Hellmut Haberland

Thin films from energetic cluster impact : a feasibility study

An intense, continuous beam of metal clusters and cluster ions is produced by combining a magnetron sputter discharge with a gas aggregation source. The average cluster size can be varied between 50 and more than 106 atoms per cluster. The sputter discharge is also used to ionize the clusters; between 30% and 80% of them carry a charge without further electron‐impact ionization. Mon− clusters with n≊1200 were separated from the neutral clusters, accelerated, and deposited on a polished Cu substrate. Above a kinetic energy of 6 keV, highly reflecting, strongly adhering thin films are formed on room‐temperature substrates. The films can be mechanically polished, which increases the reflectivity from 95% to 97% at 10.6 μm. Rutherford backscattering spectroscopy data reveal that less than 0.5% argon is incorporated into the films. The standard structure zone model of Movchan, Demchishin, and Thornton [in B. Chapman, Glow Discharge Processes (Wiley, New York, 1982)] is not applicable. The impact of an energeti...

Irregular variations in the melting point of size-selected atomic clusters

Small particles have a lower melting point than bulk material. The physical cause lies in the fact that small particles have a higher proportion of surface atoms than larger particles—surface atoms have fewer nearest neighbours and are thus more weakly bound and less constrained in their thermal motion, than atoms in the body of a material. The reduction in the melting point has been studied extensively for small particles or clusters on supporting surfaces. One typically observes a linear reduction of the melting point as a function of the inverse cluster radius,,. Recently, the melting point of a very small cluster, containing exactly 139 atoms, has been measured in a vacuum using a technique in which the cluster acts as its own nanometre-scale calorimeter,. Here we use the same technique to study ionized sodium clusters containing 70 to 200 atoms. The melting points of these clusters are on average 33% (120 K) lower than the bulk material; furthermore, we observe surprisingly large variations in the melting point (of ±30 K) with changing cluster size, rather than any gradual trend. These variations cannot yet be fully explained theoretically.

Filling of micron‐sized contact holes with copper by energetic cluster impact

A completely ionized and clustered beam of Mo or Cu is deposited with variable kinetic energy on a substrate, and the filling of micron‐sized contact holes on semiconductor devices is studied. An excellent hole filling is obtained for the impact of charged copper clusters, if they contain 1000–3000 Cu atoms and impinge with a kinetic energy of about 10 eV per atom on a substrate having a temperature of 500 K. The morphology of small hole fillings by slow and energetic cluster impact is discussed.

Experimental observation of the negatively charged water dimer and other small (H2O)−n clusters

Beams of (H2O)−n and (D2O)−n have been produced by injecting low energy electrons into a supersonic expansion of water and heavy water seeded in rare gases. Clusters with n≥2, with the exception of n=4, have been observed. The size distribution can be separated into three groups (n=2, n=6–7, and n≥10), which may be associated with different types of electron binding. The n<10 result gives a new lower limit for the number of water molecules necessary to bind an electron; the n≥10 correspond to the n=11 threshold observed earlier in pure H2O expansions.

A model for the processes happening in a rare-gas cluster after ionization

Abstract The sequence of events occurring in a rare-gas cluster after ionization is discussed, and a timescale for these events is obtained. It is argued that the positive charge is, after an initial delocalization, localized in a dimer ion. Its large binding energy of about 1 eV leads to severe cluster fragmentation. The ionized clusters cannot be considered to be small, defect-free rare-gas crystallites. They contain a point defect, a self-trapped hole, whose electronic structure is the antimorph of a V k center. This model is in agreement with many data on rare-gas cluster ionization as well as with knowledge on gaseous, liquid and solid rare gases. Finally, it is discussed that one can obtain a lower bound to fragmentation processes of any type of cluster from the study of electron- or photon-stimulated desorption processes from macroscopic surfaces.

The interaction of cesium with oxygen

Ultraviolet photoelectron spectroscopy (UPS) and electron spectroscopy by deexcitation of metastable noble gas atoms (MDS) were used to follow the continuous oxidation of Cs films. While UPS has a finite information depth, MDS probes only the properties of the outermost atomic layer. Small doses of O2 cause the formation of a ‘‘monolayer’’ of Cs11O3 in which the O2− ions are incorporated below the surface, while the surface itself consists merely of metallic Cs atoms. Further uptake of oxygen leads to a continuous transformation of the subsurface layer into Cs2O2, while a small concentration of adsorbed O atoms builds up on the surface which act as nucleation centers for subsurface oxidation. Finally, the surface layer is completely oxidized into CsO2 which then retards further oxygen penetration into the bulk. The chemical state of the surface can be directly correlated with the associated change of the work function. A monolayer of Cs adsorbed on a Cu(110) surface exhibits quite different properties.

Negative ion photoelectron spectroscopy of solvated electron cluster anions, (H2O)n− and (NH3)n−

The photodetachment spectra of (H2O)n=2−69/− and (NH3)n=41−1100/− have been recorded, and vertical detachment energies (VDEs) were obtained from the spectra. For both systems, the cluster anion VDEs increase smoothly with increasing sizes and most species plot linearly withn−1/3, extrapolating to a VDE (n=∞) value which is very close to the photoelectric threshold energy for the corresponding condensed phase solvated electron system. The linear extrapolation of this data to the analogous condensed phase property suggests that these cluster anions are gas phase counterparts to solvated electrons, i.e. they are embryonic forms of hydrated and ammoniated electrons which mature with increasing cluster size toward condensed phase solvated electrons.

Low temperature formation of benzene from acetylene on a Pd(111) surface

Abstract Experiments with UPS, metastable noble gas deexcitation spectroscopy (MDS) and thermal desorption demonstrated that C2H2 adsorbed on Pd(111) at 140 K undergoes cyclotrimerisation to C6H6 after higher (≳ 100 L) exposures. If the surface is intermediately warmed up to 300 K, the low temperature state of adsorbed acetylene transforms irreversibly into another species which is unreactive. The surface species formed by reaction was identified by comparison with the electron spectroscopic data of C6H6 adsorbed from the gas phase as well as with those of free C6H6. The molecules are only weaky held on the surface and start to desorb already at about 150 K.

A new type of cluster and cluster ion source

Combining a magnetron gas discharge with the gas aggregation technique an intense source of clusters has been developed. A large part (up to 80%) of the clusters can be generated as ions without using additional electron impact ionisation.

Clusters of water and ammonia with excess electrons

Abstract Beams of (H 2 O) − n ( n = 2, 3, ⩾ 5) and (NH 3 ) − n ( n ⩾ 35) have been produced by injecting low-energy electrons into supersonic expansions of H 2 O and NH 3 . The distribution of (H 2 O) − n is very sensitive to the carrier gas in the expansion; n ⩽ 10 are observed only when H 2 O is seeded in rare gases, n = 2, 6, 7 and n ⩾ 11 are particularly stable and may be associated with different types of electron binding. In contrast, the minimum n = 35 for (NH 3 ) − n is surprisingly large and virtually independent of expansion conditions.