Stéphane Douin

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.

Visible emission from the vibrationally hot C2H radical following vacuum-ultraviolet photolysis of acetylene: Experiment and theory

Photolysis of acetylene has been performed by vacuum-ultraviolet excitation with the synchrotron radiation via the Rydberg states converging to the first ionization potential (IP) at 11.4 eV. Only the visible fluorescence of the ethynyl radical was observed in the A 2Π–X 2Σ+ system. Excitation of several Rydberg states of acetylene over a large energy range between 9 and 11.4 eV allowed us to observe for the first time the evolution of this continuum with increasing Rydberg excitation. Intensity calculations based on accurate ab initio potential energy surfaces of C2H were performed by using a one-dimensional model accounting for the large-amplitude motion of the H atom around the C–C bond and for the overall rotation of the radical. These calculations successfully reproduce the observed visible continuum (maximum at 500 nm and blue side cutoff at 400 nm) and bring new information on the distribution of the internal energy deposited in the fragment. For most excited Rydberg states, predissociation occurs...

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.

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.

Dual time-of-flight photoelectron-photoion spectrometry on a magnetic bottle-molecular beam apparatus

Abstract A supersonic molecular beam experiment uncluding a standard “magnetic bottle” TOF photoelectron spectrometer and a TOF ion mass spectrometer has been constructed with the goal of performimg MPI studies of cooled molecules and clusters. The supersonic beam characteristics have been tested by using several pure or mixed rare-gas expansions. A rotational temperature of about 0.5 K was measured from the rotational contours of the S 1 ←S 0 transition of aniline, demonstrating the qualities of the molecular beam in the interaction region. The capability of the apparatus to form van der Waals clusters and to mass analyze them was checked by performing (1+1) REMPI on fluorostyrene—(Ar) n and aniline—(Ar) n clusters formed in a helium/argon mixture expansion. The sensitivity of the photoelectron detection was measured by recording the photoelectron spectrum of a pure xenon expansion via a (3+2) photon ionization process.

Neutral excited radicals formed by ethylene photodissociation in the 8–24 eV region

Dissociation pathways following photolysis of ethylene C2H4 in the 6.2–24.8 eV energy range, and leading to excited fragment production are investigated. The dispersed visible fluorescence of the products has been recorded for a large range of excitation energies, allowing for the determination of experimental energy thresholds for the different pathways. It is found that the C2H(A) radical is the dominant fragment, emitting in the visible range. It is formed with a considerable amount of vibrational excitation energy, as was previously reported in the photodissociation study of acetylene C2H2. From the dissociation threshold measurements, several barrierless processes are evidenced. In particular, the excited CH*(A or B) radical is formed via an intermediate isomer, ethylidene, with no excess energy. More surprisingly, all channels involving one H2 molecular elimination are observed without any barrier, unlike the dissociation processes occurring on the ground state potential energy surface (PES) of C2H4. Most probably, this observation originates from an easier relaxation of the excited state into pyramidalised states.