P. Labastie

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.

Two‐photon ionization of alkali‐halide clusters spectroscopy of excess‐electron excited states

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.

Configurational density of states from molecular dynamics simulations

Abstract 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.

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.

Isomerisation and phase transitions in small sodium clusters

Using a distance-dependent tight-binding hamiltonian, we have studied the influence of the temperature on the geometries of small alkali clusters (Na4, Na8, and Na20). We have applied a Monte-Carlo thermodynamical method which consists in performing canonical samplings for various temperatures, these samplings being reexpressed in the microcanonical ensemble. This method provides thermodynamical values such as the entropy and the specific heat. Their behaviour shows one phase transition in the case of Na4 and Na8, and two phase transitions for Na20. As concerns Na4 and Na8, the transition occurs at ≈200 K, between a solid-like phase and a phase for which the geometry of these clusters oscillates between numerous shapes. In the case of Na20, the two observed phase transitions can be described as a melting of the surface atoms (at ≈200 K) preliminarily to the fluctuation of an inner icosahedron seed (at ≈300 K).

Ab initio study of silver bromide AgnBrp(+) clusters (n⩽6,p=n,n−1)

Ab initio calculations in the framework of the density functional theory (DFT) with 19-electron pseudopotential on silver atoms are performed to study the lowest-energy isomers of silver bromide clusters AgnBrp(+) (n⩽6, p=n, n−1). The stability, the structural and electronic properties of neutral, and positively charged systems are examined. The B3LYP functional has been used. For the smallest species, the DFT/B3LYP results are very close to those obtained with multireference configuration interaction or coupled cluster methods. In AgnBrn−1 clusters, the excess electron density shows a picture intermediate between that observed in alkali halide clusters (F-center or localization on a pending alkali atom) and that suggesting partial delocalization on a dimer or trimer silver subunit. Isomer stabilities, fragmentation energies, electric dipole moments, adiabatic and vertical ionization potentials are calculated. The use of 11-electron pseudopotential on silver is discussed.

EVIDENCE FOR SURFACE MELTING IN CLUSTERS MADE OF DOUBLE ICOSAHEDRON UNITS

We study the thermodynamics of small Lennard-Jones clusters (19 ⩽ N </ 34), from both molecular dynamics and Monte Carlo data, analysed with the multiple histogram method. In the case of the 29-atom, 32-atom and 34-atom clusters, there are several indications that a narrow range of internal energies exists where the cluster remains with a solid core and a liquid-like surface. We compare these results with the known surface melting in the Mackay icosahedron LJ55.

One-electron pseudopotential study of NanFn−1 clusters (2⩽n⩽29). II. Absorption spectra, spectral signature, and classification

A one-electron pseudopotential model is used to calculate the excited states of NanFn−1 clusters in the range 2⩽n⩽29. The transition dipole moments are derived to determine the vertical absorption lines from the ground state. Monte Carlo simulations based on the Franck–Condon principle are also achieved to obtain finite temperature absorption spectra. The analysis of ground and excited orbitals in various situations illustrates the rather simple nature of the excitations which are interpreted through an analogy with s−p type transitions in atoms. The influence of electron localization in the initial state is pointed out. Comparison of the simulated spectra with experimental data provides additional keys to understand and discuss the relationship between structure, electron localization, and spectroscopic properties.

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.

Ionization potential measurement of NanFn-p for p up to 5

Abstract 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.