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The H + H2 Isotopic System

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The H + H2 Isotopic System + The H3 + H2 isotopic system Origin of deuterium astrochemistry Inaugural-Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakult¨at der Universit¨at zu K¨oln vorgelegt von Edouard Hugo aus Alenc¸on K¨oln 2008 Berichterstatter: Prof. Dr. S. Schlemmer Prof. Dr. P. Jensen Tag der m¨undlichen Pr¨ufung: 10 Februar 2009 “ either North nor South will overcome their addiction to growth without a collective and N comprehensive detoxification programme. The growth doctrine is like a disease and a drug. As Rahnema says1, Homo economicus had two strategies for taking over virgin territo- ries: one operated like HIV, the other like a drug pusher. Growth economics, like HIV, destroys societies immune systems against social ills. And growth needs a constant supply of new mar- kets to survive so, like a drug dealer, it deliberately creates needs and dependencies that did not exist before. The fact that the dealers in the supply chain, mainly transnational corporations, benefit so much from our addiction will make it difficult to overcome. But our ever-increasing consumption is not sustainable; sooner or later we will have to give it up.” Serge Latouche, Le Monde Diplomatique (english edition), November 2004 1 Majid Rahnema & Victoria Bawtree, Quand la misere` chasse la pauvrete´ English abstract Dense cold molecular clouds reckoned to be stellar nurseries are the scene of an extreme molec- 5 ular deuteration. Despite the cosmic D/H ratio of 10− , molecular species in prestellar cores are observed to contain nearly as much deuterium∼ as hydrogen. This astonishing deuterium en- + richment promoted by low temperatures is the work of H3 . It is the key species which unlocks + ⇋ + the deuterium from its HD reservoir via reactions like H3 + HD H2D + H2 and drags it + further to other species in successive reactions. For this reason, the H3 +H2 isotopic system is outstandingly critical for the astrochemistry of cold environments. However, its understanding is yet incomplete and insufficient. + This thesis thus focuses on the H3 + H2 isotopic system from a theoretical, experimental and astronomical point of view giving a particular look into the role of nuclear spins. As a first step, the stringent nuclear spin selection rules in associative, dissociative and re- active collisions are investigated. This purely theoretical study zooms into the details of the nuclear spin wavefunctions and shows that their permutation symmetry representation is nec- essary and sufficient, contrary to their angular momentum representation. Additionally, a new deterministic interpretation of nuclear spins in chemical reactions is proposed. Based on these considerations, a complete set of state–to–state rate coefficients for all + H3 + H2 isotopic variants is calculated using a microcanonical model leaned on phase space theory. An experimental study is conducted in parallel with a 22–pole ion trap apparatus in order to inspect the influences of temperature and H2 ortho–to–para ratio. The good overall agreement between experimental and theoretical results supports the validity and utility of the calculated set of rate coefficients. Furthermore, the potentiality of the 22–pole ion trap appara- tus is explored via the Laser Induced Reaction (LIR) technique applied to our system of interest. + High resolution overtone, combination and fundamental vibrational spectroscopy of H2D and + D2H is thereby achieved with cw–OPO and diode lasers. Finally, astronomical implications are inferred on an observational basis through the case of the prestellar core L183 using simple chemical models which account for the ortho, meta and + para characters of the H3 and H2 isotopologues and rely on the rate coefficients derived in this thesis. Above all, the results show that the non–thermal ortho–to–para ratio of H2 is a serious limiting factor for the enhancement of deuterium fractionations. It is a first–class parameter for the astrochemistry of very cold interstellar medium. i Deutsche Kurzzusammenfassung Dunkle und kalte molekulare Wolken sind die Geburtsst¨atten neuer Sterne und ein Ort extremer 5 molekularer Deuterierung. Trotz des kosmischen D/H-Verh¨altnisses von 10− beobachtet man in prestellaren Kernen Molek¨ule, die fast soviel Deuterium wie Wasserstoff∼ enthalten. Diese er- + staunliche Anreicherung bei niedrigen Temperaturen ist auf das H3 -Ion zur¨uckzuf¨uhren. Es ist + ⇋ + ein Schl¨usselmolek¨ul, welches Deuterium durch Reaktionen der Form H3 +HD H2D +H2 aus seinem HD Reservoir entnimmt und anderen Spezies in aufeinanderfolgenden Reaktionen + weitergibt. Aus diesem Grund ist das H3 +H2 Reaktionssystem in all seinen isotopischen Va- rianten sehr wichtig f¨ur die Astrochemie kalter Regionen. Jedoch ist dessen Verst¨andnis noch unvollst¨andig und unausreichend. + Diese Arbeit widmet sich dem H3 +H2 isotopischen Reaktionssystem aus theoretischer, ex- perimenteller und astronomischer Sicht, mit besonderem Schwerpunkt auf der Rolle der Kern- spins. Als ein erster Schritt werden die Kernspin-Auswahlregeln in assoziativen, dissoziativen und reaktiven Kollisionen