Using photoelectron spectroscopy, we have observed alkali anion photodesorption from alkali-halide cluster anions that contain two weakly bound electrons. In the alkali iodides, we have found this type of desorption in almost every (MI)(n)M- cluster we have studied (M=Na, K, Cs; n<9), although it depends on the probe laser frequency and cluster temperature. Using pump-probe techniques, we have shown that the process occurs on a picosecond time scale by way of an electronic excitation of the cluster's spin-paired electrons.

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We have studied near-threshold single-photon detachment of a model negative ion, in the presence of a second, intense radiation field. We find a simple, continuous connection between detachment in strong static fields (dc limit) and the increased energy needed to detach an electron in an intense optical field (ac limit). Below this shifted-threshold energy, net detachment is suppressed because parent systems reabsorb electrons that have been temporarily emitted. This fragile emission-absorption process is easily upset, leading to leakage detachment below the shifted threshold. PACS numbers: 32.80.Fb, 32.80.Rm, 42.50.Vk A number of recent experiments on ionization of atoms in intense radiation fields show convincing evidence for a modification of continuum electron states by the strong oscillatory field. 1 " 5 By superimposing an oscillatory motion onto each electron's drift velocity, the strong field increases that particle's average energy relative to its average momentum and creates a minimum energy U p -e 2 E 2 /4mco 2 , below which there are no continuum states. In order to leave the atom, each electron must absorb not only the simple ionization energy for the atom, but this quiver or ponderomotive "potential" energy as well. Thus the atom's ionization threshold is expected to increase by U p .Most experiments have employed a single radiation field, both to ionize the atom and to produce the ponderomotive ionization threshold shift. Clearly, this field provides all of the energy required to produce free electrons. However, three recent experiments have placed a weak detaching laser beam in the presence of a strong, lower-frequency field to directly observe the detachment threshold shift produced in CI ~ by the strong oscillatory field. 6 " 8 All three experiments show less than the full ponderomotive threshold shift U p , and one of these experiments 8 shows essentially no threshold shift due to the presence of a large standing-wave microwave field. While the outgoing electron must still oscillate in the microwave field, its quiver energy is derived entirely from the microwaves during or after detachment.To develop a theoretical basis for this absence of ponderomotive shift in the low-frequency limit, we have solved the Schrodinger equation numerically for a model one-dimensional negative ion. These calculations provide, for the first time, a continuous connection between ionization of atomic systems in strong dc fields and the full ponderomotive threshold shifts present at high frequencies. We also show that the ponderomotive effect suppresses net ionization by forcing the parent atomic systems to reabsorb electrons that they have temporarily emitted.There have been a number of numerical studies of ionization by a single, intense radiation field 9 " 14 that have focused primarily on multiphoton and above-thresholdionization effects. However, by using two separate oscillatory fields to (1) detach the electron and (2) to modify the continuum, we can fully explore the processes that produce or fail to produc...

Photoelectron spectra obtained from ͑CsX) 3 Cs Ϫ (XϭCl,Br,I͒ cluster anions are strongly dependent on the temperature of the laser vaporization source in which they were produced. Spectral features that were present in clusters emerging from the source at room temperature disappear when the source is cooled to low temperature. This effect is consistent with spontaneous thermal isomerization in the isolated clusters, a precursor to the melting transition in bulk materials.

We show that a Heisenberg model of a spin cluster can describe subdomain ferromagnetic particles, from the dynamics of individual spins to the statistical behavior of the total magnetic moment of the cluster. While an analytical solution of the quantum spin problem is given for small clusters, a classical Heisenberg numerical simulation is used to study clusters of up to 561 atoms. We have studied magnetic behavior as a function of particle size, temperature, and magnetic field and find that the magnetization of individual clusters is well described by a Langevin function, in very good agreement with recent experiments on Co clusters in a molecular beam.