Michele Alagia

Crossed beam studies of four‐atom reactions: The dynamics of OH+CO

The reaction OH+D2→HOD+D was studied in a crossed beams experiment at a collision energy of 6.3 kcal/mol. Center‐of‐mass translational energy and angular distributions were determined. The HOD product shows a very pronounced backward scattering. Thirty‐two percent of the available energy is released as product translational energy.

Electronic structure of copper phthalocyanine: An experimental and theoretical study of occupied and unoccupied levels

An experimental and theoretical study of the electronic structure of copper phthalocyanine (CuPc) molecule is presented. We performed x-ray photoemission spectroscopy (XPS) and photoabsorption [x-ray absorption near-edge structure (XANES)] gas phase experiments and we compared the results with self-consistent field, density functional theory (DFT), and static-exchange theoretical calculations. In addition, ultraviolet photoelectron spectra (UPS) allowed disentangling several outer molecular orbitals. A detailed study of the two highest occupied orbitals (having a(1u) and b(1g) symmetries) is presented: the high energy resolution available for UPS measurements allowed resolving an extra feature assigned to vibrational stretching in the pyrrole rings. This observation, together with the computed DFT electron density distributions of the outer valence orbitals, suggests that the a(1u) orbital (the highest occupied molecular orbital) is mainly localized on the carbon atoms of pyrrole rings and it is doubly occupied, while the b(1g) orbital, singly occupied, is mainly localized on the Cu atom. Ab initio calculations of XPS and XANES spectra at carbon K edge of CuPc are also presented. The comparison between experiment and theory revealed that, in spite of being formally not equivalent, carbon atoms of the benzene rings experience a similar electronic environment. Carbon K-edge absorption spectra were interpreted in terms of different contributions coming from chemically shifted C 1s orbitals of the nonequivalent carbon atoms on the inner ring of the molecule formed by the sequence of CN bonds and on the benzene rings, respectively, and also in terms of different electronic distributions of the excited lowest unoccupied molecular orbital (LUMO) and LUMO+1. In particular, the degenerate LUMO appears to be mostly localized on the inner pyrrole ring.

Crossed molecular beams and quasiclassical trajectory studies of the reaction O(1D)+H2(D2)

The dynamics of the reactions O(1D)+H2→OH+H and O(1D)+D2→OD+D have been investigated in crossed molecular beam experiments with mass spectrometric detection at the collision energies of 1.9 and 3.0 kcal/mol, and 5.3 kcal/mol, respectively. From OH(OD) product laboratory angular and velocity distribution measurements, center-of-mass product translational energy and angular distributions were derived. The angular distributions are nearly backward–forward symmetric with a favored backward peaking which increases with collision energy. About 30% of the total available energy is found to be channeled into product translational energy. The results are compared with quasiclassical trajectory calculations on a DIM (diatomic-in-molecules) potential energy surface. Related experimental and theoretical works are noted. Insertion via the 1 1A′ ground state potential energy surface is the predominant mechanism, but the role of a second competitive abstraction micromechanism which should evolve on one of (or both) the ...

Dynamics of the Simplest Chlorine Atom Reaction: An Experimental and Theoretical Study

Angular distributions and time-of-flight spectra for the reaction Cl + H2 → HCl + H obtained from a high-resolution, crossed-molecular beam experiment were compared to differential cross sections calculated by both converged quantum mechanical scattering and quasi-classical trajectory methods. Good agreement was found between the experimental results and each theoretical prediction. The results demonstrate that excellent agreement can be obtained between state-of-the-art simulations and experiments for the detailed dynamical properties of this prototype chlorine atom reaction.

Comparative dynamics of Cl(2P) and O(3P) interactions with a hydrocarbon surface

The dynamics of the interactions of atomic chlorine with the surface of a saturated hydrocarbon liquid, squalane, were investigated and compared to the results of an earlier study on analogous oxygen-atom interactions. Beams of continuous supersonic chlorine atoms were directed onto a squalane surface, and the volatile products, Cl and HCl, were observed by mass spectrometry as a function of incident angle, final angle, and incident Cl-atom energy. Both the Cl and HCl time-of-flight (from the surface to the detector) distributions revealed thermal and hyperthermal interaction channels, in analogy to the dynamical behavior of the O and OH signals observed in the previous study. The thermal HCl product may arise from two mechanisms: (i) desorption of trapped HCl product and (ii) reaction of trapped Cl atoms to form thermal HCl, which subsequently desorbs. In contrast, the reaction of O atoms with squalane led to a thermal OH signal, which could only come from desorption of trapped OH. The hyperthermal HCl s...

