Submonolayer Levels Research Articles

Core level photoelectron spectroscopy for surface crystallography

Inner shell photoionisation as a basis for surface structure determination is reviewed. During the recent decade, this field has received tremendous impetus from the advent of synchrotron radiation. Core level photoelectron spectroscopy allows the detection at submonolayer levels of foreign adsorbate atoms and the resolution of the first layers from the bulk for a clean single crystal surface as well. The surface-bulk core level binding energy shift is shown to be surface geometry dependent and may be employed to monitor clean surface reconstruction. Shifting following chemisorption, surface core levels deliver information on adsorption binding site geometry. The emission intensity is modulated by photoelectron diffraction effects along k-vector variation obtained by varying photon energy or in measurements resolved in azimuthal and polar angles. Interference patterns are sensitive to bond lengths. Finally correlation with surface extended X-ray fine structure achieved by detecting photon stimulated Auger excitation or ion emission is discussed.

Molecular dynamics of hydrated proteins

This contribution presents the first results of an application of high-resolution quasi-elastic neutron scattering to the dynamics of protein hydration. Using two samples (film stacks and 'fluffy’ powders) of biosynthetically fully deuterated C-phycocyanin extracted from blue-green algae, we have studied the Doppler-like broadening of the quasi-elastic line as a function of scattering angle 20 at several sub-monolayer H 2 O hydration levels. The backscattering spectrometer IN 10 at the Institut Laue-Langevin (I.L.L.), Grenoble, was employed to measure wet-minus-dry difference broadenings A E of up to 5 x 10 -3 cm -1 , or 0.4 geV, for momentum transfers k = (4tt/A0) sin 0 between 0.15 and 1.7 Å-1 (incident wavelength A 0 = 6 Å). The results show that A E ( k ) possesses an oscillatory structure with a first maximum between k max = 0.4 and 0.8 Å-1. The position of this maximum shifts to higher k with increasing hydration, while its intensity increases and the following minimum at k max = 0.7-1.3 Å -1 becomes progressively more shallow. Structure factor measurements indicate that line narrowing due to structure in S ( k ) is not the dominant mechanism in determining this oscillatory behaviour of A E ( k ).A Chudley-Elliott jump diffusion model was adopted as a working hypothesis to extract a characteristic length (water migration distance a ) and a characteristic time (residence time t 0 at a hydration site) from the A E ( k ) data. Values of a = 6-9 Å and r 0 = 5-30 ns were obtained for the powder sample and are shown to agree well with the average jump distances derived from topographical considerations in conjunction with hydration number estimates.

Heterogeneous Interactions of Methylamines on Porous Adsorbents. Part III. The Adsorption Characteristics of Methylamines on γ-alumina and Types '3A' and '13X' 'Molecular Sieves'

The adsorption characteristics of mono-, di-, and trimethylamine on γ-alumina have been investigated and adsorption isotherms in the regions of their boiling points determined, along with isothermal calorimetric heats of adsorption up to monolayer coverage (θ = 1). From the initial heats of adsorption of 21.6 to 32.4 kcal/mol θ = 0.05, and the strong retention of the amines adsorbed at low pressures, chemisorption is considered to occur at θ < 0.20. The maxima and minima displayed in the heat curves are discussed in terms of adsorbate–adsorbate interactions at sub-monolayer coverage levels. Adsorption isotherms for di-and tri-methylamine on Linde 'molecular sieves' '13X' and '3A' have been measured also, along with the trimethylamine heat curves for these materials.