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ETUDE EN ONDES MILLIMETRIQUES ET SUBMILLIMETRIQUES D'especes REACTIVES : Détection D'ions Et De Radicaux Libres Au Laboratoire Et Dans Le Milieu Interstellaire

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ETUDE EN ONDES MILLIMETRIQUES ET SUBMILLIMETRIQUES D'especes REACTIVES : Détection D'ions Et De Radicaux Libres Au Laboratoire Et Dans Le Milieu Interstellaire laboratoire de spectroscopie hertzienne Sc, 5:; ;., 1 No d'ordre : 684 présentée à L'UNIVERSITE DES SCIENCES ET TECHNIQUES DE LlLLE FLANDRES ARTOIS Par Claire DEMUYNCK ETUDE EN ONDES MILLIMETRIQUES ET SUBMILLIMETRIQUES D'ESPECES REACTIVES : Détection d'ions et de radicaux libres au laboratoire et dans le milieu interstellaire Soutenue le 30 Mai 1986 devant la Commission d'Examen Membres du Jury : Président : J. LEQUEUX, Astronome titulaire - Observatoire de MARSEILLE Rapporteurs : J.L. DESTOMBES, Chargé de Recherches - Université de LlLLE I E. HI ROTA, Professeur - lnstitute for Molecular Science - OKAZAKI (Japon) A. OMONT, Professeur - Université de GRENOBLE M. WINNEWISSER, Professeur - Université de GIESSEN (R.F.A.) Examinateurs : G. JOURNEL, Professeur - Université de LlLLE 1 S. LEACH, Directeur de Recherches - Université de PARIS XI B. MACKE, Professeur - Université de LILLE I R. WERTHEIMER, Professeur Emerite - Université de LlLLE 1 UNIVERSITE DES SCIENCES ET TECHNIQUES DE LlLLE FLANDRES ARTOIS - U.F.R. DE PHYSIQUE REMERCIEMENTS Ce ;t(tav&, quX. a &té aé&é à L'UnhvmiAé de LXe 7, dam Le 1abotra;tahe de Spe&oacopie ff &Zienne (U. A. 244), n'a pu &the mené à bien que ghiice au concorn de diadérted dac/tewcn davohabL~n : L' é;trtoLte cofibona;tion qui a ' uk Gdabfie avec La as.ttophyaicie~ a &té un Uément ;tauAl à 6aLt d&cinid powz Le développanent de noa ;frtavaux, & C' ut Jamu LEQUEUX qui, avec 1' en;thous&me &t géného- aaé qu' on -eLu con&, uk à L' otLigine de noa conta& avec lu comu- muté decl /radiaa;ttLanom~n~/ta.nçain. Qu'il lfitouve id L' exphennion de ma hecunMainaance, non aeuLemen;t pou avoh accepXE k phenidence de ce jmy, mdin auilai pom Ra con~iancee.t L'amLZLé qu'a noun a Roujo~ kémoigné~cl, & pou Le aou;tien commqutd a appotclté à no&e équipe e-t à L' euemble du Labom,to&e. La contau2 qu'Li2 au& .initia se aont développa avec ciid@- hem% ghoupu de hadioab&onomen dhançain, à i3ohdux, Pmi~,Meudon & GkenobLe, izt je fiem à Lem &uneh mu p4.u vida hemmc.iemem2 pou L1tUde qu'A ri' ont appotLtée aaun de mWpLu Zjome6. Je ad p&cuRiQnement neconMdinaante à Alain UMONT, qui a accepzé de huppoh- km am ce ~v~,à Michel GUELTN, avec qui nous avom de dnéquenta at ~/rucstuc)unudincu/~aiuvu, & à Pime ENCRENAZ, dont Le dqnrwninme pmanent e-t Lt accu& chaXewLeux ont &~gmn;tdavuhini? Ra mine en pkce d'une é;trroA;te ca.Uaboa.a&ion. C&e cg&b~hatian n'aW cependant pan pu a6oLLtin ai je nlavaA bëné&Lcié d'un appui aoUde au dein du Labona;tohe, Je voudhain en pantideh exphima ma ninci%e g-de à Muaieuhcl Raymond UERTffETMER, VAnectewL du La6ohcr;tuAe junqu'w 1983, QX: GWd JÙURNEL, yLU m'a& taun deux accue.iUie à mon atcnivée au Labot~cLtahe& mlon;t 6aLt bénédicim de Route Lem expihience, atvui qu'à Bhuno MACKE, actuel Rhecteu, dont Le auden amical et L' humam comaaid m' ant itaujom Ué pttécieux. 