On the combination of a low energy hydrogen atom beam with a cold multipole ion trap [Elektronische Ressource] / vorgelegt von Gheorghe Borodi
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On the combination of a low energy hydrogen atom beam with a cold multipole ion trap [Elektronische Ressource] / vorgelegt von Gheorghe Borodi

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ON THE COMBINATION OF A LOW ENERGY HYDROGEN ATOM BEAM WITH A COLD MULTIPOLE ION TRAP von der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz genehmigte Dissertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) vorgelegt von M. Sc. Gheorghe Borodi geboren am 06.10.1970 in Caianu Mic, Rumanien eingereicht am 20. Oktober 2008 Gutachter: Prof. Dr. Dieter Gerlich Prof. Dr. Manfred Albrecht Prof. Dr. Juraj Glosík Tag der Verteidigung, 09. Dezember 2008 http://archiv.tu-chemnitz.de/pub/2009/ BIBLIOGRAPHISCHE BESCHREIBUNG ON THE COMBINATION OF A LOW ENERGY HYDROGEN ATOM BEAM WITH A COLD MULTIPOLE ION TRAP Dissertation an der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz, Institut für Physik, von Gheorghe Borodi Chemnitz, 20. 10. 2008 144 Seiten inkl. 2 Publikationen in englischer Sprache mit 6 Tabellen und 66 Abbildungen Referat Der erste Teil der Aktivitäten dieser Arbeit bestand in der Entwicklung einer modernen Ionenspeicher Apparatur zur Untersuchung chemischer Prozesse mit atomarem Wasserstoff. Die Integration eines differentiell gepumpten Radikalenstrahls in eine vorhandene temperaturvariable 22-Pol Speicherapparatur erforderte größere Änderungen an dieser. Da astrophysikalische Fragestellungen im Vordergrund standen, führt die Einleitung zunächst in das Gebiet der Astrophysik und -chemie ein.

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Publié le 01 janvier 2008
Nombre de lectures 25
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ON THE COMBINATION OF A LOW ENERGY
HYDROGEN ATOM BEAM
WITH A COLD MULTIPOLE ION TRAP






von der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz
genehmigte Dissertation zur Erlangung des akademischen Grades

doctor rerum naturalium

(Dr. rer. nat.)


vorgelegt von M. Sc. Gheorghe Borodi
geboren am 06.10.1970 in Caianu Mic, Rumanien
eingereicht am 20. Oktober 2008


Gutachter:
Prof. Dr. Dieter Gerlich
Prof. Dr. Manfred Albrecht
Prof. Dr. Juraj Glosík



Tag der Verteidigung, 09. Dezember 2008
http://archiv.tu-chemnitz.de/pub/2009/








BIBLIOGRAPHISCHE BESCHREIBUNG
ON THE COMBINATION OF A LOW ENERGY
HYDROGEN ATOM BEAM
WITH A COLD MULTIPOLE ION TRAP
Dissertation an der Fakultät für Naturwissenschaften der
Technischen Universität Chemnitz, Institut für Physik,
von Gheorghe Borodi

Chemnitz, 20. 10. 2008
144 Seiten inkl. 2 Publikationen
in englischer Sprache mit 6 Tabellen und 66 Abbildungen

