Formation of small hydrocarbon ions under inter- and circumstellar conditions [Elektronische Ressource] : experiments in ion traps / vorgelegt von Igor Savić

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FORMATION OF SMALL HYDROCARBON IONS UNDER INTER- AND CIRCUMSTELLAR CONDITIONS: EXPERIMENTS IN ION TRAPS von der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz genehmigte Dissertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) vorgelegt von mr Igor Savi ć, dipl. fiz. geboren am 19.08.1970 in Priština (Jugoslawien) eingereicht am 2. Juli 2004 Gutachter: Prof. Dr. Dieter Gerlich Prof. Dr. Gotthard Seifert Dr. Juraj Glosík Tag der Verteidigung, 26. August 2004 http://archiv.tu-chemnitz.de/pub/2004/ BIBLIOGRAPHISCHE BESCHREIBUNG FORMATION OF SMALL HYDROCARBON IONS UNDER INTER- AND CIRCUMSTELLAR CONDITIONS: EXPERIMENTS IN ION TRAPS Dissertation an der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz, Institut für Physik, von Igor Savi ć Chemnitz, 2004 • 143 Seiten inkl. 1 Publikation • in englischer Sprache mit 13 Tabellen und 44 Abbildungen Referat Unter Verwendung von zwei Speicherapparaturen wurden ausgewählte, astrophysikalische wichtige Ionen - Molekülreaktionen untersucht. Durch die Kombination einer Kohlenstoffquelle mit einem Ionenspeicher, in dem so Reaktionen zwischen Ionen und Kohlenstoffmolekülen oder -atomen untersucht werden können, +wurde Neuland betreten. Es werden Ergebnisse vorgestellt für die Reaktion von D 3Ionen, die in einem Ringelektrodenspeicher gefangen sind, mit einem Strahl von heißen C (n = 1, 2, 3).

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• Astronomie

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• Publié par: technische_universitat_chemnitz
• Publié le: 01 janvier 2004
• Nombre de lectures: 18
• Langue: Deutsch
• Poids de l'ouvrage: 1 Mo

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F ORMATION OF S MALL H YDROCARBON I ONS U NDER I NTER - AND C IRCUMSTELLAR C ONDITIONS :

E XPERIMENTS IN I ON T RAPS

von der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz genehmigte Dissertation zur Erlangung des akademischen Grades doctor rerum naturalium (Dr. rer. nat.) vorgelegt von mr Igor Savić, dipl. fiz. geboren am 19.08.1970 in Priština (Jugoslawien) eingereicht am 2. Juli 2004 Gutachter: Prof. Dr. Dieter Gerlich Prof. Dr. Gotthard Seifert Dr. Juraj Glosík Tag der Verteidigung, 26. August 2004 http://archiv.tu-chemnitz.de/pub/2004/

B IBLIOGRAPHISCHE B ESCHREIBUNG

F ORMATION OF S MALL H YDROCARBON I ONS U NDER I NTER - AND C IRCUMSTELLAR C ONDITIONS : E XPERIMENTS IN I ON T RAPS Dissertation an der Fakultät für Naturwissenschaften der Technischen Universität Chemnitz, Institut für Physik, von Igor Savić Chemnitz, 2004 €143 Seiten inkl. 1 Publikation €in englischer Sprache mit 13 Tabellen und 44 Abbildungen

