Paolo Milani

Improved pulsed laser vaporization source for production of intense beams of neutral and ionized clusters

We describe an improved laser vaporization source for production of intense and stable cluster beams. Intensity and stability are notably improved compared with standard sources by the introduction of a cavity in which the vaporization takes place before expansion. This source has been successfully used for electric deflection studies of highly collimated cluster beams.

Large ion volume time‐of‐flight mass spectrometer with position‐ and velocity‐sensitive detection capabilities for cluster beams

We have developed and tested a high‐performance time‐of‐flight mass spectrometer for clusters in a molecular beam. Several important improvements over conventional designs have been implemented that include a large ionization volume, a perfected ion steering system and a large area efficient ion detector. Cluster ions that are produced within the 200‐cm3 ion volume can be simultaneously detected over a size range of from 1 to about 10u2009000 amu with a resolution of about 1000. The mass spectrometer can also be operated in a position‐sensitive mode for cluster beam deflection measurements, and in a velocity‐sensitive mode to determine the cluster velocities. The performance of the spectrometer is demonstrated with Al and Fe clusters. We present mass spectra of cluster beams and also velocity, and magnetic and electric deflection measurements of collimated iron and aluminum cluster beams.

Magnetic properties of free alkali and transition metal clusters

The Stern-Gerlach deflections of small alkali clusters (N<6) and iron clusters (10<N<500) show that the paramagnetic alkali clusters always have a non-deflecting component, while the iron clusters always deflect in the high field direction. Both of these effects appear to be related to spin relaxation however in the case of alkali clusters it is shown that they are in fact caused by avoided level crossing in the Zeeman diagram. For alkali clusters the relatively weak couplings cause reduced magnetic moments where levels cross. For iron clusters however the total spin is strongly coupled to the molecular framework. Consequently this coupling is responsible for avoided level crossings which ultimately cause the total energy of the cluster to decrease with increasing magnetic field so that the iron clusters will deflect in one direction when introduced in an inhomogeneous magnetic field. Experiment and theory are discussed for both cases.

An improved laser vaporization cluster source and time-of-flight mass spectrometer

We briefly describe an improved laser vaporization cluster source, which produces intense, stable and cold cluster beams, and a new time-of-flight mass spectrometer, which in several respects is more versatile and better suited for cluster studies than traditional designs. The mass spectrometer has a high resolution mode with a very large effective ionization region as well as position and velocity sensitive detection modes.

Electronic properties of aluminum clusters compared with the jellium model

The experimental polarizabilities, ionization potentials and electron affinities of aluminum clusters are compared with jellium predictions. It is found that the clusters have radii and work functions which are close to the jellium model predictions for clusters with more than 13 atoms. The polarizabilities of Aln correspond with the jellium only forn>40 and the shell structure features in the ionization potentials are anomalous up to 37. We conclude that nonjellium effects are important up ton=40.

Magnetic Properties of Small Iron Clusters in a Molecular Beam

The average magnetic moments of small iron clusters with less than 300 atoms per cluster have been measured from deflections of a cluster beam by a Stern-Gerlach magnet. We find that the deflections are always in the high field direction for all clusters studied. This behavior indicates that intra-molecular spin relaxation is occurring. We have measured the moments as a function of size and qualitatively as a function of temperature. The measured average moments are found to increase with increasing temperature and increasing size, and in all cases studied are found to be lower than the bulk value of 2.2 μ . We tentatively conclude than the spin relaxation process involves an exchange of angular momentum between the spin system and the rotation of the cluster.

Hyperspectral imaging with deformable gratings fabricated with metal-elastomer nanocomposites

We report the fabrication and characterization of a simple and compact hyperspectral imaging setup based on a stretchable diffraction grating made with a metal-polymer nanocomposite. The nanocomposite is produced by implanting Ag clusters in a poly(dimethylsiloxane) film by supersonic cluster beam implantation. The deformable grating has curved grooves and is imposed on a concave cylindrical surface, thus obtaining optical power in two orthogonal directions. Both diffractive and optical powers are obtained by reflection, thus realizing a diffractive-catoptric optical device. This makes it easier to minimize aberrations. We prove that, despite the extended spectral range and the simplified optical scheme, it is actually possible to work with a traditional CCD sensor and achieve a good spectral and spatial resolution.

MAGNETIC PROPERITES OF IRON CLUSTERS IN A MOLECULAR BEAM

Magnetic properties of small iron clusters in a molecular beam are investigated by examining their deflections in a Stern-Gerlach magnet. The magnetization is probed as function of magnetic field, temperature and cluster size. The clusters are either heated with light from a pulsed laser which affects the vibrational temperature, or cooled in supersonic expansions which primarily affect the rotational temperature. We find that at temperatures up to 1500 K the magnetic moments of laser heated Fe120–140 clusters are much larger than predicted either in the Heisenberg model or compared with the bulk values suggesting a stronger exchange interaction. Furthermore, rotationally cold clusters show anomalously magnetization which is non-linear with the applied field. This effect is found to be related to the anisotropy coupling of the total spin with the cluster framework. A model taking this effect into account and assuming a resonant coupling between the rotations and the Larmor precession of the spins gives go...

Metal-polymer nanocomposites for stretchable optics and plasmonics

Stretchable and conformable optical devices open very exciting perspectives for the fabrication of systems incorporating diffracting and optical power in a single element and of tunable plasmonic filters and absorbers. The use of nanocomposites obtained by inserting metallic nanoparticles produced in the gas phase into polymeric matrices allows to effectively fabricate cheap and simple stretchable optical elements able to withstand thousands of deformations and stretching cycles without any degradation of their optical properties. The nanocomposite-based reflective optical devices show excellent performances and stability compared to similar devices fabricated with standard techniques. The nanocomposite-based devices can be therefore applied to arbitrary curved non-optical grade surfaces in order to achieve optical power and to minimize aberrations like astigmatism. Examples discussed here include stretchable reflecting gratings, plasmonic filters tunable by mechanical stretching and light absorbers.

Surface and crystal field effects on the metallic properties of small systems

The aim of this paper is to present and illustrate on the basis of two different experimental studies the evolution of the metallic character of a sample when its size is increased from the isolated atom to the infinite solid. Emphasis is made on the fact that the evolution of the metallic character depends on the property studied. A distinction is also made between the case where the properties are modified by the presence of the surface as boundary, but can be otherwise described by parameters characteristic of the bulk, and the more complex case where modifications of the crystal field have to be taken into account.