Network Overview

Project Objectives 
Research Method
Work Plan

Experimental Techniques
Theoretical Methods

Task 1
Task 2
Task 3


1. Project Objectives

The major objectives of the proposed Network are to; (i) Train young researchers in the latest experimental and theoretical techniques for studying electron/positron induced chemistry by providing them with the opportunity to undertake research within internationally renowned research centres. (ii) Provide the most co-ordinated study ever undertaken, either experimental or theoretical, of the fundamental chemistry initiated by the irradiation by electrons and positrons of molecules (from diatomic to macromolecules) and (iii) To develop a new understanding of the basic processes by which such chemical reactions are induced and how such reactions are modified by their local environment. 

In particular the Network will: 

1. Study fundamental electron and positron energy transfer processes. 
2. Study electron and positron impact dissociation processes yielding reactive neutral, anionic and cationic species. 
3. Study how such processes are modified in the different phases of matter (gases, clusters and on surfaces). 
4. Probe new reaction processes initiated in denser phases of matter (e.g nucleophilic substitution). 

Different molecular compounds have been selected to illustrate these processes: halogenated hydrocarbons and fluorocarbons; simple biomolecules (e.g. DNA and RNA bases) and water. 

2. Research Method

Experimental techniques 

Authoritative investigations of electron/positron-molecule interactions requires the adoption of several different experimental methods. Within this Network all the current state-of-the-art experimental techniques for probing electron/positron induced reactions with molecules are available to the researchers. For example the UAR group has produced the first synchrotron based photo-ionisation electron source, a source that is capable of producing electron beams with resolutions of 1 meV at incident energies as low as 5 meV. With such a source it is possible to study electron impact induced rotational excitation and makes it possible to probe threshold phenomena at the onset of each mode of molecular excitation. These experiments are complemented by the recent development of new instruments by UIBK and FUB capable of producing low energy, high resolution electron beams. In addition the first analysis of product neutral fragments produced by electron impact dissociation is being undertaken at OU using laser spectroscopic analysis to determine the internal energy of the fragments. 

OU, UIBK, FUB, UPSO have also developed a variety of experimental methods for probing: 
  (i) dissociative electron attachment (resulting in negative ion fragments), 
  (ii) dissociative ionisation (producing positive ions) and 
  (iii) dissociative excitation into neutral but internally excited fragments. 
The fragmentation patterns of the dissociative processes are studied by mass spectrometric analysis of the collision products. The kinetic energy of the fragments being determined by translational spectroscopy. 

The UCLP team has developed corresponding methods for the investigation of single and multiply charged ionization (with and without positronium formation) including the first near-threshold measurements and the first complete (e,2e) type experiments with positrons. Techniques for generating positronium beams have also been developed and applied to the study of the interaction of simple atomic and molecular targets. Methods for extending these to more complex targets and probing inelastic interactions are currently under development. 

Experimental teams in Berlin (FUB) and Paris (UPSO) have pioneered the transfer of gaseous phase experimental and theoretical techniques into the study of electron molecule collisions upon surfaces and are recognized as leaders in this field. In contrast the study of positron interactions in the condensed phase (ULRS) under single collision conditions has yet to be explored and this network will provide the first opportunity for developing techniques (both experimental and theoretical) to initiate such studies. 

Theoretical Methods 

The OU/UCLP team have led the modern development of the R-Matrix method for treating electron interactions with molecules. For diatomic molecules the R-matrix method has been extended to the nuclear coordinate which has allowed for the first time full, ab initio non-adiabatic treatments of vibrational excitation and dissociative electron attachment. OU and UCLP have recently developed a novel procedure for treating near-threshold electron impact dissociation. In addition this group has recently extended their code to treat polyatomic molecules. This work is complemented by the team in Bonn (sub-node of FUB) who use a similar formalism. URLS have long employed methods based on single centre expansions to study electron interactions with polyatomic molecules. Such calculations may be used to study larger molecules still beyond the scope of the R-matrix method. Furthermore URLS have considerable experience of performing vibrational and rotational excitation calculations on polyatomics. OU, UCLP and URLS will collaborate in studies to provide cross sections for those targets for which experiments remain too difficult. The transfer of electron scattering methodologies from the gaseous phase to the treatment of inelastic electron scattering on molecules physisorbed on metal surfaces has been pioneered by the UPSO team. 