Exploring the reaction dynamics of nitrogen atoms: A combined crossed beam and theoretical study of N(2D)+D2→ND+D

In the first successful reactive scattering study of nitrogen atoms, the angular and velocity distribution of the ND product from the reaction N(2D)+D2 at 5.1 and 3.8 kcal/mol collision energies has been obtained in a crossed molecular beam study with mass spectrometric detection. The center-of-mass product angular distribution is found to be nearly backward–forward symmetric, reflecting an insertion dynamics. About 30% of the total available energy goes into product translation. The experimental results were compared with those of quasiclassical trajectory calculations on an accurate potential energy surface obtained from large scale ab initio electronic structure computations. Good agreement was found between the experimental results and the theoretical predictions.

Reactive scattering of atoms and radicals

Dynamical studies of elementary gas-phase bimolecular reactions have progressed significantly during the last few years owing to advancements in molecular beam and laser techniques as well as in theoretical methodologies. In this article we give a brief overview of the recent progress in the field of reaction dynamics and then survey recent work from our laboratory on reactions of atoms and radicals with simple molecules by the crossed molecular beam scattering method using mass-spectrometric detection. Emphasis is on three-atom (A + BC) and four-atom (AB + CD) reactions for which the interplay between experiment and theory is the strongest and the most detailed. Reactive differential cross-sections for the three-atom Cl + H2 and four-atom OH + H2 and OH + CO reactions are presented and compared with the results of quasiclassical and quantum-mechanical scattering calculations on ab initio potential-energy surfaces in an effort to assess the status of theory versus experiment. The reaction dynamics of electronically excited atoms are discussed too; the effect of electronic excitation on the reaction dynamics of atomic oxygen is examined using the reaction O(3P, 1D)+ H2S as an example.

Valence photoionization dynamics in circular dichroism of chiral free molecules: The methyl-oxirane

The dynamical behavior of circular dichroism for valence photoionization processes in pure enantiomers of randomly oriented methyl-oxirane molecules has been studied by circularly polarized synchrotron radiation. Experimental results of the dichroism coefficient obtained for valence photoionization processes as a function of photon energy have been compared with theoretical values predicted by state-of-the-art ab initio density-functional theory. The circular dichroism measured at low electron kinetic energies was as large as 11%. Trends in the experimental dynamical behavior of the dichroism coefficients D(i)(omega) have been observed. Agreement between experimental and theoretical results permits unambiguous identification of the enantiomer and of the individual orbitals.

Core-shell photoabsorption and photoelectron spectra of gas-phase pentacene: experiment and theory.

The C K-edge photoabsorption and 1s core-level photoemission of pentacene (C22H14) free molecules are experimentally measured, and calculated by self-consistent-field and static-exchange approximation ab initio methods. Six nonequivalent C atoms present in the molecule contribute to the C 1s photoemission spectrum. The complex near-edge structures of the carbon K-edge absorption spectrum present two main groups of discrete transitions between 283 and 288 eV photon energy, due to absorption to pi* virtual orbitals, and broader structures at higher energy, involving sigma* virtual orbitals. The sharp absorption structures to the pi* empty orbitals lay well below the thresholds for the C 1s ionizations, caused by strong excitonic and localization effects. We can definitely explain the C K-edge absorption spectrum as due to both final (virtual) and initial (core) orbital effects, mainly involving excitations to the two lowest-unoccupied molecular orbitals of pi* symmetry, from the six chemically shifted C 1s core orbitals.

A modular end-station for atomic, molecular, and cluster science at the low density matter beamline of FERMI@Elettra

The low density matter end-station at the new seeded free electron laser FERMI@Elettra is a versatile instrument for the study of atoms, molecules and clusters by means of electron and ion spectroscopies. Beams of atoms, molecules and helium droplets as well as clusters of atoms, molecules and metals can be produced by three different pulsed valves. The atomic and molecular beams may be seeded, and the clusters and droplets may be pure, or doped with other atoms and molecules. The electrons and ions produced by the ionization and fragmentation of the samples by the intense light of FERMI can be analysed by the available spectrometers, to give mass spectra and energy as well as angular distributions of charged particles. The design of the detector allows simultaneous detection of electrons and ions using velocity map imaging and time-of-flight techniques respectively. The instruments have a high energy/mass resolution and large solid-angle collection efficiency. We describe the current status of the apparatus and illustrate the potential for future experiments.