A.M.C. Moutinho

Negative ion formation in alkali-atom-molecule collisions

Negative ion formation in alkali-atom-molecule collisions is a powerful method to study the dynamic properties of negative ions. By studying a collision with an electron donor such as an alkali atom one has access to the electron affinity of the molecule as it is manifested in the collision. The electron affinities obtained depend on the conditions of the collision and do not have to be identical to electron affinities obtained by other means. In this way adiabatic, vertical and reactive electron affinities can be discerned. Moreover if the collision energy is in the eV region one can probe the vibrational dynamics of a negative molecular ion on a 10 fs timescale. As a theoretical introduction to negative ion formation by electron transfer in atom-molecule collisions the curve crossing model and the collision dynamics of the atom-atom case are briefly reviewed. A short survey of the experimental methods used is presented. Measurements of total, differential and double-differential cross sections as a function of the collision energy and different collision partners are discussed. In atom-molecule collisions stretching of the bond of the molecular negative ion during the collision is among the most relevant consequences of the internal degrees of freedom of the molecule. The collision dynamics explored in this way not only gives insight into the properties of negative ions, but also into the dynamics of elementary processes occurring in chemical reactions.

Identification of human calculi with time‐of‐flight secondary ion mass spectrometry

Time-of-flight secondary ion mass spectrometry was used to study four human calculi and to compare the results with those from twelve commercially available urinary calculi minerals including three organic compounds (L-cystine, uric acid and sodium urate). Phase identification of calcium phosphate compounds was carried out by considering the relative ion abundances of [Ca(2)O](+) and [CaPO(2)](+). Deprotonated [M-H](-) and protonated [M+H](+) uric acid were detected and used for component recognition in pure uric acid and in the mixed samples of struvite, calcium oxalate and uric acid. Iodine related to the medical history of a patient was also detected.

TOF-SIMS study of cystine and cholesterol stones

Two different human stones, cystine and cholesterol from the kidney and gall bladder, were examined by time-of-flight secondary ion mass spectrometry using Ga(+) primary ions as bombarding particles. The mass spectra of kidney stone were compared with those measured for the standard compounds, cystine and cysteine. Similar spectra were obtained for the stone and cystine. The most important identification was based on the existence of the protonated molecules [M + H](+) and deprotonated molecules [M-H](-). The presence of cystine salt was also revealed in the stone through the sodiated cystine [M + Na](+) and the associated fragments, which might be due to the patient treatment history. In the gallstone, the deprotonated molecules [M-H](+) of cholesterol along with relatively intense characteristic fragments [M-OH](+) were detected.

Alumina vapour condensation on Mo(110) surface and adsorption of copper and gold atoms on the formed oxide layer

Aluminium oxide vapour condensation on Mo(1 1 0) surface and subsequent adsorption of Cu and Au submonolayer films on the formed ultrathin oxide layer has been studied by X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and reflection–absorption infrared spectroscopy. AES and XPS data suggest that the oxide films formed in this way exhibit the properties characteristic for the bulk alumina, although the intramolecular bond weakening (up to 30%) compared to bulk oxide occurs at the submonolayer coverage. Both Cu and Au form 3D clusters on the surface of the oxide film regardless of its thickness (2–5 ML). At a very low coverage (<0.4 ML) Cu deposited on the oxide strongly interacts with the surface oxygen forming the species resembling the copper oxide(s). At a higher coverage the Cu deposits have metallic character. Unlike Cu, the latter, however, is the case for gold particles regardless of the coverage: No evidence of forming of the Au–O bonds was found at room temperature. Annealing of Au and Cu deposits on Al2O3 film results in metal/oxide intermixing and chemical reaction. The properties of the metals deposited on 2 and 5 ML thick oxide films are essentially the same at all coverages studied. The only observed difference is that the gold deposited on a thinner oxide film is characterised by a smaller average sizes of islands compared to Au deposited on a thicker oxide film. We tend to attribute this difference to higher defect density in thinner oxide film compared to thicker film, rather than to the influence of the underlying Mo(1 1 0) support.