Referat Unter Verwendung von zwei Speicherapparaturen wurden ausgewählte, astrophysikalische wichtige Ionen - Molekülreaktionen untersucht. Durch die Kombination einer Kohlenstoffquelle mit einem Ionenspeicher, in dem so Reaktionen zwischen Ionen und Kohlenstoffmolekülen oder -atomen untersucht werden können, + wurde Neuland betreten. Es werden Ergebnisse vorgestellt für die Reaktion von D 3 Ionen, die in einem Ringelektrodenspeicher gefangen sind, mit einem Strahl von heißen C n (n = 1, 2, 3). Die gemessenen Ratenkoeffizienten sind nur halb so groß wie die Werte, die in astrophysikalischen Modellen verwendet werden. Um die Kenntnis über alle möglichen Reaktionen, bei denen drei C-Atome beteiligt sind, abzurunden, wurden + + + zwischen 15 K und Zimmertemperatur die Reaktionen zwischen C 3 , C 3 CH und 3 H 3 Ionen mit H 2 und HD in vielen Details untersucht. Diese Experimente wurden in einer zweiten Apparatur durchgeführt, in der ein temperaturvariabler 22-Polspeicher das zentrale Element ist (VT-22PT). Berichtet werden Ergebnisse zu reaktiven Stößen, zur Deuterierung von Kohlenwasserstoffen und zur Strahlungsassoziation. In der Diskussion bleibt offen, was - in Verbindung mit der von 300 K zu 15 K zunehmenden Lebensdauer - der Grund + dafür sein kann, daß die Bildung des exothermen Produkts C 3 H anwächst. Der + Tunneleffekt scheidet aus. Bei der Reaktion C 3 + HD wurde ein Isotopeneffekt + + beobachtet, das C 3 D Produkt wird etwas häufiger gebildet als C 3 Ein Vergleich derH . + Reaktion zwischen C 3 H Ionen mit HD bzw. H 2  zeigt, daß das deuterierte Molekül wesentlich reaktiver ist. Es wurden Ratenkoeffizienten für die Strahlungsassoziation + + von H 2 Molekülen mit C 3 H und erstmals mit C 3 Ionen gemessen. Die Auswertung der Daten zeigt, dass der Prozeß langsamer abläuft, wenn der neutrale Stoßpartner deuteriert + ist. Schließlich wurde experimentell die theoretische Vorhersage überprüft, dass C 3 H 3 keinen H - D Austausch mit HD eingeht. Eine sorgfältige Analyse aller konkurrierenden -16 3 -1 Prozesse ergab, dass bei 15 K der Ratenkoeffizient kleiner als 4×s ist.10 cm

Schlagwörter Ionenfallen, Ionen - Molekülreaktionen bei tiefen Temperaturen, H-Atom-Transfer, Deuterierung, Strahlungsassoziation, kleine Kohlenstoffcluster, interstellares Medium, + + + + + + + Laborastrochemie, interstellare Moleküle: CD , C 2 D , C 3 , C 3 H , C 3 CD , 3 H 2 , C 3 HD , + + C 3 H 3 , C 3 H 2 D

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Abstract Using ion-trapping techniques, selected laboratory experiments on ion-molecule reac- tions of astrophysical interest have been performed. For the first time a carbon beam source has been integrated into an ion trapping machine for studying collisions between ions and neutral carbon atoms and molecules. Results are presented for the interaction + of D 3  ions stored in a ring-electrode trap (RET), with a beam of hot neutral carbon molecules, C n The measured reaction rate coefficients are up to a factor(n = 1, 2, 3). two smaller than values presently used in astrophysical models. In order to complete our knowledge about the ion chemistry involving three carbon atoms, detailed investiga- + + + tions of reactions of C 3 , C 3 CH and 3 H 3 H with 2  and HD have been performed be- tween 15 K and room temperature. These studies have been performed in a second ap- paratus, a variable-temperature 22-pole trap machine (VT-22PT). Results include reactive collisions, deuteration and radiative association. It is discussed + in connection with the increase in lifetime of the C 3 + H 2 collision complexes with fal- + ling temperature, what could be responsible for producing more C 3 15 K. Tunnel-H at + + ing is excluded. In C 3 collisions an isotope effect has been detected, the C+ HD 3 D + product ions being slightly more abundant than C 3 H . Comparison of the reaction of + C 3 H primary ions with HD and H 2 gas revealed that the deuterated molecules are sig- + nificantly more reactive. The process of radiative association of C 3 H and for the first + time of C 3 with hydrogen molecules has been observed. An analysis of the data shows that radiative association becomes slower, if the neutral reactant is deuterated. Finally, + the theoretical prediction from ab initio calculations that C 3 H 3 does not exchange an H for a D in collisions with HD, has been proven in an ion trap experiment. Careful analy- sis of all competing processes allows the conclusion that the rate coefficient is smaller -16 3 -1 than 4×15 K.10 cm s at

