Philippe Bréchignac

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.

Far-infrared spectroscopy of small polycyclic aromatic hydrocarbons

Both experimental and theoretical spectroscopic studies on small gas phase polycyclic aromatic hydrocarbons in the far-infrared spectral region are reported. The experimental set-up based on thermal emission and Fourier transform far infrared analysis led to the detection of relatively broad vibrational bands, unresolved in rotation, representative of each molecule. Detailed theoretical investigations were performed, including both ab initio calculations and spectral simulations. For the majority of the samples, this study provides the first detection of the vibrational modes associated with the skeleton motions.

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.

R2PI detection and spectroscopy of van der Waals complexes of 4-fluorostyrene with rare gases

Abstract One-color resonance-enhanced two-photon ionization spectra of jet-cooled 4-fluorostyrene van der Waals complexes with Ne, Ar, Kr, Xe are reported. The measured spectra display discrete structure and allow the identification of electronic spectral shirts as well as stretching and bending frequencies of the various complexes. These spectra are interpreted with the help of simple model calculations.

Site specificity of solvent shifts as revealed by ionization threshold in aniline-(argon) n clusters

We report ionization threshold measurements for different isomers of Aniline–(argon)n clusters (n=1–3). The data are well represented by simple additivity rules of ‘‘site‐specific’’ solvent shifts, which can be generalized to other systems.

Solvent shift of the ionization potential of the aniline-argon system

The resonant two-photon two-color ionization technique has been used to ionize the Aniline molecule and its Van der Waals complexes with rare gases formed in a supersonic free jet. By scanning the ionizing photon energy, ionization threshold have been determined for Aniline and its first two Van der Waals complexes with Argon. The Ar1 complex has an ionization energy 96 cm−1 lower than bare Aniline and the Ar2 complex has an ionization energy 98 cm−1 lower than the Ar1 complex (194 cm−1 lower than bare Aniline). An interesting correlation has been found between the first Ar-complex solvent shift of the IP for a series of aromatic-argon Van der Waals complexes and the IP of the bare molecule. A simple theoretical model is proposed to discuss the observed correlation.

ISOMER SPECIFIC EVAPORATION RATES : THE CASE OF ANILINE-AR2

This paper reports the results of molecular dynamics (MD) simulations of isomerization and evaporation processes of the aniline–Ar2 cluster. The trajectory results are analyzed in terms of a simple unimolecular kinetics scheme in order to extract isomer‐specific evaporation rate constants. The less stable isomer, denoted (2/0) is found to have an evaporation rate constant that is about 25% smaller than that for the more stable isomer, (1/1). This result is explained in terms of the densities of states associated with each isomer. We present preliminary results on the aniline–Ar3 cluster and connect this latter system to possible experiments.

Solvation of charge in aromatic/noble gas Van der Waals clusters

The results of an experimental study of the ionisation threshold in the various structural isomers of the Van der Waals clusters aniline-(argon)n (n = 1−5) and 4-fluorostyrene-(argon)n (n = 1–2) using resonant two-photon ionisation are reported. The data validate and generalise the site-specific modified shift additivity rule. They show an interesting influence of the localisation of argon atoms at the surface of the chromophore on the value of the net shift of the ionisation potential. Interpreatation of the results involves evaluation and balancing between the charge-induced-dipole interaction and dispersion interaction.

Estimation of the anharmonic quantum density of states in large systems

We present a mixed quantum/classical model which allows an estimation of the quantum density of vibrational states in large anharmonic systems. The construction of the model is based on the modification of the harmonic quantum density of states by using the evolution of the classical anharmonic mean energy as a function of temperature, computed from a classical canonical Nose simulation. The validity of the assumptions proposed in this model has been tested for two 1-D and 2-D model systems for which exact quantum calculations are feasible.

The hindering of the inversion motion in the van der Waals aniline-Arn clusters: An adiabatic molecular dynamics simulation for n=1–3

An adiabatic quantum molecular dynamics simulation has been performed for the van der Waals clusters aniline-Arn (with n=1–3) to understand the influence of the intramolecular inversion mode on the static properties (rotational constants). The effect of deuteration of the amino group in the chromophore (C6H5NH2 or C6H5ND2) on the rotational constants of the complexes has been evaluated. The results are in good agreement with recent experimental values [W. E. Sinclair and D. W. Pratt, J. Chem. Phys. 105, 7942 (1996)]. In the ground electronic state S0, calculations show that the −NH2 (or −ND2) plane of the aniline molecule tends to be blocked on the same side as the argon atom in the complex. On the other hand, for larger clusters (aniline-Ar2 and aniline-Ar3), it appears that the −NH2 (or −ND2) group is more localized on the side opposite to the argon atoms for the isomers containing at least one argon atom bound in a site near the nitrogen atom.