Ph. Cahuzac

Dynamics of unimolecular dissociation of sodium cluster ions

We investigate the unimolecular dissociation dynamics of energy‐rich sodium cluster ions, Na+n (5≤n≤40) by measuring the time evolution of their sequential monomer or dimer evaporative cooling. The experimental technique, tandem time‐of‐flight mass spectroscopy, measures the relative rate of competing dissociation channels from metastable ion clusters selected during an initial sampling time interval immediately following the creation of the ion cluster ensemble. Pulsed laser UV photoionization converts the distribution of neutral clusters emerging from a free‐jet expansion to the distribution of ion clusters from which the initial selection takes place. For the smaller clusters, 3≤n≤14, we compare the measured dissociation rates with those calculated from a modified version of the RRK theory of unimolecular dissociation. In applying the theory we use monomer and dimer binding energies determined from theoretical calculation. For larger clusters, 15≤n≤40, the binding energies are not known, and we invert ...

Dissociation pathways and binding energies of lithium clusters from evaporation experiments

The unimolecular dissociation of energy rich lithium cluster ions shows that Li+n dissociate by sequential atom or dimer loss. The binding energies of Li+n (n=4–42) generated in an evaporative ensemble are determined from unimolecular decay, within a well defined time window, and energy constraint. They present a sawtooth behavior vs cluster size less pronounced that it should be from a simple metal model. Odd–even alternation is superimposed on the sawtooth behavior, with odd sized cluster ions being more stable. Cohesive energies per atom of Li+n are deduced from these dissociation energies up to n=40 and from extended photo‐induced measurements up to n=95. Cohesive energies per atom of neutral clusters Lin are derived by combining these ionic cohesive energies with the literature ionization potentials. The linearity of the neutral cluster cohesive energy vs the cluster surface to volume ratio permits a volume and a surface energy to be deduced. These values are compared to the bulk values.

Collective excitation in closed-shell potassium cluster ions

Abstract Photoabsorption spectra of mass-selected potassium cluster ions K 9 + and K 21 + were obtained by photoevaporation spectroscopy in the range 0.6 to 4.7 eV. A giant resonance due to the collective dipole oscillation of the valence electrons dominates the photoabsorption spectra. For these spherical clusters, experimental evidence for the Lorentzian shape of the resonance is obtained. A blue-shift and line narrowing are observed for K 21 + . The dependence of damping on cluster size is discussed.

Morphology control of the supported islands grown from soft-landed clusters

Abstract The morphology of islands grown on surfaces from soft-landed clusters has been investigated by electron microscopy. Compact islands have been observed on amorphous carbon surfaces, whereas an evolution from compact to ramified shapes occurs on graphite surfaces as the mean size of deposited clusters increases. Moreover, by increasing the surface defect density on graphite, a continuous variation of the island morphology is observed, from extended ramified shapes to small compact shapes. In order to account for the island morphologies observed, we propose a crude model involving a competition between the time for aggregated clusters to coalesce and the time interval between successive arrivals of clusters to grow the islands. It shows that there exists a critical island size R 0 dividing island shapes into compact shapes for R R 0 and into ramified shapes for R > R 0 . This critical size R 0 varies as a function of the incident cluster size. Relying on our experimental results, we show how the morphology of the islands can be controlled by the size of the incident clusters and the presence of surface defects.

Electronic shell structure of laser-warmed Na clusters

Abstract Mass spectra are reported for large sodium clusters warmed by a continuous laser beam prior to ionization. During the 1 ms warming period, the clusters lose more than 10% of their mass by single-atom evaporation. The resulting size distribution reveals what appears to be electronic shell structure for clusters containing up to 2500 atoms.

Size dependence of inner-shell autoionization lines in mercury clusters

Abstract Inner-shell photoionization spectra of Hg n clusters ( n ⩽8) have been obtained with a molecular beam experiment using synchrotron radiation from the VUV storage ring at Orsay. The photoionization spectra exhibit two strong autoionization resonances which are red-shifted with increasing cluster size. The present results are compared with solid-state experimental data which indicate that the clusters (⩽Hg 8 ) do not yet have metallic character.

Cohesive energies of K+n 5<n<200 from photoevaporation experiments

Evaporative cooling of internal energy rich potassium cluster ions K+n (5<n<200) is investigated within two well defined but quite different time windows. One of the time windows starts 1 μs after the photoexcitation of the cluster ions isolates one step in the evaporative cooling cascade. The experimental technique insures the complete determination of the dissociation channels. Tandem time‐of‐flight mass spectroscopy measures the relative rate of competing dissociation channels from ion fragmentation patterns. The corresponding neutral fragments are unambiguously determined after the reionization. Values for the dissociation energies of K+n (up to n=25) have been deduced from the unimolecular dissociation rates using statistical methods. These values are compared to Huckel calculations. The second time window starting just after the reexcitation of mass selected K+n is used to follow the steps of the photoinduced sequential evaporation from ‘‘hot’’ clusters. The photofragmentation patterns for several p...

Adiabatic decomposition of mass‐selected alkali clusters

The decomposition of metastable photoionized mass‐selected alkali clusters is investigated using a tandem time‐of‐flight spectrometer. Na+n and K+n are found to decompose mainly by the evaporation of either a single neutral atom or a neutral dimer in a time scale of about 10 μs. The predominant fragmentation channels are found to follow the adiabatic dissociation channels associated with the lowest energies accordingly to our CI calculations. In the light of our experimental and theoretical results a comparison between the different calculations of the absolute atomization energies available in the literature is presented.

Optical excitation in small ionized sodium clusters : closed-shell and open-shell systems

Abstract Photoabsorption cross-section profiles have been obtained for closed-shell and open-shell ionized size-selected sodium clusters Na + 9 , Na + 21 and Na + 11 . The spill-out of the valence electron cloud explains the red-shifted collective excitation frequency with regard to the Mie frequency. Quantum-mechanical calculations may give energy positions of the resonant structures consistent with the experiment. However, the absolute parameters of the profiles suggest that some oscillator strength is missing.

Photodissociation of size‐selected K+n clusters

Photofragmentation studies are described for size‐selected ionized potassium‐atom clusters, generated in a supersonic expansion and analyzed with a tandem time‐of‐flight system. In the range n=3 to 41 the relative ion intensities in photofragmentation spectra reflect the relative stabilities of the ion products. Moreover the photon‐energy dependence of the fragmentation patterns suggests that photodissociation proceeds rather via a fast sequential evaporation of neutral monomers than via a fission process. A value can be deduced for the mean binding energy per atom which is increasing with the cluster size.