Thomas Pino

Electronic absorption spectrum of cold naphthalene cation in the gas phase by photodissociation of its van der Waals complexes

The electronic absorption spectrum of the naphthalene cation has been obtained in conditions relevant for comparison with the diffuse interstellar bands in astrophysics, i.e., cold species in the gas phase. The novel technique consisting to photodissociate a selectively R2P2CI-prepared PAH–argon van der Waals complex in a molecular beam [Ph. Brechignac and T. Pino, Astron. Astrophys. 343, L49 (1999)] has been used. The various aspects of the method are described in detail. The whole visible range has been explored revealing two electronic transitions displaying 28 vibronic bands. Absolute absorption cross sections have also been measured, and found much larger than reported from rare gas matrices studies. The additional information on the matrix-induced or complex-induced shifts and widths, and on the intramolecular and intermolecular processes involved in these species, is discussed. No definite conclusion about the possible presence of the cation in space can be drawn so far.The electronic absorption spectrum of the naphthalene cation has been obtained in conditions relevant for comparison with the diffuse interstellar bands in astrophysics, i.e., cold species in the gas phase. The novel technique consisting to photodissociate a selectively R2P2CI-prepared PAH–argon van der Waals complex in a molecular beam [Ph. Brechignac and T. Pino, Astron. Astrophys. 343, L49 (1999)] has been used. The various aspects of the method are described in detail. The whole visible range has been explored revealing two electronic transitions displaying 28 vibronic bands. Absolute absorption cross sections have also been measured, and found much larger than reported from rare gas matrices studies. The additional information on the matrix-induced or complex-induced shifts and widths, and on the intramolecular and intermolecular processes involved in these species, is discussed. No definite conclusion about the possible presence of the cation in space can be drawn so far.

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.

Laboratory spectra of cold gas phase polycyclic aromatic hydrocarbon cations, and their possible relation to the diffuse interstellar bands

A novel laboratory technique is described, combining the use of supersonic expansion, laser excitation and small aromatic-rare gas van der Waals (vdW) clusters properties, which was developed to access the electronic absorption spectra of the polycyclic aromatic hydrocarbon (PAH) cations in the visible. It consists in preparing vdW complexes of the PAH molecule with a rare gas in a molecular beam, to photoionize it by resonant selective two-photon ionization, then to photodissociate this ionic complex by means of a delayed laser pulse in a time-of-flight mass spectrometer. The method is illustrated by presenting the visible spectra of the Naphthalene, Phenanthrene, Fluorene and Phenylacetylene cations. Such spectra can be unambiguously compared to the astronomical spectra of reddened stars, which exhibit the so-called diffuse interstellar bands (DIBs) in absorption. An interesting feature of the technique is its ability to measure the absolute absorption cross-sections. The large values of the oscillator strengths of the transitions, which are derived, are discussed in the astrophysical context which consists in considering that the PAH cations could be carriers for some of the DIBs.

Photoionization of cold gas phase coronene and its clusters: Autoionization resonances in monomer, dimer, and trimer and electronic structure of monomer cation

Polycyclic aromatic hydrocarbons (PAHs) are key species encountered in a large variety of environments such as the Interstellar Medium (ISM) and in combustion media. Their UV spectroscopy and photodynamics in neutral and cationic forms are important to investigate in order to learn about their structure, formation mechanisms, and reactivity. Here, we report an experimental photoelectron-photoion coincidence study of a prototypical PAH molecule, coronene, and its small clusters, in a molecular beam using the vacuum ultraviolet (VUV) photons provided by the SOLEIL synchrotron facility. Mass-selected high resolution threshold photoelectron (TPES) and total ion yield spectra were obtained and analyzed in detail. Intense series of autoionizing resonances have been characterized as originating from the monomer, dimer, and trimer neutral species, which may be used as spectral fingerprints for their detection in the ISM by VUV absorption spectroscopy. Finally, a full description of the electronic structure of the monomer cation was made and discussed in detail in relation to previous spectroscopic optical absorption data. Tentative vibrational assignments in the near-threshold TPES spectrum of the monomer have been made with the support of a theoretical approach based on density functional theory.

