Ionisation et dissociation par impact électronique d'ions moléculaires d'intérêt atmosphérique et thermonucléaire

Dans les milieux ionisés existe un grand nombre d'espèces chimiques. Beaucoup sont en quantité minoritaire mais jouent pourtant, par leur grande réactivité, un rôle essentiel dans les processus de transformation chimique. Cette étude s'intéresse à la structure et à la dynamique des ions moléculaires, ainsi qu'aux collisions réactives pour des systèmes intervenant dans les atmosphères planétaires et dans le milieu interstellaire, comme les réactions mettant en jeu le monoxyde de carbone. Des modélisations numériques sont nécessaires à la préparation des expériences de fusion thermonucléaire. Enrichis par les résultats expérimentaux, concernant les ions d'hydrocarbures tels que le méthane, ils fournissent une aide pour l'interprétation des phénomènes observés dans les tokamaks. L'identification des réactions physiques à l'origine de la présence de particules dans le plasma est une étape importante afin de comprendre les environnements ionisés. Les systèmes examinés à Louvain-la-Neuve sont donc choisis pour leur intérêt immédiat en astrophysique ou en physique des plasmas. Alors que la connaissance des processus d'ionisation par impact d'électrons d'espèces atomiques ou ioniques simples est bien établie, ce n'est pas encore le cas lorsque la cible primaire est un ion moléculaire. L'existence d'au moins un degré de liberté supplémentaire rend possible la dissociation de l'ion moléculaire initial en plusieurs fragments, une partie de l'énergie potentielle est alors transférée sous forme d'énergie cinétique aux produits. En conséquence, ceux-ci sont généralement distribués dans de larges gammes angulaire et énergétique. L'analyse fine des distributions permet d'étudier les détails de la collision électronique. L'expérience a pour thématique majeure la mesure des sections efficaces absolues en fonction de l'énergie des électrons projectiles incidents, pour des réactions impliquant des espèces ioniques polyatomiques. L'observation des seuils de réaction et de l'énergie cinétique des fragments permet de déterminer la zone de Franck-Condon accessible et les états moléculaires impliqués dans les processus considérés. Les travaux réalisés au laboratoire ont démontré que l'appareillage existant est bien approprié à ce type d'étude. A un niveau plus fondamental, l'examen comparatif minutieux des présentes mesures, des résultats issus d'autres expériences et des prédictions des formalismes semi-empiriques, permet d'améliorer la connaissance des phénomènes collisionnels.(PHYS 3)--UCL, 200

Absolute cross sections and kinetic energy release distributions for electron impact ionization and dissociation of CDn+ (n=1-4)

Absolute cross-sections for electron impact dissociative excitation and ionization of CDn+ (n=1-4) leading to the formation of ionic products (CDn2+ , D+, D2+ , C+, CD+, CD2+, CD3+) are reported in the energy range from their respective thresholds to 2.5 keV. The animated crossed-beams method is used. Around the maximum, all these cross-sections are found to range from 3×10-17 cm2 to 20×10-17 cm2, except for CDn2+ and D2+, which are lower than 1.7×10-17 cm2. Kinetic energy release distributions are deduced from the analysis of ionic product velocity distributions, which are obtained for each ionic fragment, at selected electron energies. KERDs are seen to extend from 0 to a maximum of 13 eV both for both for the light and the heavy ion fragments. The comparison of the present energy thresholds and kinetic energy release with available published data allows the identification of initial or intermediate states contributing to the observed processes. Individual contributions for dissociative excitation and dissociative ionization are determined for each singly-charged product, which are of significant interest in fusion plasma edge modelling and diagnostics. A complete database including cross-sections and energies is compiled for dissociative excitation and ionization of CDn+ ions with n= 1–4, for use in fusion application

Mass Spectrometric Investigation of Anions Formed upon Free Electron Attachment to Nucleobase Molecules and Clusters Embedded in Superfluid Helium Droplets

Here we report the first mass spectrometric study of negative ions formed via free electron attachment(EA) to nucleobases (NBs) embedded in helium clusters. Pure and mixed clusters of adenine and thymine have been formed by pickup of isolated NB molecules by cold helium droplets. In contrast to EA of isolated molecules in the gas phase we observe a long-lived parent anion NB(-) and, in addition, parentcluster ions NB(-)(n) up to size n = 6. Moreover, we show that a low energy electron penetrating into a doped helium droplet causes efficient damage of the embedded nucleobases via resonant, site selective,dissociative electron attachment

Electron-impact dissociation and ionization of NH+: formation of N+ and N2+

Absolute cross sections for electron-impact dissociation and ionization of NH+ leading to the formation of N+ and N2+ products have been measured by applying the animated electron-ion beam method, in the energy range from the respective thresholds up to 2.5 keV. The maximum total cross sections are observed to be (15.7 +/- 0.7) x 10(-17) cm(2) and (11.1 +/- 0.2) x 10(-18) cm(2) for N+ and N2+, respectively. Absolute cross sections are determined separately for dissociative excitation and for dissociative ionization processes. The measurements for slow N+ ions show a noticeable contribution in the low incident electron energy range; these data are attributed to excitation processes. Dissociative excitation is assumed to play a significant role in the collision energy region close to the vertical excitation energies for the lowest electronic transitions in the Franck-Condon region. The yields of fast N+ ions have also been measured; these energetic dissociations are generally ascribed to ionization processes. Kinetic energy release distributions are seen to extend from 0 eV to 15 eV for the N+ fragments and up to 20 eV for the N2+ ones. The present energy thresholds and kinetic energy release results are compared with available published data, allowing in some cases identification of fragmentation patterns and of molecular states contributing to observed processes. The possibility of dissociative excitation of the molecular ion which could occur via a mechanism involving autoionizing resonances is discussed

