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.

Studies of cold chemistry in crossed and merged neutral beams

Silvia Tanteri

In this thesis we present the design of two setups to study low-energy collisions between neutral species: an electron velocity map imaging for chemi-ionisation reactions and a merged electrostatic guide for polar molecules scattering. In the past few years, chemi-ionisation of atoms and molecules by metastable He and Ne led to the measurement of the branching ratios between two possible reactive channels, namely Penning ionisation (PI) and Associative ionisation (AI). For molecular species, the reaction outcome is influenced by the redistribution of energy from vibrational excitation to translational energy. We designed an electron velocity map imaging spectrometer (e-VMI) to study chemi-ionisation of Kr, Ar, N2, H2 and D2 by metastable He(3S1) and Ne(3P2) atoms in a crossed beam setup. A curved magnetic hexapole was used to guide the metastable atoms, specifically selecting the He(3S1) and Ne(3P2) states. Collision energies of 60 meV were reached by individually controlling the velocities of both reactants. In these experiments, electron kinetic energy distributions obtained using the e-VMI spectrometer were related to state specific reaction products. Detecting electrons, which are released in the first reaction step, we could characterise the reaction encounter complexes. Merged beam experiments on neutral molecules allows molecular beam studies of reactive scattering below 1 K. We present the design and realization of a new merged beam setup that permits to study cold collisions between polar molecules, thus giving access to the investigation of dipole-dipole interactions in the sub-Kelvin range. The Stark effect was used to manipulate polar neutral molecules in an electrostatic guide. The electrodes structure required to merge two beams is highly complex and difficult to build by traditional methods. We used the 3D-printing metal coating approach, developed in our group, to realize a Y-shaped electrostatic guide in which two bent quadrupole guides are smoothly merging into a single hexapole guide. Simulations were performed to estimate the guide transmission as a function of the velocity in order to design the merged beams experiment. The correct functioning of the device was demonstrated by guiding ND3 molecules.

EPFL2021

Investigation of the low-energy stereodynamics in the Ne(3P2) + N2, CO reactions

Andreas Osterwalder, Junwen Zou

We report on an experimental investigation of the low-energy stereodynamics of the energy transfer reactions Ne(3P2) + X, producing Ne(1S) + X+ and [Ne–X]+ (X = N2 or CO). Collision energies in the range 0.2 K–700 K are obtained by using the merged beam technique. Two kinds of product ions are generated by Penning and associative ionization, respectively. The intermediate product [Ne–X]+ in vibrationally excited states can predissociate into bare ions (X+). The experimental ratio of the NeX+ and X+ product ion yields is similar for both molecules at high collision energies but diverge at collision energies below 100 K. This difference is explained by the first excited electronic state of the product ions, which is accessible in the case of CO but lies too high in energy in the case of N2.

2020

Etude par STM de la déposition d'agrégats d'or sur TiO2 et mesure de l'émission électronique secondaire induite par l'impact d'agrégats sur une surface

Pierre Convers

This thesis work focuses on impact and diffusion processes as well as equilibrium positions of a metal deposited on a surface. The metal is deposited in the form of clusters containing n atoms in a controlled way. Gold or silver clusters cations (Au+n and Ag+n ) are used. Their size (n ∈ [1, 9]), charge state and deposition energy (E = 10 - 4500 eV) are well defined. The surface being at room temperature during the deposition, can be annealed and transferred in a scanning tunnelling microscope (STM) after deposition. The variable temperature microscope was developed during this thesis. The approach of the tip is carried out by an axial motor, which renders the microscope very stable. The first part of this work treats of the evolution of gold deposited on a TiO2(110) surface. Position and size of gold islands created by cluster or atomic depositions are determined by STM. Compared to atomic deposition at the same energy (10 < E < 100 eV), the cluster deposition produces smaller islands with a large islands density after annealing at 800 K. Regardless the deposition conditions, gold atoms diffuse on the surface already at 300 K. The impact energy is the key parameter to reduce the diffusion by cluster or atomic implantation or defect creation. Upon a high energy deposition, part of the deposited gold is invisible to the STM. The fate of these gold atoms is unclear : they are either buried under the surface or ejected in vacuum at the impact. The control of the islands size should permit the creation of a stable system with an optimum catalytic activity (i. e. CO combustion). Research groups have shown that this activity is strongly influenced by the islands size. It is equal to zero if the diameter is greater than 5 nm. The second part of this work is devoted to the electron emission γ induced by the impact of silver clusters Ag+n on a Pt and HOPG surface. Measurements are made by varying the impact energy (and so the speed v) of size-selected clusters on a defined surface. Impact velocity ranges from 104 to 105 m/s, which is lower than the classical threshold. Nevertheless every cluster size produces an electron emission. A potential emission (γ(v=0) ≠ 0) is observed for the monomer on both surfaces, which is caused by excited ions in a metastable state. The γ(v) curves are increasing, and no mesurable oscillation are observed. This does not confirm earlier work by Meiwes-Broer et al. These authors found oscillations in γ on similar systems relating them to the electronic structure of the clusters and the surface. A model based on heating of the electron gas is developed. This model gives good agreements with the γ(v) curves. The electronic temperature is estimated to 3000-8000 K. Similar behavior on both surfaces (Pt and HOPG) depending of the cluster size is shown. This behavior is probably related to the geometrical structure of the clusters. Finally, a more pronounced molecular effect is observed for the HOPG : the number of electrons emitted by the impact of a Ag+n is up to 7 times higher than the number of electrons emitted by impacts of n independent atoms. This value is smaller than 2 for the Pt surface.