Key words ion-trap, low and high temperature ion-molecule reactions, hydrogenation, deuteration, radiative association, small neutral carbon clusters, laboratory astrochemistry, interstel- + + + + + + + + lar medium, interstellar molecules, CD , C 2 CD , 3 , C 3 H , C 3 CD , 3 H 2 , C 3 CHD , 3 H 3 , + C 3 H 2 D

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C ONTENTS 1I NTRODUCTION 1.1 The interstellar medium 1.1.1 Physical conditions in the interstellar medium 1.1.2 Processes in the interstellar medium 1.1.3 Chemistry of the interstellar medium 1.2 Overview 2C ARBON IN ISM 2.1 Observations 2.2 Carbon chemistry in ISM 2.3 Deuterium fractionation 2.4 A model for growing pure carbon chains 3E XPERIMENTAL :I ON T RAPPING A PPARATI 3.1 Ion molecule reactions at variable temperatures 3.2 Description of the machines used 3.2.1 General overview 3.2.2 The two ion traps: VT-22PT and RET 3.2.3 Selected tests and outlook 3.3 The carbon source 3.3.1 Properties of carbon 3.3.2 Technical description 3.3.3 Test measurements and outlook 4S UMMARY ,C ONCLUSION AND O UTLOOK A PPENDIX A Ion-trapping apparatus for studies on reactions between ions and neutral carbon species B Reactions of C n (n = 1 - 3) with ions stored in a temperature-variable ra- dio-frequency trap + C Low-temperature experiments on the formation of deuterated C 3 H 3 + + D Temperature variable ion trap studies of C 3 and C 3 HH + 2 and HD R EFERENCES S ELBSTSTÄNDIGKEITSERKLÄRUNG C URRICULUM V ITAE L IST OF P UBLICATIONS AND C ONFERENCE C ONTRIBUTIONS A CKNOWLEDGEMENTS

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7 7 7 7 9 11 13 13 17 25 27

31 31 34 35 44 47 50 51 55 57 61

63

71 87 103 125 135 137 139 143

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1.I NTRODUCTION

1.1. The interstellar medium (ISM) The space between the stars is not empty but filled with diffuse material, the so called interstellar medium (ISM). This material plays an important role, especially as it also provides the matter from which new stars are formed. In general it consists of atomic and ionized hydrogen, molecular gases, dust and electrons. Each of these constituents has quite complicated velocity distributions and the number density can change over a wide range of scales. In the early expansion of the universe the diffuse matter became dominant. Due to grav- ity, gas clouds started to collapse and to coagulate. In almost completely inelastic gas cloud-cloud collisions, energy of motion was transformed into heat through shocks and then radiated into space. These shocks are highly compressive and lead to formation of cold and dense phases of the ISM. These phases are necessary for star formation which occurs when the densities become sufficiently high. In the early time of the universe, all of these processes lead to the formation and later to the evolution of galaxies. Today, the ISM in galaxies is in a dynamical equilibrium concerning mass and energy flow. One of the most important evolutionary processes in the universe is the transformation of the ISM into stars which in their later stages enrich the gas with heavy elements. A fraction of these elements can condense into cosmic dust (submicron- and micron-sized solid particles).