Referat
Der erste Teil der Aktivitäten dieser Arbeit bestand in der Entwicklung einer modernen
Ionenspeicher Apparatur zur Untersuchung chemischer Prozesse mit atomarem
Wasserstoff. Die Integration eines differentiell gepumpten Radikalenstrahls in eine
vorhandene temperaturvariable 22-Pol Speicherapparatur erforderte größere
Änderungen an dieser. Da astrophysikalische Fragestellungen im Vordergrund standen,
führt die Einleitung zunächst in das Gebiet der Astrophysik und -chemie ein. Die
Grundlagen der Ionenspeicherung in temperaturvariablen Hf-Speichern sind ausführlich
in der Literatur dokumentiert. Daher ist die Beschreibung der Apparatur (Kapitel 2)
relativ kurz gehalten. Viel Mühe wurde in die Entwicklung einer intensiven und stabilen
Quelle für Wasserstoffatome aufgewandt, deren kinetische Energie variiert werden
kann. Das Kapitel 3 beschreibt dieses Modul in vielen Details, wobei der Einsatz von
magnetischen Hexapolen zum Führen der Atome und die chemische Behandlung der
Oberflächen zur Reduzierung der H-H Rekombination einen wesentlichen Platz
einnimmt.
Durch die außergewöhnliche Empfindlichkeit der Speichertechnik kann das neue
Instrument zur Untersuchung von vielen Reaktionen eingesetzt werden, die von
astrochemischer und fundamentaler Bedeutung sind. Die Ergebnisse dieser Arbeit sind
im Kapitel 4 zusammengestellt, einige Reprints und Entwürfe von Publikationen findet
+man im Anhang. Die Reaktionen von CO mit Wasserstoffatomen und -molekülen 2
erwiesen sich als sehr geeignet, um in situ H and H Dichten über den gesamten 2
Temperaturbereich der Apparatur zu bestimmen (10 K - 300 K). Zum ersten mal
+ +wurden Reaktionen von H- and D-Atomen mit den Kohlenwasserstoffionen CH , CH2 ,
+and CH bei Temperaturen des interstellaren Raums untersucht. Ein sehr interessantes, 4
noch nicht ganz verstandenes Stoßsystem ist die Wechselwirkung von protoniertem
Methan mit H-Atomen. Im Ausblick der Arbeit werden einige Ideen aufgezeigt, wie
man das Instrument verbessern kann, und es werden einige Reaktionen erwähnt, die
man als nächste untersuchen könnte.
3 BIBLIOGRAPHISCHE BESCHREIBUNG
Diese Dissertation ist einen Beitrag zum Projekt 5 der Forschergruppe Laboratory
Astrophysics: Structure, Dynamics and Properties of Molecules and Grains in Space,
die von der DFG im Zeitraum von 2000 bis 2006 unterstützt wurde.
Schlagwörter
Ionen-Atom Reaktionen, Astrochemie, Hf-Multipol Ionenfalle, niederenergetischer H-
Atomstrahl, H-H-Rekombination, Ratenkoeffizienten bei tiefen Temperaturen,
+ + + + +Dehydrierung , Deuterierung, interstellare Moleküle: CH , CD , CH , CH D , CH , 3 2 4
+ + + +CH D , CH , CH D , CO H 3 5 4 2

Abstract
The first part of the activities of this thesis was to develop a sophisticated ion storage
apparatus dedicated to study chemical processes with atomic hydrogen. The integration
of a differentially pumped radical beam source into an existing temperature variable 22-
pole trapping machine has required major modifications. Since astrophysical questions
have been in the center of our interest, the introduction first gives a short overview of
astrophysics and -chemistry. The basics of ion trapping in temperature variable rf traps
is well-documented in the literature; therefore, the description of the basic instrument
(Chapter 2) is kept rather short. Much effort has been put into the development of an
intense and stable source for hydrogen atoms the kinetic energy of which can be
changed. Chapter 3 describes this module in detail with emphasis on the integration of
magnetic hexapoles for guiding the atoms and special treatments of the surfaces for re-
ducing H-H recombination.
Due to the unique sensitivity of the rf ion trapping technique, this instrument allows one
to study a variety of reactions of astrochemical and fundamental interest. The results of
this work are summarized in Chapter 4, some reprints and drafts are reproduced in the
+appendix. Reactions of CO with hydrogen atoms and molecules have been established 2
as calibration standard for in situ determination of H and H densities over the full tem-2
perature range of the apparatus (10 K - 300 K). For the first time, reactions of H- and D-
+ + +atoms with the ionic hydrocarbons CH , CH and CH have been studied at tempera-2 , 4
tures of interstellar space. A very interesting, not yet fully understood collision system
is the interaction of protonated methane with H. The outlook presents some ideas, how
to improve the new instrument and a few reaction systems are mentioned which may be
studied next.
This thesis is a contribution to the project 5 of the research unit Laboratory Astrophys-
ics: Structure, Dynamics and Properties of Molecules and Grains in Space which has
been supported by the DFG from 2000 to 2006.
Key words
Ion-atom reactions, astrochemistry, rf-multipole ion trap, low energy hydrogen atom
beam, H-H recombination, low temperature rate coefficient, dehydrogenation, deutera-
+ + + + + + + +tion, interstellar molecules: CH , CD , CH , CH D , CH , CH D , CH , CH D , 3 2 4 3 5 4
+ CO H2
4
CONTENTS
1 INTRODUCTION 7
1.1 Physical conditions in space 7
1.2 Chemistry of the interstellar medium 7
1.3 Laboratory approaches 11
1.4 Overview 13
2 EXPERIMENTAL 14
2.1 The AB–22PT apparatus 14
2.2 Vacuum system and gas inlet 16
2.3 Description of the 22-pole ion trap machine 18
2.4 Determination of rate coefficients 21
3 COLD ATOMIC HYDROGEN BEAM SOURCE 25
3.1 Theoretical considerations 25
3.1.1 Theory of effusive gas sources 25
3.1.2 Mechanisms of H production and loss 27
3.1.3 Guiding and focusing H atoms 31
3.2 Technical description 33
3.2.1 The radio frequency dissociator 33
3.2.2 The nozzle cooling system 34
3.2.3 The hexapole magnets 36
3.3 Test measurements 38
3.3.1 Degree of dissociation measurements 39
3.3.2 Number density at trap location 45
3.3.3 Velocity distributions of atoms 53
4 REACTIONS 55
+4.1 Temperature dependence of reactions of CO 55 2
+4.2 Reactions of CH with H and D 56
+ +4.3 CH + H, CH + D 61 4 4
+ +4.4 CH + D 65 5 5
5 SUMMARY AND OUTLOOK 69
APPENDIX 71
A On the combination of a low energy hydrogen atom beam with a 71
cold multipole ion trap
+B Reactions of CO with H, H and deuterated analogues 109 2 2
C Interactions of ions with hydrogen atoms 119
D Nomenclature 129





