Fragmentation of molecular ions in slow electron collisions [Elektronische Ressource] / presented by Steffen Novotny

Dissertationsubmitted to theCombined Faculties for Natural Sciences and for Mathematicsof the Ruperto-Carola University of Heidelberg, Germanyfor the degree ofDoctor of Natural Sciencespresented byDipl.-Phys. Steffen Novotnyborn in SchwetzingenthOral examination: June 25 , 2008Fragmentation of molecular ions in slowelectron collisionsReferees:Prof. Dr. Andreas WolfProf. Dr. Thomas St¨ohlkerKurzfassungFragmentation von Moleku¨lionen in Kollisionen langsamer ElektronenDie Fragmentation positiver Wasserstoff Moleku¨lionen durch den Einfang langsamer Elektro-nen, die sogenannte Dissoziative Rekombination (DR), wurde in Speicherring-Experimenten¨am TSR, Heidelberg, durch gleichzeitige Uberlagerung zweier unabh¨angiger Elektronenstrahlenund mit hochauflo¨senden Fragmentationsabbildungsdetektoren untersucht. Die Framentations-kinematik konnte mit Hilfe kalter Elektronen bis in den Bereich einiger meV Kollisionsenergiebestimmt werden, wo ausgepr¨agte Rotations- und Vibrationsresonanzen im DR Wirkungsquer-+schnittauftreten. Fu¨rthermischangeregtesHD wurdenFragmentationswinkelalsauchdiefrei-werdendekinetischEnergieaufeinemfeinmaschigenGitterzwischenca. 10und80meVpr¨azisereingestellter Kollisionsenergie bestimmt. Die beobachtete Anisotropie, erstmals beschriebendurch Legendre-Polynome gro¨sser zweiter Ordnung, als auch die Rotationsbeitr¨age variierendabei vergleichbar mit dem rotations-gemittelten DR Ratenkoeffizienten.
Publié le : mardi 1 janvier 2008
Lecture(s) : 24
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Source : ARCHIV.UB.UNI-HEIDELBERG.DE/VOLLTEXTSERVER/VOLLTEXTE/2008/8531/PDF/PHD_THESIS_ABGABEVERSION.PDF
Nombre de pages : 162
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Dissertation
submitted to the
Combined Faculties for Natural Sciences and for Mathematics
of the Ruperto-Carola University of Heidelberg, Germany
for the degree of
Doctor of Natural Sciences
presented by
Dipl.-Phys. Steffen Novotny
born in Schwetzingen
thOral examination: June 25 , 2008Fragmentation of molecular ions in slow
electron collisions
Referees:
Prof. Dr. Andreas Wolf
Prof. Dr. Thomas St¨ohlkerKurzfassung
Fragmentation von Moleku¨lionen in Kollisionen langsamer Elektronen
Die Fragmentation positiver Wasserstoff Moleku¨lionen durch den Einfang langsamer Elektro-
nen, die sogenannte Dissoziative Rekombination (DR), wurde in Speicherring-Experimenten
¨am TSR, Heidelberg, durch gleichzeitige Uberlagerung zweier unabh¨angiger Elektronenstrahlen
und mit hochauflo¨senden Fragmentationsabbildungsdetektoren untersucht. Die Framentations-
kinematik konnte mit Hilfe kalter Elektronen bis in den Bereich einiger meV Kollisionsenergie
bestimmt werden, wo ausgepr¨agte Rotations- und Vibrationsresonanzen im DR Wirkungsquer-
+schnittauftreten. Fu¨rthermischangeregtesHD wurdenFragmentationswinkelalsauchdiefrei-
werdendekinetischEnergieaufeinemfeinmaschigenGitterzwischenca. 10und80meVpr¨aziser
eingestellter Kollisionsenergie bestimmt. Die beobachtete Anisotropie, erstmals beschrieben
durch Legendre-Polynome gro¨sser zweiter Ordnung, als auch die Rotationsbeitr¨age variieren
dabei vergleichbar mit dem rotations-gemittelten DR Ratenkoeffizienten. Rotations- und vibra-
+tionsaufgel¨osteDRExperimenteanH wurdendurcheineneuentwickelteIonenqelleermo¨glicht.2
Sowohl der DR Ratenkoeffizient als auch die Fragmentationsdynamik bei ausgewa¨hlten Reso-
nanzen niedriger Kollisionsenergie konnten selektiv in den untersten beiden Vibrations- und den
ersten drei angeregten Rotationszusta¨nden untersucht werden. Zustandsabh¨angige DR Raten
und Winkelverteilungen werden vorgestellt.