1.1.1. Physical conditions in the ISM 9 As is mentioned in [ger99a], the ISM in our galaxy has a total mass of 6×10 solar 3 masses and it is characterized by an average density of 1 hydrogen atom per cm . It con- sists of various phases with enormous differences in their thermodynamic properties. Most of the total mass of the ISM is contained in cold interstellar clouds (nebulae). There are two different types of such clouds: dense molecular clouds and diffuse clouds. Molecular clouds consist mostly of molecular hydrogen and they are characterized by a 7 kinetic temperature of 10 – 15 K and gas densities between a few hundred and 10 hy- 3 drogen molecules per cm in their dense cores. In such clouds hot molecular cores can be found which are associated with star formation having temperatures up to 200 K. Diffuse clouds consist mainly of atomic hydrogen and they have an average temperature 3 of 80 K and number densities of 50 hydrogen atoms per cm . In addition, there are pro- toplanetary disks and outflows from evolved stars which have even higher densities and temperatures. Many molecular lines and continuum radiation are observed at infrared, millimeter and radio wavelengths. This information is used for determining physical conditions such as temperature, column density, particle density or magnetic fields of ISM.

1.1.2. Processes in the ISM All constituents of the ISM (atoms, molecules, ions and grains) play an active role in its evolution. All of them and also the radiation field interact with each other in a very complex manner. The variety of molecular processes can be divided in two groups. One group contains physical processes such as heating and cooling and the other group chemical processes. Heating and cooling is based on emission of energetic photoelec-

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1.I NTRODUCTION

trons, emission from excited rotation of fine structure states and interactions with grains. A few typical heating and cooling processes are summarized in Table 1.1 [dis99]. Table 1.1.Heating and cooling processes. + - Photoelectric heating Grain/PAH + h
egrain/PAH + + - Cosmic ray heating H 2 + cosmic rayH 2 + e CO line cooling CO(J) + coll.CO(J’) + h
3 3 3 O line cooling O( P 2 ) + coll.O( P 1 )O( P 2 ) + h
+ + 2 + 2 + 2 C line cooling C ( P 1/2 ) + coll.C ( P 3/2 )C ( P 1/2 ) + h
Gas grain heating / cooling Gas + graingas’ + grain’ As can be seen from this table, carbon has a very important role in the physical evolu- tion of the ISM. In general, diffuse clouds are heated by the absorption of starlight. Car- bon is the main supplier of free electrons in diffuse clouds and contributes therefore to the heating of the interstellar gas. The energy of the interstellar radiation field also can be transferred to the gas via the photoelectric effect on dust grains. Far ultraviolet pho- tons ionize grains and the ejected fast electrons heat the gas via inelastic collisions. The heating rate cannot be directly measured, but can be estimated from observing the emis- + sion from C (see below). The comparison of this emission feature with the far-infrared emission allows an estimate of photoelectric heating efficiency of ~0.029 for translu- cent, high-latitude clouds [juv03].

Diffuse clouds are cooled by emission of photons at far-infrared and submillimeter wavelengths. As can been seen from Table 1.1 one of the major cooling processes is due to a carbon containing molecule, carbon monoxide (CO). The most abundant form of + carbon gas in cold neutral medium, the C ion, can be easily excited by collisions 2 + around 80 K. Besides the hyperfine state of the hydrogen atom, the P 3/2  state of C is the first excited state of all gas-phase constituents of the cold neutral medium. Because 2 2 + of this, the P 3/2 - P 1/2 at 158of C  emission µm is consider to be the major cooling mechanism of the cold neutral medium [wol95], [ing02]. That atomic carbon can con- tribute significantly to the thermal budget of interstellar gas has also been discussed in detailed in [ger99b]. In the case of our galactic ISM, C cools more than CO [but92], n+ [wri91]. In addition, low lying transitions of neutral (C) and multiply ionized (C ) atomic carbon are important cooling channels of the warm interstellar gases. Also, C is claimed to be the major coolant in intermediate-velocity clouds [hei01]. The chemical processes occurring in the ISM are usually separated into neutral reac- tions, ionic reactions and reactions occurring on grain surfaces. A more detailed group- ing can be found in Table 1.2 [dis99]. Table 1.2.Chemical processes. + + Ion-molecule reactions X + YZXY + Z + + Charge transfer X + YZX + YZ Neutral-neutral reactions X + YZXY + Z Radiative association X + Y(XY)*XY + h
Grain surface formation X + Y + gXY + g - - Associative detachment X + YXY + e Photodissociation XY + h
X + Y + - Dissociative recombination XY + eX + Y Collisional dissociation XY + MX + Y + M