5 REFERENCES 131
LIST OF PUBLICATIONS AND CONFERENCE CONTRIBUTIONS 137
SELBSTSTÄNDIGKEITSERKLÄRUNG 139
CURRICULUM VITAE 141
ACKNOWLEDGEMENTS 143

61 INTRODUCTION
1.1 Physical conditions in space
Most of the mater in the universe is assembled in large agglomerates of stars known as
galaxies; however, the stars occupy only a small fraction of the space. The space be-
tween stars is found to be not devoid of material but to contain so-called interstellar
medium (ISM), which consist of gas (99 %) and sub-µm sized grain particles (1 %) with
-3an average number density of 1 H atom cm [kai02]. Using various methods, astrono-
mers have been able to deduce the cosmic abundances of the elements. By far dominant
is still hydrogen; helium has about 7 % of the hydrogen abundance by number. The bio-
genic elements carbon, nitrogen and oxygen have fractional abundances with respect to
-4 -5 -4hydrogen of 4 × 10 , 9.3 × 10 , and 7.4 × 10 , respectively [her05]. Other elements like
neon, silicone, magnesium and sulfur are less copious furnishing only 0.02 % of the
hydrogen abundance. The average elemental abundances do not apply to all astronomi-
cal objects, for example, stars in certain stages of evolution can be carbon rich (contain-
ing more carbon than oxygen).
Early models [kee77] classified the ISM into three phases: the Cold Neutral Medium
(CNM) often referred to as clouds; the Warm Ionized Medium (WIM) which is consid-
ered the boundary layers of the CNM; and the Hot Ionized Medium (HIM), which is
sometimes referred to as the intercloud medium or the coronal gas. These phases are
thought to be in approximate pressure equilibrium with one another [cox05]. The CNM
itself appears to contain a variety of cloud types, spanning a wide range of physical and
chemical conditions. The densest clouds that are most protected from UV radiation from
stars are referred to as dense clouds, dark cloud or molecular clouds. They are character-
2 4 -3ized by typical number densities of 10 – 10 cm , large visible extinction, and their
kinetic temperatures are typically on the order of 10 – 50 K [sno06]. The molecules
range in size from 2 –13 atoms and are mainly organic in nature. Molecular hydrogen is
4the dominant species, having a concentration roughly 10 times than of the second most
abundant molecule – CO. The most tenuous clouds, fully exposed to starlight are usu-
ally called diffuse clouds. The concentration of diffuse matter is typically 10 – 100 at-
-3oms cm and average kinetic temperature in the range 50 – 100 K [her05]. As in all
clouds, the gas is mainly hydrogen in one form and another, betraying the fact that the
material in cloud comes from previous generation of stars, which are also mainly hy-
drogen in content.
1.2 Chemistry of the interstellar medium
The chemical complexity of the ISM has been explained for many years by gas-phase
processes driven by cosmic-ray ionization. The atoms, molecules, the dust particles and
the radiation field interact with each other in an extremely complex manner. Ion-
molecule reactions have dominated for long time the reaction models developed for
describing molecular clouds since many of them have no barriers [her73], [bla77]. At
the low density in the interstellar gas, three-body processes are unimportant, so that only
two-body reactions need to be considered. A very important mechanism by which mo-
lecular bonds can be formed at the low temperatures and densities of the ISM is radia-
tive association [ger92c]. It has recently been realized that neutral-neutral reactions in-
volving radicals or atoms can have large enough rate coefficients at low temperatures
that they may be important [her01], [kai02], [smi04]. Very relevant for understanding
the evolution of interstellar matter is the process of isotopic fractionation occurring pre-
dominantly at the low temperatures of interstellar clouds [her03], [mil03].
7 1 INTRODUCTION
One reaction of central importance which looks simple at a first glance, is the formation
of the hydrogen molecule from two hydrogen atoms. In the early universe, H is the 2
-product of the fast associative detachment reaction with atomic hydrogen H + H → H 2
- -+ e . H ion is formed by the slow radiative association reaction of H with a free electron
and that this one can be easily photodetached. There have been other mechanisms pro-
