Pierre Boissel

The profiles of the aromatic infrared bands explained with molecular carriers

Using a compilation of the photophysical properties of molecules such as coronene, C 2 4 H 1 2 , we have constructed a numerical model to calculate the IR emission spectrum of a population of polycyclic aromatic hydrocarbon(PAH) species subjected to UV photons. The model enables us to calculate not only the energy emitted in the IR bands but also the detailed band profiles. The calculated spectrum for a PAH size distribution following N - 3 . 5 C with N C m i n = 30 was found to provide a good match to the Aromatic Infrared Bands (AIBs) observed in the planetary nebula IRAS 21282+5050. The match is especially good for the intensities and for the profiles of the 6.2 and 11.3 μm bands. This work provides a coherent picture of the AIB problem supporting the PAH molecular hypothesis. It further underlines the poor spectral diversity contained in the AIB spectrum putting new constraints on the nature of the emitting population.

Ultrasensitive spectroscopy of ionic reactive intermediates in the gas phase performed with the first coupling of an IR FEL with an FTICR-MS

First example of coupling a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FTICR-MS) with an infrared Free Electron Laser (FEL) is presented. This experimental setup is ideally suited for the direct structural characterization of reactive polyatomic ions. Ultrasensitive measurements of the infrared vibrational spectrum of ionic reactive intermediate selectively prepared is allowed by the association of the high peak power of the FEL, its wide tunability, and the flexibility of FTICR-MS, where several mass selections and ion-molecule reactions can be combined. These possibilities are demonstrated in the case of Fe + complexes where two photofragmentation pathways compete. The resulting infrared spectrum is in excellent agreement, both with respect to the position and to the relative intensities of the infrared transitions, with predicted by ab initio electronic structure calculations. r 2003 Elsevier Science B.V. All rights reserved.

Infrared spectra of homogeneous and heterogeneous proton-bound dimers in the gas phase

Infrared multiphoton dissociation spectra of three homogeneous and two heterogeneous proton-bound dimers were recorded in the gas phase. Comparison of the experimental infrared spectra recorded in the fingerprint region of the proton-bound dimers with spectra predicted by electronic structure calculations shows that all modes which are observed contain motion of the proton oscillating between the two monomers. The O-H-O asymmetric stretch for the homogeneous dimers is shown to occur at around 800 cm-1. As expected, the O-H-O asymmetric stretching modes for the heterogeneous proton-bound dimers are observed to shift to significantly higher energy with respect to those for the homogeneous proton-bound dimers due to the asymmetry of the O-H-O moeity. This shift is shown to be predictable from the difference in proton affinities between the two monomers. Density functional predictions of the infrared spectra based on the harmonic oscillator model are demonstrated to predict the observed spectra of the homogeneous proton-bound dimers with reasonable accuracy. Calculations of the structure and infrared spectrum of protonated diglyme at the B3LYP/6-31+G** level and basis also agree well with an infrared spectrum recorded previously. For both heterogeneous proton-bound dimers, however, the predicted spectra are blue-shifted with respect to experiment.

Calculations of the far-infrared emission of C24H12 under interstellar conditions

This paper presents the first calculations which have been performed to predict the band profile and intensity of the low-frequency flopping modes of polycyclic aromatic hydrocarbons under interstellar conditions. The numerical simulations are based on a molecular description of the involved photophysical processes and include all available molecular data. We present the results of these calculations for the molecule C24H12, emphasizing the need for further laboratory studies. The importance of this work for future astronomical observations is underlined.

MICRA: a compact permanent magnet Fourier transform ion cyclotron resonance mass spectrometer.

MICRA,1 a compact Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer is described. The amount of miniaturisation in this device, based on a 1.24 T permanent magnet, remains compatible with genuine FT-ICR performance and analytical power in the mass range 2–1000 m/z, with a mass resolving power of 73,000 at mass 132. A first application of the transportability is the repetitive coupling of MICRA with a large-scale source of IR photons, the free electron laser CLIO.

Fragmentation of isolated ions by multiple photon absorption: A quantitative study

The photofragmentation of anthracene cations under continuous irradiation by visible light is used as a tool to measure the radiative exchanges between the isolated species and its environment. The comparison of the experimental results with a numerical simulation allows an absolute determination of the oscillator strength of the whole set of absorption lines. Experimental evidence is given that, besides infrared vibrational emission, the contribution of electronic transitions has to be accounted for in the evaluation of the cooling rate.

Gas phase formation and reactivity of iron naphthylene cations: a route for polycyclic aromatic hydrocarbon growth

Abstract The formation and reactivity of iron naphthylene cations in the gas phase are investigated by Fourier transform mass spectrometry. Fe(C 10 H 7 Cl) + and Fe(C 10 H 6 ) + are prepared by reaction of 1-chloronaphthalene with iron cations. Special attention has been paid to the reaction of iron napthylene cations with C 10 H 7 Cl. This leads to the formation of a five ring molecule which could be perylene or benzofluoranthene through spontaneous elimination of HCl. The reaction of C 10 H 7 Cl with Fe(C 10 H 7 Cl) + displays a different behaviour since the elimination of one Fe and two Cl atoms leads to the formation of a binaphthyl cation. Astrophysical implications on the growth of large PAHs are proposed.

Detailed Characterization of 2-Heptanone Conversion by Dielectric Barrier Discharge in N2 and N2/O2 Mixtures

The products of 2-heptanone conversion by dielectric barrier discharge plasma are analyzed under different conditions: alternating current (ac) or pulsed mode of excitation, variable energy, variable composition of the carrier gas. The efficiency of the conversion is higher using a pulse excitation mode than an ac mode. With a small oxygen percentage (about 2-3%) added to nitrogen, 2-heptanone is about 30% more efficiently removed than in pure nitrogen, while the 2-heptanone removal decreases with an oxygen percentage higher than 3%. A new analysis method, based on chemical ionization mass spectrometry, is used for volatile organic compound detection along with chromatography. Several products issued from 2-heptanone conversion with ac excitation are identified in nitrogen and in air, and a chemical scheme is proposed to explain their formation and their treatment by the discharge. It appears that byproducts are issued not only from oxidation reactions, but also from C-C bond cleavage by collisions with electrons or nitrogen excited states.

Infrared time-resolved study of matrix isolated SF6

Abstract An infrared-infrared double resonance technique is used to study the vibrational energy relaxation of SF 6 in the ν 3 = 1 level, trapped in Ne, Ar, Kr, Xe and N 2 matrices. With different CO 2 laser lines, fundamental and hot bands can be probed. A general scheme for intramolecular V-V transfer is proposed, consistent with the observed signals. The energy cascades from ν 3 to the lowest mode ν 6 in the nanosecond range. Then ν 6 relaxes directly to the phonons, with time constants at 8 K of 12.2 × 10 3 , 6.5 × 10 3 , 200, 7 and 2 respectively. The temperature dependence of the rate constant is discussed in the frame of direct multiphonon processes as well as of binary collisions.

The energetics of fragmentation of the naphthalene cation

Abstract The energetics of acetylene loss from the naphthalene cation and the relative stabilities of the three most probable cationic fragments have been investigated using the CIPSI method and CASSCF plus CI treatments. Theoretical results, which are in agreement with experimental data when available, point to the benzocyclobutadiene cation as a possible product of polycyclic aromatic hydrocarbon (PAH) fragmentation in the interstellar medium.