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1.I NTRODUCTION

1.1.3. Chemistry of the ISM Until today more than 120 interstellar molecules have been detected including neutrals and ions. Many of them may have rather complex structures, e.g. one assumes that polycyclic aromatic hydrocarbons (PAHs) survive. One can expect that many more will be found because of the wide variety of processes occurring in the ISM as well as the variety of physical conditions like temperature, number density, radiation fields, ener- getic cosmic particles, shocks, etc.

In general, ion-molecule reactions have no barriers ([ger93], [smi93]) and therefore their reaction rate coefficients are usually larger than those for neutral-neutral reactions. This is the reason why early reaction models (since the seventies, e.g. [her73], [bla73]) describing molecular clouds and accretion disks were already quite successful using mainly ion chemistry. At low densities and temperatures of the ISM, radiative associa- tion is also a very important reaction [ger92a]. An important class of reactions is the recombination of ions with electrons because they are the final step in the synthesis of neutrals coming from ion-molecule reactions. Today it is generally accepted that also neutral-neutral reactions can play an important role in the evolution of the ISM, espe- cially in the case of radicals. It has been shown experimentally that there are processes with large rate coefficients at low temperatures [bro97], [cha98], [kai98], [cha00], [cha01], [kai02].

In contrast to ion-molecule and neutral-neutral reactions which are, at least in general, reasonably well understood, surface reactions occurring on cold icy grains are very complex. Modern chemical networks, describing the ISM, take these processes into ac- count; however, using rather crude models in most cases. The most discussed example is the formation of hydrogen molecules via a catalytic reaction. It is commonly assumed that the two H atoms have to meet on a dust particle [hol71]; however, there is no real proof for that. In addition, it has been postulated that grain surfaces play an important role in forming some of the hydrogenated molecules [tie82], [her93].

The ion-molecule chemistry of clouds is very complicated. Chemistry of diffuse clouds begins with two different ionization processes, the ionization of H, H 2 and He by cosmic rays and photoionization of C by stellar UV radiation [smi92]. In dense clouds, which are well shielded from the UV radiation, ionization is dominated by the cosmic rays. + + + The initially formed H , H 2 and He cations are the start of a complex network of reac- tions leading to the formation of a variety of complex molecules – organic and inorganic ones. Fig. 1.1 illustrates a few steps of the chemistry occuring in diffuse and dense clouds. Inspection of the inventory list of detected interstellar molecules (see http://www.cv.nrao.edu/~awooten/allmols.html) reveals that almost 80 % of the mole- cules are carbon containing, most of them hydrocarbons and cyanopolynes. The largest molecule detected so far in the ISM is the thirteen atom HC 11 N. In addition, carbon is one of the most abundant non-volatile elements in the universe [hen98]. Its cosmic abundance is comparable to that of oxygen and nitrogen. Because of its high reactivity, high abundance (fourth most abundant element in ISM) and ability to form complex molecules with other reactants, carbon is of paramount interest for astrochemistry and astrophysics in general. Detailed studies of how the different forms of carbon ranging from atoms through complex molecules to grains are processed in ISM, will lead to a better understanding of many fundamental processes and of the evolution of the uni- verse. Some more details of the carbon and hydrogen based interstellar chemistry will be discussed below.