untersucht. Diese rein theoretische Arbeit studiert die Details der Kern- spinwellenfunktionen und zeigt, dass die Darstellung durch Permutationssymmetrien ausrei- chend und notwendig ist, die Drehimpulsdarstellung jedoch nicht. Zus¨atzlich wird eine neue deterministische Interpretation der Kernspins in chemischen Reaktionen vorgeschlagen. Auf diesen Uberlegungen¨ basierend werden mit einem mikrokanonischen Modell die zu- + standsspezifischen Ratenkoeffizienten aller isotopischen Varianten der Reaktion H3 + H2 be- rechnet. Ein paralleles Experiment mit einer 22-Pol Ionenfalle untersucht den Einfluss der Tem- peratur und des ortho-para Verh¨altnisses von H2. Die gute Ubereinstimmung¨ zwischen Experi- ment und den Rechnungen unterst¨utzt die G¨ultigkeit und N¨utzlichkeit der berechneten Ratenko- effizienten. Ausserdem wird das Potential der Ionenfalle zusammen mit der Methode der laser- induzierten Reaktionen (LIR) erkundet, um die hier auftauchenden Fragestellungen zu beant- worten. Es werden hochaufl¨osende Spektren von Oberton–, Kombinations– und fundamentalen + + Schwingungsbanden von H2D und D2H mit einem cw–OPO und Diodenlasern ausgef¨uhrt. Schliesslich wurde ein astrochemisches Modell auf die Observationen des prestellaren Kerns L183 angewendet. In diesem Modell, in dem die zustandsspezifischen Ratenkoeffizienten die- ser Arbeit ber¨ucksichtigt wurden, wurde zwischen den ortho, meta und para Zust¨anden der + H3 und H2 Isotopomere unterschieden. Die Resultate zeigen zuallererst, dass das nichtthermi- sche ortho-para-Verh¨altnis von H2 eine erhebliche Begrenzung f¨ur die Deuteriumanreicherung darstellt und somit ein wichtiger charakteristischer Parameter f¨ur kalte Wolken ist. iii Resum´ e´ franc¸ais Les nuages mol´eculaires denses, reconnus comme p´epini`eres d’´etoiles, sont la sc`ene d’une 5 deut´eration mol´eculaire extrˆeme. Malgr´ele rapport cosmique D/H de 10− , les esp`eces mol´e- culaires qui sont observ´ees dans les noyaux pr´estellaires contiennent presque∼ autant de deut´erium que d’hydrog`ene. Cet enrichissement stup´efiant en deut´erium promu par les basses temp´eratures + ( 10 K) est l’oeuvre de H3 . C’est l’esp`ece cl´equi lib`ere le deut´erium de son r´eservoir HD, par ∼ + ⇋ + le biais de r´eactions du type H3 + HD H2D + H2, puis le transfert `ad’autres esp`eces lors + de r´eactions successives. De ce fait le syst`eme isotopiqueH3 +H2 est de premi`ere importance pour l’astrochimie des milieux froids. Pourtant sa compr´ehension est aujourd’hui incompl`ete et insuffisante. + Cette th`ese focalise donc sur le syst`eme isotopique H3 + H2 depuis des points de vue th´eorique, exp´erimental et astronomique avec une attention particuli`ere pour le rˆole des spins nucl´eaires. En premier lieu, les r`egles de s´election de spins nucl´eaires lors de collisions associatives, dis- sociatives et r´eactives sont examin´ees. Cette ´etude th´eorique zoome sur les fonctions d’onde de spin nucl´eaire d´emontrant ainsi que leur repr´esentation de sym´etrie de permutation est n´ecessaire et suffisante, contrairement `aleur repr´esentation de moment angulaire. De plus, une nouvelle interpr´etation d´eterministe des spins nucleaires pour les r´eactions chimiques est propos´ee. Sur la base de ces consid´erations, un jeu complet de vitesses de r´eaction d’´etat-`a-´etat pour + toutes les variantes isotopiques H3 +H2 est calcul´ed’apr`es un mod`ele microcanonique repo- sant sur la th´eorie de l’espace des phases. Parall`element, une ´etude exp´erimentale est conduite avec un pi`ege `aions `a22 pˆoles afin d’inspecter les influences de la temp´erature et du rapport ortho/para de H2. Le bon accord entre r´esultats th´eoriques et exp´erimentaux soutient la vali- dit´eet l’utilit´edu jeu de coefficients calcul´e. Par ailleurs, le potentiel du dispositif exp´erimental est ´etendu avec la technique de Reaction Induite par Laser appliqu´ee a notre syst`eme d’int´erˆet, + + r´ealisant ainsi de la spectroscopie vibrationnelle de H2D et D2H avec des lasers de type diode ou OPO. En dernier lieu, les implications astronomiques sont d´eclin´ees sur une base observationnelle du noyau pr´estellaire L183 en utilisant des mod`eles chimiques simples qui tiennent compte du + caract`ere ortho, meta ou para des isotopologues de H3 et H2 et faisant usage des vitesses de r´eactions calcul´ees dans cette th`ese. Les r´esultats montrent que le rapport ortho/para deH2 ´etant loin de l’´equilibre thermique, il est un facteur limitant s´erieux pour le fractionnement deut´er´e + de H3 . C’est donc un param`etre de premier ordre pour l’astrochimie des milieux froids. v Contents English abstract i Deutsch Kurzzusammenfassung iii Resum´ e´ franc¸ais v List of Figures xii List of Tables xiv 1 Introduction 1 1.1 Astrochemistry .................................. 2 1.1.1 Chemicalnetworks............................ 2 1.1.2 Hydrogen,theubiquitousnucleus . ... 3 + 1.1.3 Trihydrogen cation H3 .......................... 4 1.2 Deuteriumastrochemistry. .... 5 1.2.1 Coldspaceandstarformation . 6 1.2.2 Extremedeuteration ........................... 9 + 1.2.3 H3 isotopologues............................. 10 1.2.4 ortho and para .............................
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