1h ont accepté de juga ce krtavd, et je Ru en ttmacie vivement. La tté~u&ah p~&bent&iGL aunt Le d'un Long ;Drav& d' équhpe au corn duqueX pWodu datu & moiu ~a~tuae aunt nuccédéa. C'ut la cohébion de noae équipe quh MOU a pamh de dépuha Lu puadu &u "maim ~a~itu",& ,UmceX BOGEY ta& comme Jean-Luc VESTOMBES aunt aaodCh de taop pb à ce ;Dtav& pou que je pdae Lu ttemmea ici : havent qu&u aetanancu de débohea cst d' enthousdme nou avom pamkgéu ! Je ne suain aubfim Havé BULVIN, Mmc VENTS, Havé VUBUS & Banatrd LEMOINE, que je ttemade de Lmamicde co&bo~on, ni Muhieum Pime RUSSEELS Jean-Matrc LAPAUW, dont L'intwvemXon a &té décinive pou la mine au point du dinpuh.Ltid exptk3nen;tae. Je vou&ain égdemevct as~o&aà cu hemmcieme& taun ceux qui ont pamXcipé aux dévetoppemeru2 in?ldxumeMkaux, c'ut-à-d&e Lu membttu du diddetLe~a avicu itechniquu du Labohatohe de 1' U. F. R. de Phgaique, Le Cwuke Commun de Mamu de L'UnivmLté de LüXe 7 qCLi, en nou ptré/tavct un mcino;titan, a appohté un éLément ;tOU/t a ~aLtuaen- fiet au apecA;iromBa%e que nous avom min au point, cst le gtLoupe de Radioas&onomie de L' Obaavatohe de Meudon, qCLi nous a oum mi Lu diodu Scho;tthy. Endin, je adexAtèmemevtt ~euibleà L' honnem que me dont Lu p~oduaeumWINNEWISSER eA HIROTA en accephnt d' &e happonteum de ce IfrtavaLL, aimi que Momiewr Sydney LEACH, qui a bien va& pm- cipm au jmy, et je voudtrd Lem exphuna id tode ma heconnainaance. Je fiem égaLemevtt à nemmcla Mandted et Btrenda CiIINNElrlISSER pawr Lem encauhagemevu2 chaeew~uet btLnt~i?t comaknt qu'& pahtent à no6 kuvaux, & tout pm,ticuRiè.4.ement &enda qui a'ut ahabLement chmgée d'un impatLtant &UV& de ;titaduoti~n, Il me ttute Ù expnimw mu ttemeircimenf~ lu phbinchu Ù ;tau ceux qui ont coWbué à lu ttéaeindan rnat@Ù&e de ce m&uhe, Madmoh&e WALASZAK qui a dadydogttdphLé le manudp.t, Manniu RAFFAUD qui a ttéa-45.~é lu ,X.bUom, eA Mesaieum FAU2UEMBERGUE & CAREY qui ae aan;t changéb du a2uge. TABLE DES MATIERES ................................................................... INTRODUCTION CHAPITRE 1 : SPECTROSCOPIE MILLIMETRIQUE ET RADIOASTRONOMIE MOLECULAI RE 1-A) RECHERCHE ET 1 DENTIFICATION DE RAI ES MOLECULAIRES 1-A- 1) Recherche directe d'une molécule 1-A-2) Relevés systématiques de spectres et détections accidentelles 1-B) MISE EN EVIDENCE D'IONS MOLECULAIRES 1-B-1) Importance des ions moléculaires 1-B-2) Problèmes observationnels et expérimentaux - 1-C) DETECTION DE FORMES 1 SOTOPIQUES RARES 1-C-1) Fractionnement isotopique 1-C-2) Aspects observationnels REFERENCES DU CHAPITRE 1 CHAPITRE II : INTERPRETATION THEORIQUE DES SPECTRES II-A) MOLECULES DIATOMIQUES A COUCHES FERMEES II-A-1) Correction de Dunham II-A-2) Corrections adiabatiques II-A-3) Corrections non adiabatiques II-A-4) Expression générale des coefficients de Dunham II-A-5) Application au monosulfure de carbone II-B) MOLECULES POLYATOMIQUES A COUCHES FERMEES II-B-1) Structure par substitution II-B-2) Influence des masses atomiques sur les effets vibrationnels II-B-3) Structure par "dépendance des masses" + II-B-4) Structure des ions moléculaires HCO+ et HOC II-C) MOLECULES DIATOMIQUES A COUCHES OUVERTES II-C-1) Forme générale du hamiltonien effectif II-C-2) Structure hyperfine II-C-3) Etat doublet ll d'un hydrure léger II-C-4) Dépendance vibrationnelle et isotopique des paramètres moléculaires REFERENCES DU CHAPITRE II -- -- CHAPITRE III : ASPECTS EXPERIMENTAUX -- - III-A) LE SPECTROMETRE III-A-1) Fonctionnement dans la gamme millimétrique III-A-2) Extension au domaine submillimétrique III-B) LES TECHNIQUES DE PRODUCTION III-B-1) Spectroscopie millimétrique au sein de