Mass spectra obtained upon ion-beam-induced desorption of homologic series of SAMs on Au(111) (see picture) show an odd-even effect following odd-even changes in the SAM chain length. By comparing the measured results with existing data obtained by other experimental techniques, a new methodology for tracing systematic changes in the stability of AuS and SC bonds in SAMs is demonstrated.

Using laser ionization in combination with time-of-flight mass spectrometry, we have studied ion-induced desorption of neutral particles from self-assembled monolayers (SAMs) of ω-(4'-methylbiphenyl-4-yl) alkane thiols (CH3(C6H4)2(CH2)nSH, BPn, n = 2, 4, 6) formed on Au(111) substrates. Because BPn/Au(111) SAMs with n = even exhibit polymorphism, the effect of purely structural changes on emission yield and fragmentation pattern could be studied without interference from changes in the chemical composition. In spite of the high energy of the primary ion beam (15 keV), the mass spectra reveal a striking sensitivity of the desorption process to rather subtle changes in the structure of the layer. Depending on the SAM structure, substantial differences in the ratio between the cleavage of the molecule−substrate and the C−S bonds are observed. For applications of SAMs as resists in ion beam lithography, the results demonstrate that well-defined removal of molecules requires exact control of the SAM structure.

The structures of neutral and cationic Cr3On0,+ (n = 0-3) clusters are calculated with density functional theory employing the BLYP and BP86 functionals. Gas-phase CrnOm clusters are produced by laser vaporization and characterized with time-of-flight mass spectrometry. The ionization energies of Cr3On (n = 0-2) are determined with threshold photoionization spectroscopy using tunable laser light in the 4.5-5.60 eV range. On the basis of a comparison between experimental and calculated ionization energies, ground-state structures were assigned. The influence of sequential addition of oxygen on the exchange coupling between the chromium atoms is investigated providing evidence for enhanced ferromagnetic coupling of chromium atoms in both the neutral and cationic Cr3On0,+ clusters. This evidence of superexchange interaction through oxygen extends earlier ideas to control the magnetic interactions in the chromium dimer via chemical reactions with oxygen toward larger chromium clusters.

Size dependent stabilities, fragmentation pathways and dissociation energies of a series of gas phase cationic doped gold clusters, Au(n)X+ (3 < or = n < or = 20; X = Y, Er and Nb), and pure Au(n)+ clusters were investigated in photofragmentation experiments. Size dependent stability patterns were obtained and the branching between monomer and dimer evaporation was studied. For bare gold, the competing neutral monomer and dimer evaporation channels were found to be in agreement with earlier studies. For doped clusters, monomer evaporation is the most likely fragmentation channel with the exception of Au18Y+ and Au20Y+ for which gold dimer evaporation is also observed. Relations between the evaporative activation energies and both the experimental abundances and the fragment yield were derived based on unimolecular rate constants. The dissociation energies from this analysis show an odd-even staggering and enhanced stabilities for certain cluster sizes, in agreement with simple electronic shell model predictions.

General principles for designing stable highly symmetrical clusters are proposed. This approach takes advantage of both the extra stability of cage aromaticity and the good geometrical balance between the outer cage and the endohedral atom. The applicability of these design principles was confirmed by gas-phase experimental observations on group 14 element cages with endohedral Al's and also is illustrated by many literature examples of diverse systems.

We report on the investigation of the double-resonant photoionization of Sr atoms out of the ground state 5s(1)(2)S(0), and the metastable triplet states 5s5p P-3(j0). Specifically, the saturation behavior of both the excitation steps to intermediate states, and the ionization steps to auto-ionizing states have been studied, and the photoionization cross-sections from the intermediate levels into auto-ionizing states were determined. The ionization schemes were applied in saturation conditions to estimate the ratio of atoms sputtered in the ground and metastable states during Ar+ ion-beam sputtering from a polycrystalline strontium target. (C) 2000 Elsevier Science B.V. All rights reserved.

We report on the production of small and medium size lithium and lithium oxide clusters by a laser vaporization cluster source. The isotopomeric distribution of natural lithium allowed to identify LikO clusters as the most abundant components in the mass spectrum. Photoionization efficiency curves of LikO clusters with photon energies from 3.4 to 4.7 eV were measured for 8 ≤ k ≤ 27. Using linear extrapolation of the increase in photoionization efficiency with photon energy, ionization potentials were extracted. With the chemical bond of the O2- anion to two Li atoms, leaving n = k-2 valence electrons in the (Li2O)Lin clusters, clear shell closure effects are present at n = 8 and n = 20.

A systematic NMR decoration study of the field inhomogeneity in the mixed state of high-T-c cuprates is presented. A thin layer of silicone oil formed around individual grains due to wetting was used as the NMR probe, We have measured the temperature dependence of the additional NMR line broadening Delta w, caused by the field modulation in the mixed state. Using the relation Delta w = 1/lambda(2)(T) we have determined the temperature dependence of the London penetration depth lambda(T). The lambda(T) variation can be characterized by a simple power law, lambda(2)(0)/lambda(2)(T) = 1-(T/T-c)(n), We have obtained for YBa2Cu3O7, Bi2Sr2Ca2Cu3O10 and Bi2Sr2CaCu2O8 the powers n = 3, 2.35 and 2.26 and the values lambda(0) = 2500, 2400 and 3350 +/- 10 Angstrom, respectively. These lambda(0) values, derived from the fit, correspond to an average over different grain orientations. The power n decreases with an increasing anisotropy of the studied high-T-c cuprates.

Back side Raman measurements were performed on Ge/Pd/n-GaAs ohmic contacts. The analysis was carried out on as-deposited and annealed Ge/Pd/60 nm GaAs structures, and complemented by measurements on isolated 60 nm GaAs slabs. Ohmic behavior is found to coincide with the presence of a confined, highly doped (approximately 1-3 X 10(19)/cm3) region in the GaAs surface layers. This result can be interpreted as giving direct experimental evidence for the importance of doping in the ohmic behavior for this contact.

The structural and electronic properties of the intermixed interfaces of layered Au/Te/n-GaAs structures were investigated by the combined application of129I Mössbauer spectroscopy (in the decay of129mTe), X-ray diffraction, Raman spectroscopy and electrical measurements. The transition from Schottky-type to ohmic contact by high fluence pulsed laser irradiation was examined and compared to rapid time furnace heat treatment. Distinctly different ohmic contact formation mechanisms have been observed. Whereas for furnace alloying the formation of a crystalline (graded As doped) Ga2Te3 interface layer is crucial, strong evidence is adduced that the ohmic character of high-fluence laser mixed structures is correlated to the formation of a high density of defect complexes in the GaAs top layer.

We report the first application of an ultrasensitive detection scheme in on-line collinear-laser spectroscopy. It is based on radioactive detection of optical depopulation pumping, using state-selective charge exchange as an intermediate step. This extends the calcium isotope shift measurements beyond the f7/2 shell closure. The extracted Ca-50 mean-square charge radius constitutes the first experimentally determined one of a short-lived neutron-rich isotope beyond the N = 28 shell closure. The steep increase in radius after the doubly magic Ca-48 nucleus indicates a strong coupling between the P3/2 neutrons and the core protons.