Ph. Bréchignac

Rotational relaxation of CO by collisions with H2 molecules: A comparison between theory and experiment

Abstract A comprehensive study of rotational relaxation of CO(X 1 Σ + , v = 1) due to collosions with H 2 perturbers is presented. A time-resolved infrared double resonance experiment is described, which yields transfer and decay rates for the rotational levels around j = 10 at liquid nitrogen and room temperatures. Physical conditions of the experiment are discussed in order to justify the approximation used in the analysis. Close coupling calculations, using a previously computed ab-initio potential energy surface, are developed for the CO + H 2 ( j = 0) collision. The CO + H 2 ( j = 1) collision rates are limited to low rotational levels, and an extension to high rotational levels using the sudden approximation is presented. Spherical tensor formalism is used to derive cross sections suitable for analysing the polarization effects in the experiment together with the pressure-broadening data. Comparison between theory and experiment shows a reasonable agreement, particularly for the ratios of cross sections, in spite of some discrepancies in the absolute values, which are most noticeable at low temperatures.

Ion irradiation of carbonaceous interstellar analogues - Effects of cosmic rays on the 3.4 μm interstellar absorption band

Context. A 3.4 μm absorption band (around 2900 cm-1), assigned to aliphatic C-H stretching modes of hydrogenated amorphous carbons (a-C:H), is widely observed in the diffuse interstellar medium, but disappears or is modified in dense clouds. This spectral difference between different phases of the interstellar medium reflects the processing of dust in different environments. Cosmic ray bombardment is one of the interstellar processes that make carbonaceous dust evolve. Aims. We investigate the effects of cosmic rays on the interstellar 3.4 μm absorption band carriers. Methods. Samples of carbonaceous interstellar analogues (a-C:H and soot) were irradiated at room temperature by swift ions with energy in the MeV range (from 0.2 to 160 MeV). The dehydrogenation and chemical bonding modifications that occurred during irradiation were studied with IR spectroscopy. Results. For all samples and all ions/energies used, we observed a decrease of the aliphatic C-H absorption bands intensity with the ion fluence. This evolution agrees with a model that describes the hydrogen loss as caused by the molecular recombination of two free H atoms created by the breaking of C-H bonds by the impinging ions. The corresponding destruction cross section and asymptotic hydrogen content are obtained for each experiment and their behaviour over a large range of ion stopping powers are inferred. Using elemental abundances and energy distributions of galactic cosmic rays, we investigated the implications of these results in different astrophysical environments. The results are compared to the processing by UV photons and H atoms in different regions of the interstellar medium. Conclusions. The destruction of aliphatic C-H bonds by cosmic rays occurs in characteristic times of a few 108 years, and it appears that even at longer time scales, cosmic rays alone cannot explain the observed disappearance of this spectral signature in dense regions. In diffuse interstellar medium, the formation by atomic hydrogen prevails over the destruction by UV photons (destruction by cosmic rays is negligible in these regions). Only the cosmic rays can penetrate into dense clouds and process the corresponding dust. However, they are not efficient enough to completely dehydrogenate the 3.4 μm carriers during the cloud lifetime. This interstellar component should be destroyed in interfaces between diffuse and dense interstellar regions where photons still penetrate but hydrogen is in molecular form.

Nanostructuration of carbonaceous dust as seen through the positions of the 6.2 and 7.7 μm AIBs

Context. Carbonaceous cosmic dust is observed through infrared spectroscopy either in absorption or in emission. The details of the spectral features are believed to shed some light on its structure and finally enable the study of its life cycle. Aims. The goal is to combine several analytical tools in order to decipher the intimate nanostructure of some soot samples. Such materials provide interesting laboratory analogues of cosmic dust. In particular, spectroscopic and structural characteristics that help to describe the polyaromatic units embedded into the soot, including their size, morphology, and organisation are explored. Methods. Laboratory analogues of the carbonaceous interstellar and circumstellar dust were produced in fuel-rich low-pressure, premixed and flat flames. The soot particles were investigated by infrared absorption spectroscopy in the 2−15 μm spectral region. Raman spectroscopic measurements and high-resolution transmission electron microscopy were performed, which offered complementary information to better delineate the intimate structure of the analogues. Results. These laboratory analogues appeared to be mainly composed of sp 2 carbon, with a low sp 3 carbon content. A cross relation between the positions of the aromatic C=C bands at about 6.2 micron and the band at about 8 micron is shown to trace differences in shapes and structures of the polyaromatic units in the soot. Such effects are due to the defects of the polyaromatic structures in the form of non-hexagonal rings and/or aliphatic bridges. The role of these defects is thus observed through the 6.2 and 7.7 μm aromatic infrared band positions, and a distinction between carriers composed of curved aromatic sheets and more planar ones can be inferred. Based on these nanostructural differences, a scenario of nanograin growth and evolution is proposed.

Photoinduced products from cold coronene clusters : A route to hydrocarbonated nanograins?

Free cold pure coronene clusters have been formed in a gas aggregation source and irradiated with excimer laser pulses. Analysis of the photoproducts thanks to a reflectron time-of-flight mass spectrometer showed that new ionic compounds are formed. These species may include Polycyclic Aromatic Hydrocarbons (PAHs) larger than coronene, PAH-coronene clusters, as well as coronene clusters branched with unsaturated aliphatic chains. The relevance of these results in the context of interstellar medium chemistry, and in particular the carriers of the so-called aromatic infrared emission bands (AIBs), is discussed.