la colonne positive III-B-2) Spectroscopie millimétrique au sein d'une décharge radio-fréquence III-B-3) Spectroscopie millimétrique au sein de la lumière négative REFERENCES DU CHAPITRE II 1 CONCLUS 1ON SOMMAIRE DES ARTICLES PUBLIES INTRODUCTION Le véritable développement de la spectroscopie millimétrique d'ions moléculaires et de radicaux très réactifs date d'une dizaine d'années, lorsque fut démontrée la possibilité d'effectuer ce type d'expérience au sein d'un plasma faiblement ionisé. Jusqu'alors, seuls quelques radicaux à durée de vie relativement longue étaient connus par spectroscopie microonde, et les espèces les plus réactives n'avaient pu être caractérisées qu'à partir de leurs spectres optiques. Par ailleurs, la spectroscopie millimétrique était apparue très tôt comme un complement indispensable de la radioastronomie molé- culaire, puisqu'elle ne met en jeu que le niveau vibronique fondamen- tal, le seul qui soit peuplé de façon significative dans le milieu interstellaire. La mise en évidence d'ions et de radicaux libres dans les nuages moléculaires, et la découverte de leur rôle-clé dans la synthèse des molécules plus complexes a constitué pour la spectroscopie en ondes millimétriques une motivation extrèmement vive, au point que certains y ont même vu une source unique de renouveau pour cette disci- pline qu'ils considéraient comme vieillissante. La réalité est toute autre, et la spectroscopie millimétrique et infra-rouge d'ions et de radicaux libres apporte à notre connaissance de ces espèces à très courte durée de vie des éléments indispensables : en effet, les spec- tres électroniques des ions moléculaires sont souvent situés dans l'ultra-violet lointain, et leur observation, quand elle est possible, pose de délicats problèmes expérimentaux. observation de leurs spectres millimétriques et/ou infra-rouges est donc une des rares techniques donnant accès à leur structure moléculaire et permettant de tester la validité des calculs ab-initio. En ce qui concerne les radi- caux, la haute résolution de la spectroscopie millimétrique constitue un moyen privilégié d'investigation de la structure fine et hyperfine : le dédoublement A, ou la structure complexe des niveaux d'énergie résul- tant de la présence simultanée d'un spin électronique et d'un ou plu- sieurs spins nucléaires, sont en effet directement accessibles à partir des spectres millimétriques, avec une précision qui justifie des développements théoriques élaborés. Par ailleurs, les décharges élec- triques permettent d'atteindre des températures de vibration très élevées, et d'observer ainsi des espèces vibrationnellement très excitées. LGtude des spectres de rotation dans des états de vibration élevés permet alors de déterminer avec précision la structure moléculaire à l'équilibre ainsi que la dépendance vibrationnelle des paramètres molé- culaires. Le travail présenté dans ce mémoire est composé de deux parties. La première partie expose les différents aspects du travail qui a été effectué. Le premier chapitre rassemble quelques éléments d'astrophysique poléculaire, destinés à mettre en évidence les liens profonds qui unissent la spectroscopie millimétrique et la radioastronomie molécu- laire : le problème de l'identification des raies, l'importance des ions moléculaires, et le rôle des molécules isotopiquement substituées sont abordés en particulier.
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