Ramiro Moro

Metastability of free cobalt and iron clusters: a possible precursor to bulk ferromagnetism.

The cobalt and iron clusters CoN, FeN (20<N<150) measured in a cryogenic molecular beam are found to be bistable with magnetic moments per atom both {\mu}N/N 2{\mu}B in the ground states and {\mu}N */N {\mu}B in the metastable excited states (for iron clusters, {\mu}N ~3N{\mu}B and {\mu}N* N{\mu}B). This energy gap between the two states vanish for large clusters, which explains the rapid convergence of the magnetic moments to the bulk value and suggests that ground state for the bulk involves a superposition of the two, in line with the fluctuating local orders in the bulk itinerant ferromagnetism.

Electrostatic deflection of the water molecule: A fundamental asymmetric rotor.

An inhomogeneous electric field is used to study the deflection of a supersonic beam of water molecules. The deflection profiles show strong broadening accompanied by a small net displacement towards higher electric fields. The profiles are in excellent agreement with a calculation of rotational Stark shifts. The molecular rotational temperature being the only adjustable parameter, beam deflection is found to offer an accurate and practical means of determining this quantity. A pair of especially strongly responding rotational sublevels, adding up to ≈25% of the total beam intensity, are readily separated by deflection, making them potentially useful for further electrostatic manipulation.

Nonclassical dipoles in cold niobium clusters

Electric deflections of niobium clusters in molecular beams show that they have permanent electric dipole moments at cryogenic temperatures but not higher temperatures, indicating that they are ferroelectric. Detailed analysis shows that the deflections cannot be explained in terms of a rotating classical dipole, as claimed by Andersen et al.. The shapes of the deflected beam profiles and their field and temperature dependence indicates that the clusters can exist in two states, one with a dipole and the other without. Cluster with dipoles occupy lower energy states. Excitations from the lower states to the higher states can be induced by low fluence laser excitation. This causes the dipole to vanish.

Amino-acid and water molecules adsorbed on water clusters in a beam

Water clusters (H2On and (D2On (n<or=15) are produced by supersonic expansion and then pick up an additional heavy or light water molecule, respectively, or an amino-acid molecule (glycine or tryptophan). The products are analyzed by electron bombardment ionization mass spectrometry. Ionization proceeds via the well-known loss of an OH or OD group, but these turn out to have a strong predilection to come from the guest, rather than the host, molecule: between 30% and 60% of the time the loss originates in the picked-up molecule, even for large n. In fact, the magnitude of this fraction depends on the guest, but is largely insensitive to the cluster size. The observations suggest that the host clusters are frozen into compact annealed shapes, and the adducts reside on the surface and form an inhomogeneity where dissociative ionization tends to localize. It is also notable that no significant amino-acid fragmentation is observed beyond the hydroxyl group loss, in contrast to other measurements employing electron-impact ionization.

Pick-up cell for cluster beam experiments

In experiments in which a cluster beam picks up guest molecules by passing through a cell, the detection efficiency may be limited because of cluster beam scattering by the molecular vapor. We present a modified pick-up cell design that reduces cluster deflections and thereby improves the detection of mixed clusters. Its performance is illustrated with the help of a beam of water clusters picking up amino-acid or heavy-water molecules.

Loss of chlorine in mass spectra of DCl picked up by water clusters in a beam

Water clusters (H(2)O)(n), n<or=16, are produced by supersonic expansion of water vapor into vacuum, and then pick up a DCl molecule. The resulting mixed clusters are analyzed by electron bombardment ionization mass spectrometry. In all cases observed, the chlorine atom is lost in the ionization process, producing a deuterated water cluster cation [(H(2)O)(n)D(+)]. This suggests that the chlorine atom stays on the surface and has a weaker bond to the host cluster.

Electric Dipole Moments of Nitric Acid-Water Complexes Measured by Cluster Beam Deflection

Water clusters embedding a nitric acid molecule HNO3(H2O)_{n=1-10} are investigated via electrostatic deflection of a molecular beam. We observe large paraelectric susceptibilities that greatly exceed the electronic polarizability, revealing the contribution of permanent dipole moments. The moments derived from the data are also significantly higher than those of pure water clusters. An enhancement in the susceptibility for n=5,6 and a rise in cluster abundances setting in at n=6 suggest that dissociation of the solvated acid molecule into ions takes place in this size range.