Fragmentation studies with stored beams of small polyatomic ions [Elektronische Ressource] / presented by Lutz Lammich

Sujets

• Physik

Informations

• Publié par: ruprecht-karls-universitat_heidelberg
• Publié le: 01 janvier 2004
• Nombre de lectures: 24
• Langue: Deutsch
• Poids de l'ouvrage: 2 Mo

Extrait

Dissertation
submitted to the
Combined Faculties for the Natural Sciences and for Mathematics
of the Ruperto-Carola University of Heidelberg, Germany
for the degree of
Doctor of Natural Sciences
presented by
Diplom-Physicist Lutz Lammich
born in Ludwigshafen am Rhein
Oral examination: 12. May 2004Fragmentation Studies with Stored Beams
of Small Polyatomic Ions
Referees: Prof. Dr. Andreas Wolf
Prof. Dr. H.-Jur¨ gen KlugeKurzfassung
Fragmentations-Studien an gespeicherten Strahlen kleiner mehratomiger Molekulionen¨
Die Struktur kleiner Molekule¨ hangt¨ wie auch ihre Reaktionen eng mit den fundamentalen
Mechanismen zusammen, die allen chemischen zugrundeliegen. Experimentelle
Untersuchungen molekularer Fragmentationsprozesse liefern daher einen wichtigen Beitrag
zum detaillierten Verstandnis¨ dieser grundlegenden Mechanismen. Als geeignete Umgebung
fur¨ solche Studien an Molekulionen¨ stehen heute Schwerionen-Speicherringe zur Verfugung,¨
mit denen bisher hauptsachlich¨ zweiatomige Molekulionen¨ unter wohldefinierten Bedingun-
gen und mit effizientem Nachweis der produzierten Fragmente untersucht wurden. Fur¨
eine genaue Untersuchung mehratomiger Systeme mussen¨ die in fruheren¨ Fragmentationsstu-
dien an zweiatomigen Ionen etablierten Techniken in mehreren Aspekten erweitert werden,
hauptsachlich¨ bezuglich¨ der Identifikation der entstehenden Fragmente und der Auswertung
der hier auftretenden multi-dimensionalen Fragmentationsgeometrien. In der vorliegenden Ar-
beit wird die Erweiterung experimenteller Methoden auf dreiatomige Molekule¨ fur¨ zwei Falle¨
studiert: Fur¨ die Dissoziative Rekombination des H -Kations und seiner Isotopomere mit
langsamen Elektronen, mit dem Schwerpunkt auf Fragmentationsgeometrien und Isotopeffek-
ten, sowie die Fragmentation des LiH -Anions nach Entfernen eines Elektrons, hier mit dem
Schwerpunkt auf der chemischen Zusammensetzung der Produkte.
Abstract
Fragmentation Studies with Stored Beams of Small Polyatomic Ions
The structure of small molecules, as well as their reactions, is closely related to the funda-
mental mechanisms governing all chemical reactions. Experimental investigations of molecular
fragmentation processes thus yield important data contributing to the detailed understanding of
these basic mechanisms. As an advantageous environment for such studies on molecular ions
nowadays heavy ion storage rings are available, where so far mainly diatomic molecules were
studied under well defined conditions and with efficient detection of the fragments produced.
For a detailed investigation of polyatomic systems, the standard techniques established for pre-
vious fragmentation studies on diatomic ions have to be extended in several aspects, mainly
connected to the identification of the emerging fragments and to the analysis of the occurring
multi-dimensional breakup geometries. In this work, the extension of experimental methods on
triatomic molecules is studied for two cases: The dissociative recombination of the H cation
and its isotopomers with slow electrons, focusing on fragmentation geometries and isotope ef-
fects, and the fragmentation of the LiH anion following electron detachment, here focusing on
the chemical composition of the products.