Infrared spectroscopy of aniline (C6H5NH2) and its cation in a cryogenic argon matrix

Infrared absorption spectra of aniline, trapped in a low-temperature 5 K argon matrix, have been obtained in the spectral region from 500 to 4000 cm−1. The aniline cation C6H7N+, was formed inside the matrix by UV laser irradiation. We report the first IR absorption spectroscopy of this cation in an argon matrix: five fundamental vibrational bands were observed.

Electronic spectroscopy of a cyclopentafused PAH cation, the fluorene+: comparison between gas phase and matrix spectra

Abstract The D3←D0 electronic transition of the cold fluorene+ cation, C 13 H 10 + , has been measured in the gas phase and in a neon matrix. The gas phase spectrum exhibits broad vibronic bands of Lorentzian lineshape ( full width at half maximum ( FWHM )=140 cm−1) revealing a very short lifetime for the D3 state. The comparison between the spectra taken in these two media, which are extremely similar, brings new information on the perturbation by the neon matrix. An upper limit of the relaxation rate due to intermolecular dynamics in the matrix has been derived and no effect due to solvation on the non-adiabatic coupling is observed.

Identification of anthracene in Comet 1P/Halley

Context. Polycyclic aromatic hydrocarbons (PAHs) have been detected in many interstellar medium (ISM) sources. They are considered among the most abundant organic compounds in these environments. Aims. We aim at identifying the carriers of the near UV fluorescence bands that appear in the spectra of 1P/Halley’s inner coma. Methods. Near UV spectra of 1P/Halley were recorded on March 9, 1986 by the three-channel spectrometer onboard the Vega2 spacecraft at projected distances of between 421 and 932 km. We compare these data to laboratory spectra obtained under laser-induced fluorescence conditions in a jet-cooled molecular beam. Results. The cometary spectral features are found to be consistent with the laboratory fluorescence spectrum of anthracene. Four main peaks coincide at 363, 367.5, 373 and 382.5 nm. We then report the identification of anthracene, a three-ring PAH in the inner coma of 1P/Halley at projected distances of less than thousand km. We derive an abundance relative to water of 5 × 10 −5 to 1 × 10 −4 for this molecule. This new detection adds to the previously reported UV identifications of phenanthrene and pyrene in the innermost coma of 1/P Halley. Conclusions. Recently, three small PAHs, namely naphtalene, phenanthrene and pyrene, were identified in dust grains collected by the Stardust probe in the environment of Comet 81P/Wild2. These findings, together with the present identification of anthracene in 1P/Halley’s near UV spectra, confirm that similarities exist between the composition of comets and that of the ISM.

Experimental evidence for anharmonic and rotational effects in the evaporation of sodium clusters

Abstract Time-of-flight mass spectrometry is used to measure the translational kinetic energy release during the unimolecular decay of Na n + ( n =5–21) clusters. The detection of neutral fragments of evaporation allows measurement of translational kinetic energies as small as 10 meV. The kinetic energy release predicted in the frame of a harmonic model is compared with the measured value. The discrepancy, interpreted in the frame of a novel extension of phase-space theory, emphasizes the effect of the sub-cluster rotation in the kinetic energy release and the role of vibrational anharmonicity.

Rotational relaxation of CO by collisions with H2, He, Ne molecules: a comparison between diode laser spectroscopy and infra-red double-resonance data

SummaryPressure broadening in infra-red lines of CO (v=1←0) perturbed by H2, He and Ne is studied at low temperature using a tunable diode laser in the 4.7μm region. A new liquid-nitrogen-cooled absorption cell is used. CO−H2 results will be discussed and compared to previous double-resonance data and calculations of inelastic cross-sections.RiassuntoSi studia l’allargamento per pressione delle righe infrarosse del CO (v=1←0) perturbate da H2, He e Ne a bassa temperatura facendo uso di un laser a diodo accordabile nella regione di 4.7 μm. Si è usata una nuova cellula d’assorbimento raffredata con azoto liquido. Si discutono i risultati del sistema CO−H2 e li si confronta con i dati sulle doppie risonanze precedenti e con i calcoli delle sezioni d’urto anelastiche.РезюмеИспользуя настраиваемый диодный лазер в области 4.7 μм, исследуется уширение под действием давления инфракрасных линий CO (v=1←0) в результате соударений с молекулами H2, He и Ne. Используется новая абсорбционная ячейка, охлаждаемая жидким азотом. Обсуждаются результаты для CO−H2 и проводится сравнение с двойным резонансом данными и вычислениями неупругого поперечного сечения.

Laboratory analogues of hydrocarbonated interstellar nanograins

Carbonaceous extraterrestrial matter is observed in a wide variety of astrophysical environments. The spectroscopic signatures revealed a large variety of chemical structure illustrating the rich carbon chemistry that occurs in space. In order to produce laboratory analogues of carbonaceous cosmic dust, a new chemical reactor has been built in the Laboratoire de Photophysique Moleculaire. It is a low pressure flat burner providing flames of premixed hydrocarbon / oxygen gas mixtures, closely following the model system used by the combustion community. In such a device the flame is a one-dimensional chemical reactor that offers a broad range of combustion conditions and sampling which allows production of many and various by-products. In the present work, we have studied: i) the infrared transmission spectra of thin film deposit samples whose nature ranges from strongly aromatic to strongly aliphatic materials; ii) the resonant two-photon photoionisation spectra of gas phase PAHs formed in the flame.