-posed but in most circumstances the H pathway dominates [glo03]. Unfortunately there
is considerable variation in the rate coefficients of this fundamental reaction. For exam-
ple, published values for the associative detachment reaction differ by nearly an order of
magnitude introducing significant uncertainties into the H formation rate. The cosmo-2
logical implications have been discussed recently [glo06b], [glo08a]. The only way to
remove these uncertainties will be to obtain more accurate rate coefficients for the rele-
vant associative detachment processes at cosmologically relevant collision energies
supplemented by further theoretical calculations.
In interstellar clouds, it is generally assumed that, hydrogen molecules are produced on
the grain surfaces. A large body of theoretical work exists [pir00] stretching back to the
pioneering work of Gould & Salpeter [gou63] and Hollenbach, Werner, & Salpeter
[hol71]. So far no gas phase processes are known, e.g. ternary or radiative association or
chemical reactions, which could explain the observed H abundances. Some quantum 2
mechanical calculation have suggested that molecular hydrogen may be formed by reac-
tions between hydrogen atoms and positively charged PAHs, resulting in a simultaneous
dehydrogenation of the aromatic ions. Since regeneration of the original cation by addi-
tion of atomic hydrogen to the dehydrogenated positive ion is thermodynamically al-
lowed, the process can be cycled [che94], [her99]. In general catalytic cycles such as
+ + + +XY + H → XYH followed by XYH + H → XY + H are possible schemes. Potential 2
+ + + + +candidates for XY are C , CH , C H , or C H . Some of the relevant steps have been 3 2 2 3 2
studied in this work.
A special very interesting subfield of interstellar chemistry is the formation and destruc-
tion of hydrocarbons. The gas-phase chemical networks describing the most important
formation routes of carbon-bearing molecules in local ISM have been discussed exten-
sively in the literature [smi92], [hol97], [dis99], [her01], [her05]. Because hydrogen is
more abundant than any other element, reactions with H and H dominate the networks 2
if they are exothermic. In dense clouds the ion-chemistry is initiated by the action of
+ + + +cosmic rays on the major constituents producing H , H , and He . The produced H 2 2
+ion reacts fast with molecular hydrogen to form the stable H ion. This ion plays a piv-3
otal role in the subsequent ion-molecule chemistry through proton transfer. The reaction
+ +of H with C to form CH initiates the hydrocarbon chemistry through the chain of re-3
+ + + + +actions CH + H → CH → CH . Because the reaction of CH with H to form CH 2 2 3 3 2 4
+is endothermic only slow radiative association reaction leading to CH can occur 5
[ger92c]. Once protonated methane is synthesized, it is depleted by dissociative recom-
bination leading mainly to the production of methyl radical rather than methane and by
reaction with abundant CO molecules leading to methane. As indicated in Fig. 1.1 some
+ + + depletion of CH occurs via the H atom transfer reaction CH + H → CH + H. This 5 5 4
reaction channel is more important in diffuse clouds where the abundance of H and H 2
are equal. Motivated by the important of this reaction for interstellar chemistry and fun-
damental research it has been studied in the present work at collision energies relevant
for dark interstellar clouds.
Once simple hydrocarbons such as methane are formed, the formation of more complex
hydrocarbons occurs via three types of reactions [her01]: (i) carbon insertion reactions
+ + + + (e.g. C + CH → C H + H, C H + H ); (ii) condensations reactions (e.g. CH + CH4 2 3 2 2 2 3 4
8 1 INTRODUCTION
+ + +→ C H +H ); and (iii) radiative association reactions (e.g. C + C → C +h υ). In 2 5 2 n n+1
+general, carbon insertion with C is thought to be the dominant route. Because the larger
+ions C H do not react rapidly with H , low-temperature gas-phase chemistry produces n m 2
strongly unsatured hydrocarbons, in agreement with observations of dark clouds. Recent
+laboratory work revealed that many C H ions that are unreactive in a molecular hy-n m
drogen environment became reactive in an H-atom environment. For example, the syn-
thesis of benzene in interstellar clouds is initiated by the slow radiative association reac-
+ + tion C H + H → C H + h υ [ewa99]. 4 2 4 3

