Stereodynamics of Ne(3P2) reacting with Ar, Kr, Xe, and N2

Stereodynamics experiments of Ne(3P2) reacting with Ar, Kr, Xe, and N2 leading to Penning and associative ionization have been performed in a crossed molecular beam apparatus. A curved magnetic hexapole was used to state-select and polarize Ne(3P2) atoms which were then oriented in a rotatable magnetic field and crossed with a beam of Ar, Kr, Xe, or N2. The ratio of associative to Penning ionization was recorded as a function of the magnetic field direction for collision energies between 320 cm−1 and 500 cm−1. Reactivities are obtained for individual states that differ only in Ω, the projection of the neon total angular momentum vector on the inter-particle axis. The results are rationalized on the basis of a model involving a long-range and a short-range reaction mechanism. Substantially lower probability for associative ionization was observed for N2, suggesting that predissociation plays a critical role in the overall reaction pathway

Study of He*/Ne*+Ar, Kr, N2, H2, D2 Chemi-Ionization Reactions by Electron Velocity-Map Imaging

The chemi-ionization of Ar, Kr, N2, H2, and D2 by Ne(3P2) and of Ar, Kr, and N2 by He(3S1) was studied by electron velocity map imaging (e-VMI) in a crossed molecular beam experiment. A curved magnetic hexapole was used to state-select the metastable species. Collision energies of 60 meV were obtained by individually controlling the beam velocities of both reactants. The chemi-ionization of atoms and molecules can proceed along different channels, among them Penning ionization and associative ionization. The evolution of the reaction is influenced by the internal redistribution of energy, which happens at the first reaction step that involves the emission of an electron. We designed and built an e-VMI spectrometer in order to investigate the electron kinetic energy distribution, which is related to the internal state distribution of the ionic reaction products. The analysis of the electron kinetic energy distributions allows an estimation of the ratio between the two-reaction channel Penning and associative ionization. In the molecular cases the vibrational or electronic excitation enhanced the conversion of internal energy into the translational energy of the forming ions, thus influencing the reaction outcome

Quantum-state-controlled channel branching in cold Ne(3P2)+Ar chemi-ionization

A prerequisite to gain a complete understanding of the most basic aspects of chemical reactions is the ability to perform experiments with complete control over the reactant degrees of freedom. By controlling these, details of a reaction mechanism can be investigated and ultimately manipulated. Here, we present a study of chemi-ionization—a fundamental energy-transfer reaction—under completely controlled conditions. The collision energy of the reagents was tuned from 0.02 K to 1,000 K, with the orientation of the excited Ne atom relative to Ar fully specified by an external magnetic field. Chemi-ionization of Ne(3P2) and Ar in these conditions enables a detailed investigation of how the reaction proceeds, and provides us with a means to control the branching ratio between the two possible reaction outcomes. The merged-beam experimental technique used here allows access to a low-energy regime in which the atoms dynamically reorient into a favourable configuration for reaction, irrespective of their initial orientations

Crossed molecular beams and theoretical studies of the O(3P)+ 1,2-butadiene reaction: Dominant formation of propene+CO and ethylidene+ketene molecular channels

Detailed understanding of the mechanism of the combustion relevant multichannel reactions of O( 3 P) with unsaturated hydrocarbons (UHs) requires the identification of all primary reaction products, the determination of their branching ratios and assessment of intersystem crossing (ISC) between triplet and singlet potential energy surfaces (PESs). This can be best achieved combining crossed-molecular-beam (CMB) experiments with universal, soft ionization, mass-spectrometric detection and time-of-fiight analysis to high-level ab initio electronic structure calculations of triplet/singlet PESs and RRKM/Master Equation computations of branching ratios (BRs) including ISC. This approach has been recently demonstrated to be successful for O( 3 P) reactions with the simplest UHs (alkynes, alkenes, dienes) containing two or three carbon atoms. Here, we extend the combined CMB/theoretical approach to the next member in the diene series containing four C atoms, namely 1,2-butadiene (methylallene) to explore how product distributions, branching ratios and ISC vary with increasing molecular complexity going from O( 3 P)+propadiene to O( 3 P)+1,2-butadiene. In particular, we focus on the most important, dominant molecular channels, those forming propene+CO (with branching ratio ∼0.5) and ethylidene+ketene (with branching ratio ∼0.15), that lead to chain termination, to be contrasted to radical forming channels (branching ratio ∼0.35) which lead to chain propagation in combustion systems. © 2019 Chinese Physical Society

Raccomandazioni cliniche in odontostomatologia

L’Organizzazione Mondiale della Sanità considera la salute orale come parte integrante, non solo dello stato di salute generale, ma anche della qualità della vita di ciascun individuo. In tale ottica, in accordo e con il sostegno del mondo professionale e della comunità scientifica di questo settore, le raccomandazioni cliniche in odontostomatologia contenute nel presente volume forniscono indicazioni e definiscono standard di intervento per la prevenzione e la cura delle più comuni patologie del cavo orale nonché per l’identificazione di percorsi terapeutici appropriati a supporto degli operatori pubblici e privati. Esse costituiscono, inoltre, uno strumento utile per mantenere alto il livello di qualità delle cure in questo periodo storico, nel quale, se da un lato crescono le opportunità tecniche di cura e la potenziale domanda delle stesse, dall’altro la riduzione delle risorse economiche a disposizione dei cittadini tende a limitare l’accesso all’offerta professionale privata e, al contempo, i processi di razionalizzazione dell’allocazione dei fondi nel SSN rendono più difficile l’accesso alle strutture pubbliche. L’individuazione di raccomandazioni cliniche chiaramente definite e rese pubbliche può facilitare la condivisione degli obiettivi e dei percorsi terapeutici tra odontoiatra e paziente, migliorare la comunicazione e, in ultima analisi, accrescere la fiducia sia nei riguardi del proprio specifico professionista che del “sistema delle cure” più in generale. La revisione delle “Raccomandazioni cliniche in odontostomatologia” si è resa necessaria in considerazione del cambiamento dell’evidenza scientifica, dell’immissione in commercio di nuovi materiali e dell’utilizzo di nuove tecnologie, specie in ambito protesico. Come per la precedente, alla realizzazione di questa edizione ha contribuito un ampio gruppo di docenti ed esperti delle singole branche odontoiatriche insieme alle più importanti e rappresentative Associazioni professionali e ai componenti della Commissione Albo Odontoiatri (CAO) nazionale