The C−H Stretch Intensities of Polycyclic Aromatic Hydrocarbon Cations. Origins and Astrophysical Implications

Infrared vibrational transition intensities of polycyclic aromatic hydrocarbons are known to depend strongly on the charge state. The detailed understanding of this effect for the C-H stretching modes has been approached by applying the quantum theory of atoms in molecules. Several benchmark calculations were undertaken in order to disentangle charge and size effects, from benzene (C(6)H(6)) up to the ovalene (C(32)H(14)) molecule. Upon decomposition of the dipole moment derivative along a C-H stretch into charge, charge flux, and dipole flux terms, it is found that it is the competition between the sum of the first two terms and the latter which drives the intensity, due to their opposing signs. Additionally, while the dipole flux term changes very little with size and charge, the other terms are strongly sensitive to these. This effect leads to a very weak C-H stretch intensity for cation sizes close to pyrene (C(16)H(10)) and comparable intensities between neutral and cations for the much larger ones. The astrophysical implications are discussed.

Visible photodissociation spectra of the 1- and 2-methylnaphthalene cations: laser spectroscopy and theoretical simulations.

The electronic absorption spectra of the two methyl derivatives of the naphthalene cation were measured using an argon tagging technique. In both cases, a band system was observed in the visible range and assigned to the D2 ← D0 electronic transition. The 1-methylnaphthalene(+) absorption bands revealed a red shift of 808 cm(-1), relative to those of the naphthalene cation (14,906 cm(-1)), whereas for 2-methylnaphthalene(+) a blue shift of 226 cm(-1) appeared. A short vibrational progression, similar to the naphthalene cation, was also observed for both isomers and found to involve similar aromatic ring skeleton vibrations. Moreover, insights into the internal rotation motion of the methyl group were inferred, although the spectral resolution was not sufficient to fully resolve the substructure. These measurements were supported by detailed quantum chemical calculations. They allowed exploration of the potential energy curves along this internal coordinate, along with a complete simulation of the harmonic Franck-Condon factors using the cumulant Gaussian fluctuations formalism extended to include the internal rotation.

R2PI Spectroscopy of Aromatic Molecules Produced in an Ethylene-Rich Flame

Laser spectroscopy, combined with mass spectrometry, was applied to study the spectra of aromatic molecules produced in a premixed ethylene-rich flat flame. These studies produce new gas-phase electronic spectra of polyaromatic compounds, which ultimately will guide the understanding of the chemical processes that lead to polycyclic aromatic hydrocarbon (PAH) growth or PAH formation locking. Resonant two-photon ionization (R2PI) spectra of all species detectable in a specific fuel-rich flame were recorded simultaneously during a single scan of the laser wavelength, within the 220-330 nm range. Comparison with spectra available in the literature allowed us to identify 16 aromatic species. In the PAH forming region of this flame, we found that the main PAHs are accompanied by a great diversity of other species, including in particular various side-chains on aromatic networks. We also show that this technique allows, at least in some cases, to distinguish between different isomers associated with the same mass peak, although the extracted PAHs are only cooled down to room temperature.

Interstellar and interplanetary solids in the laboratory

The composition of the interstellar matter is driven by environmental parameters (e.g. elemental abundance, density, reactant nature, radiations, temperature, time scales) and results also from extreme interstellar medium physico-chemical conditions. Astrochemists must rely on remote observations to monitor and analyze the composition of interstellar solids. These observations give essentially access to the molecular functionality of the solids, rarely elemental composition constraints and isotopic fractionation only in the gas phase. Astrochemists bring additional information from the study of analogues produced in the laboratory, placed in simulated space environments. Planetologists and cosmochemists can have access and spectroscopically examine collected extraterrestrial material directly in the laboratory. Observations of the diffuse interstellar medium (DISM) and molecular clouds (MC) set constraints on the composition of organic solids and large molecules, that can then be compared with collected extraterrestrial materials analyses, to shed light on their possible links.

Ion-pair dissociation of highly excited carbon clusters, size and charge effects

Ion-pair dissociation of a highly excited molecule is a relaxation process giving rise to emission of anionic and cationic fragments. We present first measurements of ion-pair dissociation of carbon clusters. We found that ion- pair relaxation is an ubiquitous, although very small, relaxation channel common to all sizes and charges of Cq+n species produced in high velocity C+n-He collisions. Quantitative interpretation of measured branching ratios is conducted on the basis of a statistical approach i.e through listing of all possible final states.