Coordination du silicium aux hydrocarbures aromatiques polycycliques : modélisations et expériences dans les conditions du milieu interstellaire

Une des découvertes majeures de l'astronomie infrarouge est la présence de molécules polycycliques aromatiques hydrogénées dans les milieux interstellaires et circumstellaires. Ces macromolécules, porteuses des Bandes Aromatiques Infrarouges (AIB) qui illuminent de nombreux objets interstellaires, jouent un rôle essentiel dans la physique et la chimie de ces milieux. L'hypothèse d'une chimie organométallique impliquant ces macromolécules et les éléments lourds tels que le fer, le silicium et le magnésium a été proposée pour expliquer à la fois la forte déplétion de ces éléments dans la phase gazeuse du milieu interstellaire diffus et les variations observées des AIB. Ce travail de thèse a d'abord permis d'obtenir les propriétés structurales, énergétiques et vibrationnelles de complexes p [SiPAH]0/+ d'intérêt astrophysique, en combinant calculs de structure électronique ab initio et basés sur la théorie de la fonctionnelle de la densité (DFT). Les espèces porteuses des AIB étant vibrationnellement excitées, l'influence de la température sur les modes de vibration caractéristiques des complexes [SiPAH]+ a ensuite été évalué par des simulations de dynamique moléculaire Born-Oppenheimer. Enfin, des résultats expérimentaux préliminaires sur la réactivité du système {Si+ ; C24H12} ont été obtenus avec le dispositif PIRENEA, un piège à ion dédié à l'astrochimie. Les complexes-p [SiC24H12]+ ont été formés efficacement et un mécanisme d'isomérisation original a été proposé à l'aide de calculs DFT.The presence of polycyclic aromatic hydrocarbons (PAHs) in interstellar and circumstellar regions is one of the major discoveries of the infrared astronomy. These macromolecules, which are responsible for the aromatic infrared bands (AIBs) that illuminate numerous interstellar objects, play an essential role in the physics and chemistry of these media. The hypothesis of an organometallic chemistry that involves these macromolecules and heavy elements such as iron, silicon or magnesium has been proposed to account for both the strong depletion of these element in the gas phase of the diffuse interstellar medium and the variations observed in the AIBs. The work in this thesis has uncovered the structural, energetic and vibrational properties of [SiPAH]0/+ p-complexes of astrophysical interest by combining ab initio and density functional theory (DFT) approaches to the calculations of electronic structure. Since the species responsible for the AIBs are vibrationally excited, the influence of temperature on the characteristic vibration modes of [SiPAH]+ complexes has been evaluated by Born-Oppenheimer molecular dynamics simulations.Preliminary experimental results on the reactivity of the system {Si+ ; C24H12} have been obtain with the PIRENEA set-up, an ion trap dedicated to astrochemistry. [SiPAH]+ p-complexes were formed efficiently and a novel isomerization mechanism has been proposed based on DFT calculations

Isaac Newton learns Hebrew: Samuel Johnson's Nova cubi Hebræi tabella

This article concerns the earliest evidence for Isaac Newton’s use of Hebrew: a manuscript copy by Newton of part of a work intended to provide a reader of the Hebrew alphabet with the ability to identify or memorize more than 1000 words and to begin to master the conjugations of the Hebrew verb. In describing the content of this unpublished manuscript and establishing its source and original author for the first time, we suggest how and when Newton may have initially become acquainted with the language. Finally, basing our discussion in part on an examination of the reading marks that Newton left in the surviving copies of Hebrew grammars and lexicons that he owned, we will argue that his interest in Hebrew was not intended to achieve linguistic proficiency but remained limited to particular theological queries of singular concern.Michael Joalland’s work was supported by the University of Suwon, 2013. The images are reproduced courtesy of the Syndics of The Fitzwilliam Museum, Cambridge, and of the Syndics of the Cambridge University Library

The PAH hypothesis after 25 years

The infrared spectra of many galactic and extragalactic objects are dominated by emission features at 3.3, 6.2, 7.7, 8.6 and 11.2 \mu m. The carriers of these features remained a mystery for almost a decade, hence the bands were dubbed the unidentified infrared (UIR) bands. Since the mid-80's, the UIR bands are generally attributed to the IR fluorescence of Polycyclic Aromatic Hydrocarbon molecules (PAHs) upon absorption of UV photons -- the PAH hypothesis. Here we review the progress made over the past 25 years in understanding the UIR bands and their carriers.Comment: 13 pages, 7 figures. To appear in the proceedings of IAU symposium 280 "The Molecular Universe