The group at URLS have developed several dedicated scattering codes for treating the quantum dynamics of low-energy positron beams in a molecular gas, specifically one containing polyatomic molecules. These codes are now to be shared and developed jointly with ICP. This Network therefore contains the only research teams currently able to evaluate annihilation rates in polyatomic gases and who are able to evaluate total and differential elastic cross sections below the Ps formation threshold. 

3. Work Plan 

The work plan is set out in terms of three inter-linked scientific strands namely the study of : 
Electron and positron induced reactions in the gaseous phase. 
Reactions in molecular clusters and aggregates and 
Reactions in the condensed phase. 

For each of these strands four specific molecular targets have been chosen since these provide the opportunity to determine the reactivity as a function of particular site and/or chemical bond. The targets selected are : 

Halogenated hydrocarbons (CHxRy where R is any halogen species F, Cl, Br and I and x +y = 4 and the fluorocarbons (CFxRy where R is Br,Cl (or H) and x +y = 4).  

Simple bio-molecules that form the bases of DNA and RNA (adenine, cytosine, guanine, thymine and uracil) often called the building blocks of life. 

  Water, the universal solvent. 

Collaborative research programmes in each task will be coordinated by a designated strand leader. Milestones/deliverables, are given in Table 3.1 with Tables 3.2 and 3.3 giving the research effort devoted by each team and the collaborative structure respectively. 

Task 1. Electron and positron induced reactions in the gaseous phase 

Experimental Programme: The different fragmentation patterns induced by low energy electrons and positrons will be explored for the five membered halogenated hydrocarbons (CHxRy where R is any halogen) and fluorocarbons (CFxRy where R is Br,Cl or H). Yields of both anionic and cationic fragments will be monitored together with their kinetic energy. Total electron scattering cross sections will likewise be compared for these targets since at low energy (< 1eV) such cross sections provide a complementary means of determining the probability of a specified molecule to undergo dissociative attachment. In addition positron interactions with water will be measured. 

Secondly using target sources developed at UIBK we will study electron (and possibly positron) induced anionic and cationic production from the bio-molecule adenine, cytosine, guanine, thymine and uracil. 

Thirdly we will measured compare electron and positron fragmentation patterns in gaseous phase water and for the first time study positronium (a bound complex of electron and positron) interactions with the water molecule. 

Theoretical Programme: Methodology and general purpose codes, built upon existing codes constructed by teams in the network, will be developed to study fundamental electron interactions with halogenated hydrocarbons and fluorocarbons leading to excitation and dissociation. 

The formation of temporary anion states leading to fragmentation of halogenated hydrocarbons and fluorocarbons 

Electron and positron induced dissociation of the water molecule. 

Task 2. Reactions in clusters and aggregates 

The goals of this research programme are to compare the electron induced reactions in the gas phase with those observed in simple cluster or aggregates and to determine the importance of intra-cluster scattering phenomena (electron transfer between the cluster components) in both primary and secondary chemical reactions. Preparing clusters of halogenated hydrocarbons and fluorocarbons we propose to study the role of such reactions as a function of cluster size and chemical composition. These studies will then be extended to pioneering studies of electron attachment to bio-molecules and to targets consisting of a bio-molecule clustered either with water or simple organic molecules. The methodology for extending of such studies using positrons will also be investigated. 

Task 3. Reactions in the condensed phase 

Electron induced reactions from molecules absorbed upon surfaces will be studied both theoretically and experimentally. In particular the network will study: 

Experimental studies on the formation and evolution of negative ion resonances in electron scattering from halogenated hydrocarbons and fluorocarbons absorbed on either metallic or ice surfaces. 

Theoretical modelling of these systems will be developed using a new R-matrix method to investigate how the environment influences the molecular inelastic scattering properties i.e. vibrational excitation, electronic excitation and dissociative attachment processes. 

Investigation for extending techniques developed for electron studies to using positrons to probe surfaces.