M. Vollmer

Analysis of fractional order thermal desorption

Sodium on a LiF(100) single crystal surface has been studied by thermal desorption. With increasing coverage the spectra exhibit a pronounced shift to higher desorption temperatures. We find that the results can be explained by fractional order desorption kinetics where the order of desorption is x=0.79±0.08. In addition, activation energies have been extracted. They vary between 0.55 and 0.8 eV for coverages ranging from 5×1013 to 1017 atoms/cm2. An analysis of fractional order thermal desorption is presented and the desorption energies are discussed. The results are interpreted in the framework of a microscopic model where sodium atoms desorb from the edges of Nan-clusters formed by adatom diffusion on the LiF insulator surface.

Metal particles on surfaces-desorption, optical spectra, and laser-induced size manipulation

Desorption induced by electronic excitation with laser light is discussed. Emphasis is placed on nonthermal desorption where surface plasma excitation in small particles precedes the rupture of the surface chemical bond. A scenario for the mechanism underlying such a process is proposed. In this context, calculations of the electronic spectra of small sodium particles are presented and the influence of different multipole orders of the collective electron oscillation, of different shapes of the clusters and of the substrate are outlined. Furthermore, manipulation of the size distribution of metal particles on supports is described as an application of the effect. This allows the preparation of very special surfaces with novel physical and chemical properties. Methods to characterize such adsorbate-substrate combinations, especially by use of the optical spectra of the particles, are also discussed. Finally, prospects for future experiments in this field are outlined.

Characterization of large supported metal clusters by optical spectroscopy

Small sodium and silver particles were generated on dielectric substrates like LiF, quartz and sapphire under ultrahigh vacuum conditions. The optical transmission spectra of the clusters were measured as a function of cluster size and shape, for low and high substrate temperatures as well as for s- and p- polarization of the incident light. Excitation of dipolar surface plasmon oscillations in the directions normal and parallel to the substrate surface could be identified. Furthermore, optical spectra for Na and Ag clusters were calculated with the classical Mie theory. The measured spectra vary strongly if the experimental conditions are changed and can be exploited, for example, to characterize the particles with regard to their size and shape. In particular, the axial ratio of the spheroidal clusters could be determined. Its value is considerably different for the two investigated metals and depends on the substrate material. Furthermore, the temperature of the substrate has a pronounced influence on the shape of the particles. At low temperature of T=100 K two-dimensional island growth is predominant. The particles extend only little in the direction perpendicular to the surface and coalesce readily at small coverage of metal atoms. In contrast, the clusters are truly three-dimensional at T=300 K. At this stage, sodium particles still exhibit a rather small axial ratio whereas silver clusters appear almost spherical. Thus, measurements of the optical spectra permit direct in situ monitoring of cluster growth during the nucleation of adsorbed atoms and of temperature induced shape variations. In addition to investigations of the shape of the particles, the quadrupolar surface plasmon mode was observed for Ag clusters.

Ablation of metal particles by surface plasmon excitation with laser light

Abstract Small sodium particles supported on an insulating LiF crystal are illuminated with visible light of an Ar+ or Kr+ laser. As a result the emission of neutral metal atoms from the particle surface is observed. the strongly resonant character of this emission, centered at about 490 nm, indicates that a surface plasmon excitation precedes the ejection. Time-of-flight measurements show large mean velocities of the emitted atoms. Continued laser illumination of the metal particles leads to an ablation of typically 20% of the initial coverage. The process is explained by a nonthermal mechanism.

Towards monodisperse neutral clusters—concepts and recent developments

(1990). Towards monodisperse neutral clusters—concepts and recent developments. Phase Transitions: Vol. 24-26, No. 2, pp. 737-784.

Desorption of metal atoms with laser light of different polarization

Laser-induced desorption of metal atoms from the surface of small metal particles has been investigated as a function of the shape of the particles and the polarization of the incident laser light. The particles were supported on LiF, quartz or sapphire substrates. In a first set of experiments, the shape of the particles was determined by recording optical transmission spectra with s- and p-polarized light incident under an angle of typically 40° with respect to the surface normal. The metal particles turn out to be oblate, the ratio of the axes perpendicular and parallel to the substrate surface being on the order of 0.5. This ratio decreases with increasing particle size. Also, the particles change shape if the temperature is raised. In further experiments, s- and p-polarized light has been used to stimulate desorption of atoms via surface plasmon excitation. It is found that the desorption rate markedly depends on the polarization of the light. This is explained by excitation of the collective electron oscillation along different axes of the non-spherical particles.

Laser-stimulated desorption of potassium atoms

Desorption of K atoms by laser-excitation of surface plasmons in small K particles is reported. The desorption rate has been measured for different laser wavelengths and particle sizes. Time-of-flight measurements reveal a kinetic energy of the desorbed atoms of Ekin=0.13(3) eV. From the experimental data it is concluded that the desorption mechanism is non-thermal in nature. Comparison of the results reported here with our earlier work on Na desorption is made.

Optical spectra and laser-induced dissociation of supported Na particles

This paper reports new results on the optical spectra of Na particles and on laser-induced photodissociation of Na atoms from the surface of these particles. In continuation of our earlier studies we have performed experiments to elucidate the mechanism of thenonthermal dissociation process. Furthermore, theoretical calculations have been carried out with the goal to correlate the wavelength dependence of the photodissociation yield with the optical absorption spectra of the metal particles. In addition, manipulation of the size distribution of metal particles on supports is outlined as an application of the effect. This allows for the preparation of very special surfaces with novel physical and chemical properties.

Interplay between collective and single electron excitations in large metal clusters

Collective and single electron excitations of large metal clusters supported on transparent substrates are investigated. The applied experimental techniques include extinction spectroscopy and laser induced dissociation accompanied by the ejection of individual atoms. The optical spectra depend on the electromagnetic far field and reflectcollective electron excitations of the conduction electrons, i.e. surface plasmons. Dissociation, however, is correlated to repulsivesingle electron energy levels. The characteristics of these localized excitations indicate a strong influence of collective excitations. In particular, it is found that nonlocal optical effects are important. In this picture surface plasmons catalytically enhance the number of single electron excitations and therefore of the metal atoms ejected as a result of the absorption of visible light. Results will be presented, which illustrate this interplay between collective and single electron excitations.

Desorption Stimulated by Collective Electron Excitation

In recent years laser-induced desorption processes have found a continuously growing interest [1–3]. Desorption stimulated with laser light can take place by different mechanisms. Most frequently it is simply promoted by thermal heating. Light is absorbed at the substrate surface, the temperature rises and the particles desorb. Bond breaking at the surface can also be stimulated by vibrational excitation of the adsorbate molecules with infrared laser light. Coupling of the vibrational excitation to the phonon bath of the substrate is the dominant channel for energy relaxation. As a result, the surface temperature rises causing desorption. This means that the energy is absorbed by a resonant process initially, but the desorption is thermal, a phenomenon denoted as “resonant heating”. It is responsible for the lack of molecular or isotopic selectivity in such experiments. In addition to vibrational excitation laser induced photodesorption can also be accomplished by electronic excitation. Even though the study of such processes is a new field and the number of experiments is still small compared to that of classical DIET processes [4–6] stimulated by electron bombardment or synchrotron radiation, different types of excitation have been observed ana different mechanisms established. Electronic excitation may either be accomplished in the adsorbate, in the substrate or in the adsorbate-substrate complex. The majority of experiments have been performed with high intensity pulsed lasers, where the available energy of the photons usually does not exceed about 6 eV.