Abstract
Fragmentation of molecular ions in slow electron collisions
Thefragmentationofpositivelychargedhydrogenmolecularionsbythecaptureofslowelectrons,
the so called dissociative recombination (DR), has been investigated in storage ring experiments
at the TSR, Heidelberg, where an unique twin-electron-beam arrangement was combined with
high resolution fragment imaging detection. Provided with well directed cold electrons the
fragmentation kinematics were measured down to meV collision energies where pronounced ro-
+vibrational Feshbach resonances appear in the DR cross section. For thermally excited HD the
fragmentation angle and the kinetic energy release were studied at variable precisely controlled
electron collision energies on a dense energy grid from 10 to 80 meV. The anisotropy described
ndfor the first time by Legendre polynomials higher 2 order and the extracted rotational state
contributions were found to vary on a likewise narrow energy scale as the rotationally averaged
+DR rate coefficient. Ro-vibrationally resolved DR experiments were performed on H produced
2
in distinct internal excitations by a novel ion source. Both the low-energy DR rate as well as
the fragmentation dynamics at selected resonances were measured individually in the lowest
two vibrational and first three excited rotational states. State-specific DR rates and angular
dependences are reported.Contents
1 Introduction 1
2 Dynamics in molecular fragmentation 5
2.1 General concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 Potential energy surfaces and molecular states . . . . . . . . . . . . 5
2.1.2 Fragmentation processes of molecules . . . . . . . . . . . . . . . . . 7
2.1.3 Fragmentation by dissociative recombination . . . . . . . . . . . . . 9
2.2 Fragment angular dependences. . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2.1 Angular distributions in the axial-recoil limit . . . . . . . . . . . . . 14
2.2.2 Anisotropies in electron-neutral molecule fragmentation . . . . . . . 15
2.2.3 Angular dependence in dissociative recombination . . . . . . . . . . 17
2.3 Dissociative recombination of the hydrogen cation . . . . . . . . . . . . . . 19
2.3.1 Low-energy DR resonances of the hydrogen cation . . . . . . . . . . 20
3 Fast beam fragment imaging 25
3.1 The ion storage ring technique . . . . . . . . . . . . . . . . . . . . . . . . . 25
3.2 Neutral fragment imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2.1 2-body fragmentation kinematics . . . . . . . . . . . . . . . . . . . 27
3.2.2 Transverse distance information . . . . . . . . . . . . . . . . . . . . 29
3.3 Experimental setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.3.1 The twin-merged beam facility at the TSR . . . . . . . . . . . . . . 33
3.3.2 The multi-hit 2D and 3D fragment imaging detector. . . . . . . . . 36
+4 Product kinematics at resonances of HD DR 51
4.1 Controlled ion beam experiments . . . . . . . . . . . . . . . . . . . . . . . 51
4.2 Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4.2.1 Rotational state contributions to the DR rate . . . . . . . . . . . . 57
4.2.2 Fragment angular distributions . . . . . . . . . . . . . . . . . . . . 66
4.3 Angular distribution models . . . . . . . . . . . . . . . . . . . . . . . . . . 74
4.3.1 Partial wave approach . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.3.2 MQDT description of the angular dependence . . . . . . . . . . . . 81
4.3.3 Beyond the axial-recoil description . . . . . . . . . . . . . . . . . . 84
4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
ICONTENTS
+5 State-selective DR of H 932
+5.1 Production of H ions in defined states . . . . . . . . . . . . . . . . . . . . 932
5.1.1 Electron impact ion beam production . . . . . . . . . . . . . . . . . 94
5.1.2 The laser ion source (LISE) . . . . . . . . . . . . . . . . . . . . . . 98
5.2 State selective measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 105
5.2.1 DR rate coefficients of selected ro-vibrational states . . . . . . . . . 106
5.2.2 Angular distributions of selected ro-vibrational states . . . . . . . . 112
5.2.3 Comparison to model angular distributions . . . . . . . . . . . . . . 120
+5.2.4 Comparison to HD fragmentation dynamics . . . . . . . . . . . . . 125
5.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
6 Summary & Outlook 129
6.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
6.2 Future goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
Appendix 133
A Transverse distance distribution . . . . . . . . . . . . . . . . . . . . . . . . 135
B Electron energy distribution . . . . . . . . . . . . . . . . . . . . . . . . . . 137
References 139
II1
Introduction
Collisions of molecules are the starting point for chemical and physical changes of matter
in a variety of environments ranging from low density plasmas in interstellar space to
living tissue of a human body. They can be associated with a transfer of energy between
the reactants, cause a rearrangement of the molecular geometry or simply give rise that
themoleculefallsapartthereafter. Infundamentalprocessesthemoleculeinteractswitha
photonoranelementaryparticle,suchasanelectronoraproton,butthecomplexityofthe
collision is not limited and can comprise reactions with abundant molecular constituents
aswell. Thisriseslargeinterestandopensupawiderangeformolecularphysicsresearch.
