained in this laboratory,
exhibit propensities relevant to reveal nature and selective
role of the elastic and inelastic scattering events, participating in the overall
mechanisms which lead to
molecular alignment and cooling. The present analysis shows
that the dynamics of such phenomena crucially depends on the initial and final rotational state,
on the collision energy, on the involved orbital angular momentum and therefore alternative routes are possible for
molecular polarization and relaxation. These routes lead to scattering into
specific angular cones and therefore observations from different experiments provide complementary
pieces of information which, exploiting studies of various molecular systems under diverse
experimental conditions, can be correlated in a single mosaic.

Molecular beam scattering of aligned oxygen molecules. The nature of the bond in the O2-O2 dimer
Journal of American Chemical Society, 121, 10794 (1999)
Vincenzo Aquilanti, Daniela Ascenzi, Massimiliano Bartolomei, David Cappelletti, Simonetta Cavalli, Miguel de Castro Vitores, Fernando Pirani

Abstract: Molecular beam experiments are reported for collisions between oxygen molecules. Total integral cross sections have been measured as a function of the collision energy and with the control of molecular alignment. The low collision energy (in the thermal and subthermal range), and the high angular resolution permit observation of the "glory" effect, manifestation of quantum-mechanical interferenc e, which allows an accurate probe of intermolecular interactions. This first complete experimental characterization of the interaction yi elds a ground (singlet) state bond energy of 17.0 +- 0.8 meV for the most stable dimer geometry (the two oxygen m olecules lying parallel at a distance of 3.56 +- 0.07 A). Also the splittings among the singlet, the triplet and the quintet surf aces are obtained and a full representation of their angular dependence is reported via a novel harmonic expansion function al form for diatom-diatom interactions. These results indicate that most of the bonding in the dimer comes from van der Waals forces but chemical (spin-spin) contributions in this open-shell--open-shell system are not negligible (~15 % of the van der Waals component of the interaction).

Production, characterization and scattering of a sulfur atom beam:
Interatomic potentials for the rare-gas sulfides, RS (R = Ne, Ar, Kr, Xe).
Physical Chemistry Chemical Physics, 2 4081-4088 (2000).
V. Aquilanti, D. Ascenzi, E. Braca, D. Cappelletti, and F. Pirani

An intense and stable continuous  beam of S atoms and SO radicals is produced from a microwave discharge source operating in a SO2-He mixture. The S-atom beam, characterized by coupling mechanical velocity selection with magnetic analysis and detected by a quadrupole mass filter,  has been employed in scattering experiments. The transmittance across a Stern-Gerlach magnetic selector, measured mass-spectrometrically at m/e = 32 and 34 as a function of the beam velocity and of the deflecting field strength, indicates that the sulfur atoms are mainly in their electronic ground state 3Pj, with fine structure levels j=0,1,2 populated according to their degeneracies. Total integral cross sections for collisions of S (3Pj) atoms with Ne, Ar, Kr, and Xe have been measured in the beam velocity range 1.0 - 2.4 Km/s. The scattering data exhibit a glory interference effect with a quenching in the amplitude of the glory pattern which increases along the rare-gas series from Ne to Xe. The results are analysed to yield a characterization of the spherical and anisotropic components of the interaction,  providing lengths and strengths of the bonds in the six low lying states of the rare-gas sulfides NeS, ArS, KrS and XeS.

Coupling by Charge Transfer: role in bond stabilization for open-shell systems and
ionic molecules and in harpooning and proton attachment processes
Molecular Physics 98, 1749-1762 (2000).
F. Pirani, A. Giulivi, D. Cappelletti, and V. Aquilanti

Abstract: A variety of phenomena of apparently different nature can be compacted and described within a unifying picture by taking into account the role of the charge transfer interaction. Relevant information on this interaction is obtained by the analysis of bond stabilization in halides, oxides, sulfides and ionic d imers of rare--gases. Most of this information comes from recent molecular beam experiments: when combined with the analysis of processes occurring at crossings between covalent and ionic states in alkali halides it leads to the characterization of the dependence of the charge--transfer matrix element on basic physical properties of the interacting partners. The magnitude of the coupling matrix element is correlated to polarizabilities and charges. Its exponential decreasing with the intermolecular distance is given in terms of ionization potentials and electron affinities, in the spirit of the study by Grice and Herschbach [Mol. Phys., 1973, 27, 159.] on the long-range configuration interaction of ionic and covalent states. A proper representation is obtained both for the transition from van der Waals to chemical bonds and for the behaviour of different families of compounds, such as those of alkali halides and of rare-gas protonated systems. This paper is dedicated to the memory of Professor Roger Grice (1941--1998).

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