Contents
1 Introduction 3
2 Fragmentation of molecules 5
2.1 General concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 Molecular structure: The concept of potential energy surfaces . . . . . 5
2.1.2 Fragmentation processes . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Experimental studies of molecular fragmentation . . . . . . . . . . . . . . . . 9
2.2.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.2 Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.3 Electron induced fragmentation reactions . . . . . . . . . . . . . . . . . . . . 11
2.3.1 Cations: Dissociative excitation and recombination . . . . . . . . . . . 11
2.3.2 Anions:ve e and electron detachment . . . . . . . . 15
3 Fast beam molecular fragmentation studies 18
3.1 The ion storage ring technique . . . . . . . . . . . . . . . . . . . . . . . . . . 18
3.2 Accessible properties of fragmentation reactions . . . . . . . . . . . . . . . . . 20
3.3 Experimental procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.3.1 The heavy ion storage ring TSR . . . . . . . . . . . . . . . . . . . . . 22
3.3.2 Fragment imaging measurements . . . . . . . . . . . . . . . . . . . . 25
3.3.3 Total and partial rate . . . . . . . . . . . . . . . . . . . 30
4 Dissociative recombination of H and its isotopomers 34
4.1 General background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
4.1.1 Properties and interactions of H . . . . . . . . . . . . . . . . . . . . 34
4.1.2 Fragmentation studies of the H / H system . . . . . . . . . . . . . . 36
4.1.3 Energy considerations for the H DR . . . . . . . . . . . . . . . . . . 37
4.2 Experimental setup and procedures . . . . . . . . . . . . . . . . . . . . . . . . 40
4.3 Handling and representation of imaging data . . . . . . . . . . . . . . . . . . . 40
4.3.1 Separation of reaction channels and assignment of fragment masses . . 41
4.3.2 Representation of three-body fragmentation data . . . . . . . . . . . . 45
4.3.3 Monte-Carlo simulation . . . . . . . . . . . . . . . . . . . . . . . . . 54
1




Contents
4.3.4 Simulation results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
4.4 Experimental Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
4.4.1 Kinetic energy release in the three-body channel . . . . . . . . . . . . 63
4.4.2 Breakup geometry in the channel . . . . . . . . . . . . . . 66
4.4.3 Kinetic energy release in the two-body channel . . . . . . . . . . . . . 70
4.4.4 Rate coefficient measurements . . . . . . . . . . . . . . . . . . . . . . 71
4.5 Comparison to theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.5.1 Rotational excitations . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
4.5.2 Breakup dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
4.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
5 Electron-impact detachment from LiH 75
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
5.1.1 Previous studies of the LiH system . . . . . . . . . . . . . . . . . . . 75
5.1.2 Energy levels of the LiH / LiH system . . . . . . . . . . . . . . . . 76
5.2 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
5.3 results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
5.3.1 Total cross section for production of neutral fragments . . . . . . . . . 84
5.3.2 Branching ratios between different channels . . . . . . . . . . . . . . . 88
5.3.3 Observations at short storage times . . . . . . . . . . . . . . . . . . . 94
5.4 Comparison to theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
5.4.1 Calculation of potential energy surfaces . . . . . . . . . . . . . . . . . 99
5.4.2 The dominance of the LiH + H channel . . . . . . . . . . . . . . . . . 102
5.4.3 Possible nature of the transient . . . . . . . . . . . . . . . . . 103
5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
6 Discussion and outlook 107
Appendix 111
A Properties of Dalitz plots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
A.1 Derivation of the Dalitz coordinates . . . . . . . . . . . . . . . . . . . 111
A.2 Energetically allowed region . . . . . . . . . . . . . . . . . . . . . . . 113
A.3 Restrictions through momentum conservation . . . . . . . . . . . . . . 113
B Reconstruction of branching ratios in the fragmentation of LiH . . . . . . . . 115
C Decay modes of the transient state of LiH . . . . . . . . . . . . . . . . . . . 118
References 120
2





1. Introduction
Fragmentation is a fundamental phenomenon occurring in many fields of physics and chemistry.
Besides an interest in the actual fragmentation dynamics itself, in many cases the fragmenta-
tion of a system is also initiated on purpose for studying structural properties which cannot be
addressed in the undisturbed system.
Examples reach from high energy physics, where the fragments emerging from very high
energetic collisions are studied to gain insight in the properties of the particles and interactions
involved in the reaction, over nuclear physics, where the most prominent example is the large-
scale technical use of energy released in the fragmentation of heavy nuclei, to the wide field of
atomic and molecular physics with its applications in chemistry and biology. Considering larger
systems, even the phase transition of a liquid to its gaseous form, or the breakup of a macro-
scopic compound by mechanical force, revealing properties of interest in material science, can
be viewed as examples of fragmentation reactions in a most general sense.
In atomic and molecular physics, the spectrum of fragmentation reactions includes pro-
cesses like the ionisation of an atom, whose dynamics especially is investigated in modern ex-
per