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1.I NTRODUCTION

For understanding in detail of the evolution of the matter of the ISM, a concerted effort of astronomers, physicists and chemists is required to coordinate astronomical observa- tion, astrophysical modeling, laboratory experiments, and basic theory. There exist al- ready very detailed astrophysical models where the parameters can be tuned to explain specific observational facts; however, they should be improved. They must be based on a detailed knowledge of ionization processes, chemical reactions and grain evolution processes. Basically, two different classes of gas-phase models exist: steady-state mod- els and time-dependent models. Steady-state models are depth dependent models. In these models abundances of the molecules do not change with time but with distances. Time-dependent models are depth independent. Molecular abundances are calculated as a function of time at a given position deep inside the cloud. Diffuse clouds: H 2 + + H 2 H 3 H 2 C He + + cosmic rays He CH H H 2 2 CO . H u.v C H 2 2 + + + CH C CH 2 3 radiation CH CH H H 2 2 + + + C C H 2 2 C 2 H 2

Dense clouds:

cosmic rays

H 2 He

+ H 2 + H + He

H 2

CO

+ H 3

+ C

C

H 2

+ CH + CH 2

H 2

+ CH 3

Fig. 1.1.First steps of chemistry in diffuse and dense clouds. Concerning gas-grain models, there exist basically two different approaches [dis98]: the “accretion-limited” regime and the “reaction-limited” regime. In the first approach, the time for a mobile species to scan the surface is much shorter than the accretion time of the other reactant. Therefore, reactions are limited by the accretion rate of new species. In the second case, a species is trapped in a site and can react only with a migrating molecule that visits that site. The next critical action in gas-grain models is how mole- cules return into the gas phase. If there would be no desorption included, then, for typi- 4 -3 cal dark cloud with molecular densities of about 10 cm , most molecules would disap- 6 pear from the gas phase in less then 10 years. This is inconsistent with observations. Therefore there must be, also in dark clouds, some efficient desorption mechanisms, e.g. thermal evaporation by cosmic ray spot heating, explosive heating due to exothermic

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1.I NTRODUCTION

reactions between radicals which can be triggered by cosmic rays or by grain-grain col- lisions at velocities greater than 0.1 km/s. For details see [dis98] and references therein.

1.2. Overview In the following Section 2.1, an overview of observational facts is presented in order to emphasize more on the importance of carbon in the ISM. The Section 2.2 describes in more detail the gas phase interactions of hydrogen, carbon and hydrocarbons which are of importance for the physical and chemical evolution of the ISM. In Section 2.3, recent observations and models are mentioned which have shown that the incorporation of D instead of H atoms is also a universal tracer for the interstellar environments and there- fore also of importance for understanding the fundamental microscopic processes. Sec- tion 2.4 describes a model for growing pure carbon chains. With the aim to improve our knowledge in these fields, selected laboratory experiments have been performed in this thesis. Several measurements have been made utilizing two different ion trapping machines which allow one, on one side, to simulate the conditions prevailing in cold interstellar clouds and to study isotope enrichment in collisions with HD, and on the other side, to provide a temperature variable environment and a carbon target for understanding the interaction of ions with carbon atoms or molecules under inter and circumstellar conditions. After a short overview of the experimental methods used in the field of laboratory astro- chemistry given in Section 3.1, the common features of the two instruments used in this work are described in 3.2. Since it was the first time that a carbon beam source was in- tegrated into a trapping machine, the details of this combination are described in the separate Section 3.3 and separate publications. First results have already been published [cer02], and the paper is reproduced in Appendix A. A manuscript of a detailed publica- tion describing the apparatus and results for the deuteron transfer reaction + + + D 3 + C 3 C 3 D + D 2 and reaction C 3 + C 3 can be found in the Appendix B. Dedicated investigations of reactions between simple carbon ions with H 2 and HD have been performed in the Variable Temperature 22-Pol Trap apparatus (VT-22PT). The + rate coefficients for forming deuterated C 3 H 3 are given in Appendix C. Interesting new + + results for the hydrogenation of C 3 and C 3 reacting with HH ions 2 have been obtained at different temperatures. They are presented in Appendix D. The papers printed in the appendices B, C and D are manuscripts which have been sub- mitted or are ready for submission.

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