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Contrary to the claims in a recent letter by Michaelianand Santamar´ia-Holek (MSH) [1], heat capacities can benegative in atomic clusters. This effect has been care-fully examined many times in the past [2–7]. Negativeheat capacities can be found in noncanonical ensembles,and have been extensively investigated in microcanoni-cal ensembles, whereas they are always positive in thecanonical ensemble. The finite size of the system is a suffi-cient condition for statistical ensembles to be inequivalent.This inequivalence has also been proved for systems in thethermodynamic limit [8], providing various examples withan “S-bend” in the caloric curve. The Gibbs equationinvoked by MSH does not hold in the microcanonicalensemble.On the other hand, both true non-ergodicity andthe failure to achieve ergodicity due to a finite obser-vation time may also cause S-bends in caloric curves.Intermediate states with the solid and liquid phases incontact can be thermodynamically unfavourable becauseof the free-energy cost of introducing the interface [5].The related energy barriers between the phases increasewith system size. In macroscopic systems they maybecome too high to overcome on the experimental timescale, leading to phase separation, superheating, andsupercooling. Various models have been constructed

Molecular simulations are very convenient to get insight into the structure and thermal behaviour of small atomic clusters. In this paper, we show how the classical Monte Carlo method can be used to tackle both the global optimization problem of molecular structure and the finite temperature properties. Smarter schemes to compute the caloric curves are also illustrated in the example of the histogram reweighting method.

We propose a new method to study metastable fragmentation of clusters in a reflectron time-of-flight mass spectrometer. This method does not require any modification of the standard apparatus. No preselection of parents is needed: all fragmentation reactions can be analyzed by recording a few numbers of spectra obtained by changing only the repelling voltage of the reflectron Vref. The evolution of a peak as a function of 1/Vref is a straight line. The extrapolation of this line at 1/Vref=0 gives the mass of the parent M and its slope gives the mass of the fragment m: the whole decay channel set for all parents can be determined unambiguously. The intensity of a peak is stable when scanning Vref: rather accurate branching ratios can be determined. Application to the fragmentation of clusters containing Ag, H2O, and SF6 is presented. Masses are measured to an accuracy of about 0.5%, allowing for example to study independently the dissociation channels involving the two natural isotopes of silver, 107Ag and...

We investigate the thermodynamical behavior of small neutral sodium fluoride clusters using both Monte Carlo (MC) and molecular dynamics (MD) simulations. The three clusters studied, (NaF)n with n = 6, 7, and 13, all display thermodynamical phases of a solidlike nature at low temperature and liquidlike at higher temperatures. However, unlike van der Waals clusters, the melting process appears to be a multistage phenomenon: The clusters undergo successive transitions toward more fully fluid states. We relate this behavior to the isomerization processes occurring in these systems and the topography of the potential energy surface. As one of our indicators, we calculate the largest Liapunov exponent λ as a function of the total energy E. The slope of log λ(E) strongly drops at the onset of first isomerization. The subsequent fluid transitions are not associated with mere changes in this slope. In view of this fact, we conclude that the degree of chaos in the dynamics of the system is not sensitive to the to...

This Letter examines new types of localization sites for an excess electron in finite alkali-halide clusters resulting from defects on cuboidal structures, namely {open_quotes}edge states,{close_quotes} R center, and other surface defects. We present theoretical calculations on Na{sub n}F {sub n{minus}1} clusters with one excess electron. Comparisons with experimental results are presented for different cluster sizes (n=17 , 23, 28, and 29). Structures with edge or surface defects are relevant for n=23 , 28, and 29. {copyright} {ital 1997} {ital The American Physical Society}

We study the thermodynamics of small Lennard-Jones clusters (19 ⩽ N

Na n F n - p clusters were produced in a laser vaporization source. They were ionized by one photon and detected in a high-resolution time of flight mass spectrometer. Only odd values of p were observed, apart from Na 4 F 2 , Na 8 F 6 , Na 14 F 12 and Na 23 F 21 . Ionization potentials (IP) were measured for n up to 30 and p up to 5. The IPs of the Na 14 F 14- p sequence are in good agreement with existing theoretical predictions. Na 15 F 12 and Na 24 F 21 have a surprisingly low IP, which indicates that the metallization sequence is of a different nature in this case.

The multiple histogram method has been adapted to treat distributions of potential energies taken from classical molecular dynamics simulations carried out at different total energies. For clusters with a Lennard-Jones pairwise interaction, it is compared to the results of the original method, which was developed for constant temperature Monte Carlo simulations. The same caloric curves are found in both cases. Since the Monte Carlo algorithm is always cheaper, this makes it a method of choice for studying clusters at thermodynamic equilibrium.

NanFn−1 clusters are produced in a laser vaporization source. They are ionized by one‐photon or resonant two‐photon ionization, and detected in a high resolution time of flight mass spectrometer. By scanning the ionization wavelength, it is possible to obtain ionization potentials and absorption spectra of these clusters, for n ranging from 2 to 42. The results we obtain show many discrepancies with the cuboid model, which has proven valid for small sizes. The possible origin of these disagreements is discussed.