Novel approach to the semi-empirical universal theory for secondary electron yield

The universal semi-empirical equation has been commonly used to quantitatively describe the energy dependence of the secondary electron yield (SEY). It is even used as a first reliability test for experimental data. The equation is based on the assumption that the stopping power is constant along the electron trajectory. In this article, we derive a novel analytical expression based on a more advanced model which considers linear stopping power dependence on penetration depth. Although coinciding with the universal equation at low energies, the novel function has lower intensity in the higher energy range. The models were compared with experimental SEY data of different metals, taken from literature, as well as freshly cleaved highly oriented pyrolytic graphite, measured in the frame of this work. It is confirmed that the novel expression better describes the experimental data.

Adsorption dynamics of water on the surface of TiO2 (110)

Rutile titanium dioxide TiO2 is used in a number of technological areas. Therefore, in surface science, it has become the most studied oxide surface. Water adsorption on rutile TiO2 (110) has been investigated using the X-ray photoelectron spectroscopy (XPS) and the work function study (WF): water adsorption induces formation of a dipole layer, which locally changes the work function. This can be experimentally observed as the onset shift of the secondary electron energy spectrum. While XPS seems to be insufficiently sensitive to monitor water adsorption on TiO2, there is a clear work function change undoubtedly attributed to the water adsorption. The measurements were done for different water vapour pressures, exposure times, sample temperatures and general surface conditions. Time evolutions of the work function change and the H2O partial pressure, enable us to successfully model the adsorption dynamics and help us understand the observed results. The analysis clearly shows existence of at least three different adsorption sites. Their interplay governs the work function time evolution, while the relative contributions depend on the surface temperature and, presumably, its topography. These results will be discussed in the light of several recent experimental and theoretical studies of this system done by other authors.

Influence of the patch field on work function measurements based on the secondary electron emission

A work function study based on the onset shift (i.e., following low energy cut-off) of secondary electron spectra has been used for the last four decades to monitor the deposition and adsorption in real time, measure the dipole momentum and polarizability of the surface layer, and determine the lateral distribution of the work function. In this work, we show that the onset shift depends on both the coverage of adsorbed species that change the work function and the size of low work function patches. Additionally, the extraction field, which is always applied in these measurements, may also influence the onset shift. Numerical calculations of the potential distributions above different non-uniform surfaces were performed in order to quantitatively determine each of these influences. Depending on the patch size, we define three measurement regimes in which the onset position is related to either the surface average of the work function (small patches), the minimum local work function (large patches), or a va...

High performance temperature controlled UHV sample holder.

A requirement of many surface science studies is the capability to alter a sample temperature in a controlled mode. Sample preparation procedures such as heating or cooling ramps, high temperature spikes, fast annealing, or simply maintaining a sample at a very high, or very low, temperature are common. To address these issues, we describe the design and the construction of a multipurpose sample holder. Key points of this design are operation in an extended temperature range from liquid nitrogen (LN(2)) temperature to approximately 1300 K, temperature control during heating and cooling, low thermal inertia with rates up to 50 K s(-1) (heating) and -20 K s(-1) (cooling), and small heated volume to minimize background problems in thermal desorption spectroscopy (TDS) spectra. With this design the sample can be flash heated from LN(2) temperature to 1300 K and cooled down again in less than 100 s. This sample holder was mounted and tested in a multitechnique apparatus and adds a large number of sample preparation procedures as well as TDS to the list of already available surface analysis techniques.

MASS SPECTROMETRY OF ALIPHATIC AZIDES

Electron impact (EI) mass spectra of N3CH2COOH, N3CH2COCH3 and N3CH2CH2OH are reported. Molecular ions of these compounds are observed. The fragmentation mechanisms observed for each compound are discussed. Copyright

Ion-pair formation in some potassium—molecule collisions. Dependence on the vibrational energy

Abstract State-specific cross sections for ion-pair formation in collisions of potassium with halogen and methylhalogen molecules are obtained by means of an inverse Laplace transform. This transform was applied to the total cross sections measured as a function of the temperature of the molecules and the translational energy. The efficiencies of the translational energy and internal energy are compared. The behaviour of the post-threshold region is discussed assuming strong stretching or even dissociation in the ionic channel.