Fig. 1.1. Initial steps in the gas-phase carbon chemistry in diffuse and dark clouds
[dis99]. Of specific importance for this work is the dehydrogenation of ions (see for ex-
+ample CH ) in collisions with H which, simultaneously leads to H formation. 5 2
Due to the intensity of the stellar UV radiation in diffuse clouds a substantial fraction of
the matter is in the atomic form. Thus the major constituents are H atoms and H mole-2
cules, with C, N, and O atoms as minor constituents. In the outer part of the diffuse
clouds where the UV is more intense, the hydrocarbon chemistry begins with photoioni-
+ +zation of C atoms by stellar UV, producing C ions and electrons. Formation of CH
+ + through the reaction C + H → CH + H does not proceed at low temperature because 2
+the reaction is endothermic by about 0.4 eV. In particular, C undergoes a slow radiative
+ + association reaction with H : C + H → CH + h υ which has been measured to occur 2 2 2
-15 3 -1with a rather small rate coefficient of ~ 10 cm s [ger92c]. Deep inside diffuse clouds
the UV radiation field is reduced and a fraction of carbon remains in its neutral form.
Therefore, similarly to dark clouds, an alternative route for hydrocarbon synthesis in
+ + diffuse clouds is the reaction H + C → CH + H . 3 2
9 1 INTRODUCTION
At high temperatures of ~ 100 - 4000 K such as encountered in photon-dominated re-
gions (PDR) or shocked regions, gas-phase reactions with H and H become significant 2
[dis99]. PDR chemistry differs from normal ion-molecule chemistry in a number of
ways. Obviously, because of the high FUV flux, photoreactions are very important, as
are reaction with atomic hydrogen. Likewise, vibrationally excited H is abundant and 2
can play a decisive role in the PDR chemistry. If the gas get very warm ( ≥ 500 K), the
activation barrier of reactions of atoms and radicals with H can be easily overcome and 2
this type of reactions can dominate. Electron recombination and charge exchange reac-
tions are important for the ionization balance. Finally, the FUV flux keeps atomic O
very abundant throughout the PDR and hence burning reactions are effective. The most
important reactions in the chemistry of carbon compounds are schematically shown in
Fig. 1.2.

Fig. 1.2. Reactions of central importance for the carbon-bearing molecules included in the
chemical network of dense photodissociation regions (PDRs) (adapted from [hol97]).
The PDR surface layer consists largely of neutral or cationic atoms created by photodis-
sociation and ionization reactions. Hydrocarbon chemistry in PDR begins with
+ photoionization of atomic carbon followed by the endothermic abstraction reaction C +
+ + * + + H → CH + H and C + H → CH + H. The CH ions rapidly formed by these reac-2 2
+ tions are removed by reactions with H , electrons, and the reverse reaction CH + H → 2
+ C + H which has been measured also in this work (see Sec. 4.2). Likewise, a small 2
*fraction of the neutral C reacts with H to form CH which is destroyed by the reverse 2
reaction with H. Photoreactions are important for C, OH, and CO but not for the small
hydrocarbon radicals and cations.
In addition to H, which is the most abundant atom in the Universe, there is a density of
-5D atoms, which is smaller by a factor of about 10 compared with the density of H at-
oms in regions where the gas is primarily atomic. Despite the small abundances of D
atoms about 30 deuterated molecules have been detected to date in the interstellar me-
dium including doubly and even triply deuterated species [lis07]. Surprisingly, the
measured abundance ratios of the singly deuterated molecules to their undeuterated
4counter parts tend to be factor of up to 10 greater than expected from the D/H elemen-
-5tal ratio of about 1.5 ×10 . Owing to the studies of low-temperature ion-molecule reac-
tions it is now known that this enrichment is due to the phenomenon of ‘isotope frac-
tionation’. In molecular clouds most of the deuterium is contained in HD. Therefore, it
10