We have measured the single and double photoionization efficiency of Hgn mass selected clusters as a function of photon energy between 30 and 110 eV for masses ranging from n=1 to 36 for the singly ionized and from n=5 to 70 for the doubly ionized clusters, using the light beam issued from a wiggler at the BESSY synchrotron. The results show in all cases a spectrum similar to the 5d→ef shape resonance previously observed in the isolated atom. In contrast, the expected 5p→6p transition allowed by hybridization of the 6s and 6p orbitals is not observed. We also compared the mass spectra of singly and doubly charged mercury clusters ionized by electron impact (30–90 eV) to those of clusters ionized by synchrotron radiation (30–70 eV). We found that the ratio Hg2+n/Hg+n was higher for photoionization at low energy and for electron impact ionization at high energy. Finally, we estimated the critical stability size of the triply charged 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.

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.

The ionization potentials of Lin and Nan 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.

International audience; We present a joint theoretical and experimental investigation of small neutral and anionic silver fluoride clusters View the MathML source(AgF)n(-) with n ⩽ 6. We have measured the electron detachment energies of the anions View the MathML source(AgF)n- and determined the adiabatic electron affinities of neutral (AgF)n clusters. Ab initio calculations in the framework of the density functional theory have been performed to investigate the electronic and structural properties of both neutral and anionic species. The localization of the excess electron in the anion is discussed.

Small Na 3 F n-p clusters can be considered as formed of n Na + ions and n-p F - ions, which tend to form a rock-salt lattice, together with p excess electrons. The electronic arrangement, and hence the ionization potential, is thus very structure dependent. In this respect, it may happen that different isomers of the same cluster present a significant difference between their ionization potential, allowing one to monitor selectively the passage through one specific isomer in time-resolved experiments. Indeed, the ionization probability may become time-dependent, provided the probe pulse energy is low enough to photoionize only one isomer while the molecule is excited in such a way that it wanders through several structures. Na 3 F is such a cluster, with two isomers: the lowest one with a quasi-planar C 2v structure and vertical ionization energy of roughly 4.9 eV, while the second one with 3D C 3v structure and vertical ionization energy of 4.4 eV. Such a large difference in vertical ionization energies makes this cluster an ideal system to test the above ideas.

In this article we present the first measurement of the electric dipole susceptibility of one excess electron NanFn–1 clusters. The static electronic polarizability and the permanent electric dipole of these clusters have been calculated with a one-electron model. Experimental values for the susceptibility are clearly related to the calculated values of the permanent dipole. The size evolution of the dipole moments is interpreted in terms of the interplay between the lattice and electron properties. This study outlines that the response of the cluster to the electric field cannot be fully understood with only equilibrium structure calculations and that the coupling between the permanent dipole and the vibrational motion of the cluster has to be taken into account

The abundance spectra and the fragmentation channels of silver bromide clusters have been measured and analyzed. The most abundant species are AgnBrn - 1+ and AgnBrn + 1- and Ag14Br13+ is a magic number, revealing their ionic nature. However, some features depart from what is generally observed for alkali-halide ionic clusters. From a certain size, AgnBrn - 1+ is no more the main series, and AgnBr n - 2, 3+ series become almost as important. The fast fragmentation induced by a UV laser makes the cations lose more bromine than silver ions and lead to more silver-rich clusters. Negative ions mass spectra contain also species with more silver atoms than required by stoichiometry. We have investigated the metastable fragmentation of the cations using a new experimental method. The large majority of the cations release mainly a neutral Ag3Br3 cluster. These decay channels are in full agreement with our recent ab initio DFT calculations, which show that Ag+-Ag+ repulsion is reduced due to a globally attractive interaction of their d orbitals. This effect leads to a particularly stable trimer (AgBr)3 and to quasi-planar cyclic structures of (AgBr)n clusters up to n = 6. We have shown that these two features may be extended to other silver halides, to silver hydroxides (AgOH)n, and to cuprous halide compounds.

Metastable fragmentation channels of free silver bromide clusters (AgnBrp)+ are reported. The large majority release a neutral trimer (AgBr)3. This is interpreted in the frame of ab initio DFT calculations with 19-electron pseudopotential on silver atom. The stability, the structural and electronic properties of (AgnBrp)(+) (n = 6, p = n, n - 1) are examined. The structures of the clusters are different from expected in electrostatic models. The 3D transition has not yet happened: the calculated ground state structures of (AgBr)n? 6 clusters are quasi two-dimensional. Our work rationalizes the part of covalence in the Ag-Br interaction and the role of d electrons of silver. The known abundance of trimers in the vapor of silver bromide, but also of other silver and cuprous halides is for the first time explained as an indirect consequence of the covalent interaction between metal ions.

The LinHm clusters are produced in a seeded molecular beam. The lithium vapor is expanded in the vacuum with a mixture of 99% of argon and 1% of H2. The formed clusters are analyzed by a time of flight mass spectrometer and the clusters electronic structure is probed through ionization potential measurements. The electronic structure of LinH and LinH2 may be interpreted by assuming that the bonding of one hydrogen atom localizes one electron of the lithium atoms and that the other electrons remain delocalized.