Ph. Dugourd

Direct measurement of the electric polarizability of isolated C60 molecules

We present the first direct measurement of the electric polarizability of isolated C60 molecules by molecular beam deflection technique. We have obtained a value of 76.5±8.0 A3 which is consistent with most of the recent calculations and slightly lower than the value of the polarizability of C60 measured in fullerite crystals.

Measurements of lithium cluster ionization potentials

The ionization potentials of Lin clusters are obtained by photoionization. Shell effects are observed at n = 8 and n = 20, as well as strong odd/even alternations. The results are compared with the spherical shell model and the ab initio calculations.

Static dipole polarizability of small mixed sodium–lithium clusters

We have measured the static dipole polarizability of Nay−xLix clusters (with y=2, 3, 4, and 8) by molecular beam deflection technique. For a given size, the polarizability of pure lithium clusters is smaller than the polarizability of pure sodium clusters. For mixed clusters, a smooth decrease in the polarizability is observed as the proportion of lithium atoms increases. For the NaLi molecule, both experimental permanent dipole and average polarizability have been obtained. Experimental results are compared to results of density functional theory and configuration interaction single and double (CISD) ab initio calculations.

Ionization potenital measurements of hydrogenated lithium clusters

Ionization Potentials of LinHm clusters have been measured by photoionization. As in Lin, odd-even alternations and shell closing effects are observed. In a first approximation, we find that LinH clusters behave like Lin−1 and LinH2 like Lin−2. The results may be interpreted by assuming that the bonding of one hydrogen localizes one electron and that the other electrons remain delocalized.

Vibronic structure of the Li3 ground state

Abstract The two-photon ionization spectra of the Li 3 molecule have been systematically investigated. A new spectroscopic system corresponding to the C state has been observed. The hot bands of the A←X and C←X transitions have been recorded at various temperatures. The assignment of these bands allows us to determine completely the ground-state potential surface. The results obtained lead to a quasi-free pseudo-rotation motion even in the lowest vibronic level.

Metal-insulator segregation in lithium rich LinHm + clusters

The metallicity of lithium rich LinHm + cluster ions [1⩽m⩽6, n⩽22, and (n–m)>3] is studied by measuring unimolecular dissociation rates. These clusters are found to decompose by evaporation of a Li atom or a Li2 molecule. The evaporative rates of mixed clusters display features characteristic of metallic clusters. This confirms and extends to a larger size range the previous results obtained by photoionization and absorption cross-section measurements. The simulation of the evaporative rates, with a statistical Rice–Ramsperger–Kassel (RRK) model which has been adapted to mixed clusters, confirms that there is a clear separation between a metallic Lin–m  + part and an insulating (LiH)m part.

Experimental and theoretical investigations of ionization potentials and structures of mixed sodium lithium clusters

Abstract We have measured the ionization potentials (IP) of Na n − m Li m clusters (3⩽ n ⩽21 and 1⩽ m ⩽3) generated in a supersonic beam. The IPs of mixed clusters are close to the values obtained for pure sodium and pure lithium clusters. They display the features of a metallic system with n delocalized electrons. For clusters containing 2–6 atoms, we have performed density functional theory calculations. The IPs calculated for the lowest-energy structures are in good agreement with experiment. The geometries of the heteronuclear clusters, in particular the 2D–3D transition, are discussed in detail.

Electronic properties and geometric structures of Li4H and Li9H from optical absorption spectra

Optical absorption spectra of Li4H and Li9H clusters have been recorded by depletion spectroscopy in the visible range. From comparison with ab initio calculations, geometries of both clusters are identified. The hydrogen atom assumes a peripheral position bridging two and three Li atoms in the planar and three‐dimensional structures of Li4H and Li9H, respectively. Na4F and Na9F clusters are also theoretically studied and it is shown how the strong electronegativity of the F atom leads to different geometries than in lithium hydrids. Finally, the metallic character of these clusters is discussed and in both cases, the hydrogen or fluorine atom localizes one valence electron. However, the optical absorption spectra are much broader than in pure Lin and Nan clusters due to the lower symmetry.

First observation of an excited state of Li2H

Abstract We report the first experimental optical spectra of Li 2 H. Vibrational structure of the Li 2 H B state has been obtained by two-photon ionization spectroscopy, as well as high-resolution rotational spectra. Simulations of the rotational structure in the first vibrational band allowed us to determine the geometry of Li 2 H in the ground and excited B states: LiLi = 2.52 A, LiH = 1.715 A in the ground A 1 state and LiLi = 3.24 A, LiH = 2.29 A in the B 1 state. The geometry found for the ground state is in excellent agreement with several ab initio calculations. The ionic character of both states is discussed.

High resolution spectroscopy of small metal clusters

The optical absorption spectrum of small lithium clusters has been measured up to Li8. In Li3 high resolution Two Photon Ionization (TPI) spectra have been recorded allowing us to determine the geometry and potential surfaces of the ground and excited states. In larger clusters, the excited states are dissociative and the absorption spectra have been obtained by Depletion Spectroscopy. Vibronic resolution is still achieved in Li4, but not in larger clusters. The measured spectra exhibit a rather small number of transitions to electronically excited states. In Li7, only one intense band is observed in the blue region, while in Li8, an intense band is also observed in the blue region and a much weaker band in the red region. All the obtained results are in very good agreement with the ab initio calculation of Bonacic-Koutecky et al. This demonstrates that molecular effects are always present in these small clusters. The semi-classical models of surface plasma resonances are also discussed.