The largest effect on the environment and its subsequent development will result from a
fragmentationofthemoleculefollowingthecollision, thatisbreakingthechemicalbonds.
The basic phenomenon initiating the breakup of a molecule is the resonant creation of
an excited, unstable state in the collision. The molecule, finding itself in this new state,
will start separating and proceeds upon a potential energy surface from an initial bound
system at short internuclear distances to fragments well separated on the macroscopic
scale. The evolving fragments carry the information on the reaction pathway and hence
their study provides direct access to the fundamental fragmentation mechanisms.
From the experimental side, the investigation of the kinetic energy distributed among the
products yields a ”rough” picture of the fragmentation process as the energy balance will
allow to infer the final product states. Further insights, revealing the distinct reaction
path, can be obtained by comprising in addition the study of directional properties. For
instance, the dependence of the reaction cross section on the orientation of the molecule
canberelatedtotheelectronicsymmetrycharacterizingtheparticipatingpotentialenergy
surface. Consequently,thestudyofmolecularfragmentationprocesseswillrequirehighest
control on the reaction parameters and aims at a full energy and momentum resolved
picture of the escaping fragments. Particularly sensitive experiments in this respect are
1CHAPTER 1. INTRODUCTION
those utilizing the high energy resolution of photon induced interactions [1] or applying
electrons to study the creation of charged fragments in resonant collisions with neutral
molecules [2].
The present work focuses on the fragmentation dynamics in binary collisions of positively
charged diatomic molecular ions with electrons. Caused by the capture of the electron,
the molecular cation by resonant energy transfer forms an excited neutral compound
state which is unstable against dissociation into neutral atomic fragments. The excess
energy released in the destruction process is distributed over internal excitations of the
fragments, asfaraspossible, andoverthekineticdegreesoffreedomofthefinalproducts.
The initiated neutralization process, generally referred to as dissociative recombination
(DR) [3] (Chapter 2.1.3), attracts strong interest in modeling low-density ionized media
such as atmospheric layers [4] or astrophysical environments [5, 6], but presents also an
important process in various laboratory plasmas [7].
In view of the importance of the DR process, both theoretical and experimental interest
has continuously risen over the past decades addressing mainly the molecular fragmen-
tation by electrons through rate measurements in event-by-event counting experiments.
Recently, in particular the experimental studies have benefited greatly from the storage
ring technique becoming available to the field of molecular physics [8]. Within this setup
both electrons and molecular ions can be brought together at well defined kinematics
down to small relative collision energies in the sub-eV range. Compared to the fast ion
beam (reaching energies of up to a few MeV/nucleon) the kinetic energy released among
the escaping fragments is small (in the range of eV) so that the trajectories of all neutral
products are constraint to a narrow cone in forward direction. This greatly simplifies the
detectionsetupwhichisrequiredtoefficientlycollecttheneutralrecombinationproducts.
Taking advantage of these technical developments, experiments focusing on the DR-
induced fragmentation dynamics are now becoming possible. They combine the storage
ringsetupwithafragmentimagingdetectionsystemtoperformmeasurementswithunidi-
rectionalmonochromaticelectronimpactunderstableionbeamconditions. Inthepresent
experiments a multi-hit 2D and 3D imaging detector has been utilized to determine both
the information on the kinetic energy release as well as the fragmentation direction from
the simultaneous measurement of the relative fragment impact positions. The details on
the fragment imaging technique are described in Chapter 3.
+ +The hydrogen molecular ion H and thedeuterated relative HD have found much atten-2
tion in numerous experiments investigating the electron-ion interaction in merged beam
setups at storage rings (e.g. [9, 10, 11, 12, 13]). Their simple structure favors the study
also from the theoretical side [14, 15, 16] so that the molecular system has become a
2

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