Electron-impact ionization of singly-charged neon ions

Absolute cross sections for electron-impact single and multiple ionization of Ne+ leading to the formation of Neq+ (q = 2-5) are reported. The animated crossed beams method is applied in the energy range from the respective thresholds up to 2.5 keV. The maximum cross sections are found to be (4.12 +/- 0.21) x 10(-17) cm(2), (1.42 +/- 0.07) x 10(18) cm(2), (2.57 +/- 0.14) x 10(-20) cm(2), (8.78 +/- 0.54) x 10(-22) cm(2) for the multiply-charged products Neq+ (q = 2-5), respectively. The corresponding threshold energies are measured to be (41.0 +/- 0.5) eV, (105 +/- 1) eV, (205 +/- 5) eV and (345 +/- 15) eV. Present data are compared with available experimental and theoretical results

Electron impact multiple ionization of argon ions

Absolute cross sections for electron impact single and multiple ionization of argon ions leading to the formation of Arq+ (q = 2-8) are reported. The animated crossed beam method is applied in the energy range extending from the respective thresholds up to 2.5 keV. The maximum cross sections for the multiply charged products Arq+ (q = 2-8) are measured to be (12.6 +/- 0.6) x 10(-17) cm(2) for Ar2+, (4.96 +/- 0.21) x 10(-18) cm(2) for Ar3+, (5.50 +/- 0.23) x 10(-19) cm(2) for Ar4+, (8.10 +/- 0.33) x 10(-20) cm(2) for Ar5+, (8.2 +/- 0.7) x 10(-21) cm(2) for Ar6+, (8.9 +/- 0.8) x 10(-22) cm(2) for Ar7+ and (5.5 +/- 1.6) x 10(-23) cm(2) for Ar8+. The corresponding threshold energies are determined to be (27.4 +/- 0.5) eV, (68.7 +/- 0.5) eV, (135 +/- 2) eV, (250 +/- 10) eV, (310 +/- 10) eV, (480 +/- 20) eV and (640 +/- 40) eV for the production of Arq+ (q = 2-8), respectively. The direct process is seen to dominate for q = 2-3, while the indirect processes are dominant for charge states 4-8

Radio Interface for High Data Rate Wireless Sensor Networks

This paper gives an overview of radio interfaces devoted for high data rate Wireless Sensor Networks. Four aerospace applications of WSN are presented to underline the importance of achieving high data rate. Then, two modulation schemes by which High Data Rate can be achieved are compared : Multi carrier approaches, represented by the popular Orthogonal Frequency Division Multiplexing (OFDM) and Single carrier methods, represented by Single Carrier Frequency division Equalization and its application for multiple access Single Carrier Frequency division multiple Access (SC-FDMA). SC-FDMA, with a very low Peak Average Power Ratio (PAPR), is as strong alternative to the OFDM scheme for highly power constraint application. The Chosen radio interface will be, finally, tested by a model based design approach based on Simulink and FPGA realization. SC-FDMA, with a very low Peak Average Power Ratio (PAPR), is as strong alternative to the OFDM scheme for highly power constraint application. The Chosen radio interface will be, finally, tested by a model based design approach based on Simulink and FPGA realization

Isotope effects in electron-impact dissociation of D(2)H(+)

Absolute cross sections have been measured using the crossed electron-ion beams method for electron-impact dissociation of the D(2)H(+) molecular ion yielding H(+), D(+), HD(+) and D(2)(+) fragments. The collision energy ranges from a few eV up to 2.5 keV. Around the maximum, cross sections are found to be of a similar amplitude: (3.25 +/- 0.08) x 10(-17) cm(2) for H(+), (3.56 +/- 0.15) x 10(-17) cm(2) for HD(+) and (3.33 +/- 0.11) x 10(-17) cm(2) for D(2)(+), except for D(+) for which the maximum cross section is (4.62 +/- 0.09) x 10(-17) cm(2). Individual contributions for dissociative excitation (DE) and for dissociative ionization (DI) are determined for each product. Close analysis of present data brings into evidence isotope effects in the fragmentation pattern of the D(2)H(+) target. Ejection of the lightest isotope is generally favoured: for resonant DE (H(+) over D(+)), for DE (molecular ions D(2)(+) and HD(+), associated with ejection of H and D, respectively) and for dissociative ionization, but not for H(+) and D(+) produced via direct dissociative excitation. Present DE cross sections for D(2)H(+) are found to be significantly lower than those measured for D(3)(+), although those of DI agree well together