A chirped-pulse Fourier-transform microwave/pulsed uniform flow spectrometer. I. The low-temperature flow system

We report the development of a new instrument that combines chirped-pulse microwave spectroscopy with a pulsed uniform supersonic flow. This combination promises a nearly universal detection method that can deliver isomer and conformer specific, quantitative detection and spectroscopic characterization of unstable reaction products and intermediates, product vibrational distributions, and molecular excited states. This first paper in a series of two presents a new pulsed-flow design, at the heart of which is a fast, high-throughput pulsed valve driven by a piezoelectric stack actuator. Uniform flows at temperatures as low as 20 K were readily achieved with only modest pumping requirements, as demonstrated by impact pressure measurements and pure rotational spectroscopy. The proposed technique will be suitable for application in diverse fields including fundamental studies in spectroscopy, kinetics, and reaction dynamics.National Science Foundation (U.S.) (Award MRI-ID 1126380

[SiPAH]+ pi-Complexes in the Interstellar Medium

We investigate the presence of silicon atoms adsorbed on the surface of interstellar polycyclic aromatic hydrocarbons (PAHs) to form SiPAH pi-complexes. We use quantum chemistry calculations to obtain structural, thermodynamic and mid-IR properties of neutral and cationic SiPAH complexes. The binding energy was found to be at least 1.5 eV for [SiPAH]+ complexes whereas it is roughly 0.5 eV for their neutral counterparts. From the spectral analysis of the calculated IR spectra, we found that the coordination of silicon to PAH+ does not strongly affect the intensities of the PAH+ spectra, but systematically introduces blueshifts of the C-C in-plane and the C-H out-of-plane bands. The thermodynamic data calculated for [SiPAH]+ complexes show that these species are stable and can be easily formed by radiative association of Si+ and PAH species that are known to be abundant in photodissociation regions. Their mid-IR fingerprints show features induced by the coordination of silicon that could account for (i) the blueshifted position of the 6.2 micron AIB and (ii) the presence of satellite bands observed on the blue side of the 6.2 and 11.2 micron AIBs. From such an assignment, we can deduce that typically 1% of the cosmic silicon appears to be attached to PAHs.Comment: Accepted for publication in Astronomy and Astrophysic

Coupled Blind Signal Separation and Spectroscopic Database Fitting of the Mid Infrared PAH Features

The aromatic infrared bands (AIBs) observed in the mid infrared spectrum are attributed to Polycyclic Aromatic Hydrocarbons (PAHs). We observe the NGC 7023-North West (NW) PDR in the mid-infrared (10 - 19.5 micron) using the Infrared Spectrometer (IRS), on board Spitzer. Clear variations are observed in the spectra, most notably the ratio of the 11.0 to 11.2 micron bands, the peak position of the 11.2 and 12.0 micron bands, and the degree of asymmetry of the 11.2 micron band. The observed variations appear to change as a function of position within the PDR. We aim to explain these variations by a change in the abundances of the emitting components of the PDR. A Blind Signal Separation (BSS) method, i.e. a Non-Negative Matrix Factorization algorithm is applied to separate the observed spectrum into components. Using the NASA Ames PAH IR Spectroscopic Database, these extracted signals are fit. The observed signals alone were also fit using the database and these components are compared to the BSS components. Three component signals were extracted from the observation using BSS. We attribute the three signals to ionized PAHs, neutral PAHs, and Very Small Grains (VSGs). The fit of the BSS extracted spectra with the PAH database further confirms the attribution to ionized and neutral PAHs and provides confidence in both methods for producing reliable results. The 11.0 micron feature is attributed to PAH cations while the 11.2 micron band is attributed to neutral PAHs. The VSG signal shows a characteristically asymmetric broad feature at 11.3 micron with an extended red wing. By combining the NASA Ames PAH IR Spectroscopic Database fit with the BSS method, the independent results of each method can be confirmed and some limitations of each method are overcome

Roadmap on dynamics of molecules and clusters in the gas phase

This roadmap article highlights recent advances, challenges and future prospects in studies of the dynamics of molecules and clusters in the gas phase. It comprises nineteen contributions by scientists with leading expertise in complementary experimental and theoretical techniques to probe the dynamics on timescales spanning twenty order of magnitudes, from attoseconds to minutes and beyond, and for systems ranging in complexity from the smallest (diatomic) molecules to clusters and nanoparticles. Combining some of these techniques opens up new avenues to unravel hitherto unexplored reaction pathways and mechanisms, and to establish their significance in, e.g. radiotherapy and radiation damage on the nanoscale, astrophysics, astrochemistry and atmospheric science