M. de Frutos

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.

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...

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.

Coulombic fission and evaporation of antimony cluster ions

The preferential dissociation channels of singly and doubly charged antimony clusters have been determined from the unimolecular dissociation of energy rich cluster ions, using an ion stopping technique. It is found that singly charged Sb+n clusters with 5≤n≤80 dissociate by loss of neutral molecules. Neutral dimer loss is observed for n=5, 6, 7 whereas for n≥8, Sb+n loses Sb4. The fragmentation of doubly charged Sb++n has been investigated above the critical size n++c=24 from which doubly charged clusters are detectable in mass spectra. On the time scale of the experiment, which is 1 μs≤t≤100 μs with respect to photoionization, the delayed Coulombic fission into two singly charged clusters competes with the evaporation of Sb4. It is shown that for the smaller Sb++n clusters with 26≤n≤36 the fission to two singly charged clusters is of relatively asymmetrical character, leading to the detachment of five and seven atom cationic fragments. In larger clusters n≥40 the fission is of more symmetrical character...

Bacteriophage T5 DNA Ejection under Pressure

The transfer of the bacteriophage genome from the capsid into the host cell is a key step of the infectious process. In bacteriophage T5, DNA ejection can be triggered in vitro by simple binding of the phage to its purified Escherichia coli receptor FhuA. Using electrophoresis and cryo-electron microscopy, we measure the extent of DNA ejection as a function of the external osmotic pressure. In the high pressure range (7-16 atm), the amount of DNA ejected decreases with increasing pressure, as theoretically predicted and observed for lambda and SPP1 bacteriophages. In the low and moderate pressure range (2-7 atm), T5 exhibits an unexpected behavior. Instead of a unique ejected length, multiple populations coexist. Some phages eject their complete genome, whereas others stop at some nonrandom states that do not depend on the applied pressure. We show that contrarily to what is observed for the phages SPP1 and lambda, T5 ejection cannot be explained as resulting from a simple pressure equilibrium between the inside and outside of the capsid. Kinetics parameters and/or structural characteristics of the ejection machinery could play a determinant role in T5 DNA ejection.

Is the In Vitro Ejection of Bacteriophage DNA Quasistatic? A Bulk to Single Virus Study

Bacteriophage T5 DNA ejection is a complex process that occurs on several timescales in vitro. By using a combination of bulk and single phage measurements, we quantitatively study the three steps of the ejection-binding to the host receptor, channel-opening, and DNA release. Each step is separately addressed and its kinetics parameters evaluated. We reconstruct the bulk kinetics from the distribution of single phage events by following individual DNA molecules with unprecedented time resolution. We show that, at the single phage level, the ejection kinetics of the DNA happens by rapid transient bursts that are not correlated to any genome sequence defects. We speculate that these transient pauses are due to local phase transitions of the DNA inside the capsid. We predict that such pauses should be seen for other phages with similar DNA packing ratios.

Simple metal clusters

The response of alkali cluster ions to an optical excitation is investigated for two different photon energy domains. Below the ionization potential giant resonances in the photoabsorption cross-section are observed for closed shell species. Above the ionization potential, the ionization process competes with the photofragmentation process. The number of valence electrons determines both the behavior of the photoabsorption spectrum and the evolution of the ionization cross-section with the cluster size. The stability of the clusters against an excess of charge is examined through the observation of an asymmetric fission of Nan++. Experimental results are discussed in term of an electrostatic model giving an estimate of the critical size of stability and of the height of the coulombic barrier.

Alkali-metal clusters as prototypes of metal clusters

This paper presents three experiments on mass-selected alkali-metal cluster ions: the unimolecular dissociation after photoexcitation in order to determine the binding energies, the photoabsorption cross-section in an extended energy range 0.6–4 eV, and the ionization potentials of doubly and triply charged clusters. These results are well understood assuming that the valence electrons of alkali-metal clusters constitute a Fermi electron gas in a potential well.

Thermodynamical properties of ionized lithium oxide clusters, Li2n+pO+n

Lithium oxide clusters Li2n+pOn+ are generated by combining reactive nucleation in a gas aggregation source and photoionization. Unimolecular dissociation of mass selected cluster ions provides evidence that the excess of metal atoms evaporates first leading to the most stable species Li+(Li2O)n, which then evaporate Li2O molecules. The evaporation rate behavior as a function of cluster size demonstrates that Li+(Li2O)n can be prepared with different temperatures. It is discussed how metal evaporation from metal‐rich oxide clusters leads to oxygen saturated clusters with a lower temperature. An estimate of the dissociation energies of Li+(Li2O)n are given for small sizes n≤10 from photoevaporation experiment.

Dissociation energies of tellurium cluster ions from thermoevaporation experiments

The dissociation of tellurium cluster ions containing up to 40 atoms has been studied using unimolecular decay of thermoexcited clusters. Clusters with less than 10 atoms dissociate mainly by Te2 loss. Larger clusters fragment by loss of Te5, Te6, or Te7 species. As the cluster size increases, Te5 loss becomes the predominent channel, showing a dissociation which smoothly evolves to the bulk behavior. The dissociation energies of mass selected Te+n, with n=5–35, are deduced from the relative branching ratios of the competitive fragmentation channels. The changes in the observed neutral products are correlated to changes in the dissociation energies which are minima for cluster ion parents containing 13–25 atoms.