Peter Gardner

Structure determination of Ni(111)c(4 × 2)-CO and its implications for the interpretation of vibrational spectroscopic data

Abstract A detailed quantitative structure determination of the Ni(111)c(4 × 2)-CO structure has been undertaken using scanned energy mode photoelectron diffraction from the C 1s state over a wide range of emission angles. Analyses of these data by approximate direct methods, and by two independent multiple scattering trial-and-error fitting optimisations lead to a consistent structure in which the CO occupies both types of hollow site on the surface in equal amounts with a C-Ni top layer spacing of 1.29 ± 0.05 A. This structure is therefore essentially the same as that for Ni(111)c(4 × 2)-NO, and provides further evidence that simple use of the intramolecular stretching frequencies of such adsorbed molecules, which had been interpreted in both cases as indicative of bridge site adsorption, is not always a reliable indicator of local adsorption site.

Single local site structure for vibrationally distinct adsorption states : NO on Ni(111)

Abstract Scanned energy mode N and O 1s photoelectron diffraction data collected from NO on Ni(111) under conditions corresponding to three distinct N—O vibrational frequencies, previously assigned to different (bridge and atop) local adsorption configurations, show that NO in all cases occupies the same site. Comparison of these data with the results of theoretical simulations indicates that the common site is a threefold coordinated hollow. These results indicate that there is a need to reevaluate the range of applicability of the widely used method of adsorption site assignment based on the numerical values of internal molecular vibrational frequencies.

Is the frequency of the internal mode of an adsorbed diatomic molecule a reliable guide to its local adsorption site

Abstract Although the C-O stretching frequency for the system Ni>{111}c(4>x2)-CO falls in the so-called bridge region, a structure determination with photoelectron diffraction shows that the CO molecules are actually adsorbed in threefold hollow sites. These, and other recent data, seriously call into question the practice of adsorption site assignment solely on the basis of the stretching frequency of an internal mode.

The effect of anisotropic molecular vibrations in photoelectron diffraction of adsorbed species

Anisotropic adsorbate vibrations can strongly influence the intensity modulation in energy-scan photoelectron diffraction. For adsorbed molecules the frustrated translations and rotations parallel to the surface are expected to be much softer than the vibrations of the molecule against the surface. In the present paper we therefore specifically include the effect of emitter vibrations in the multiple-scattering theory for photoelectron diffraction by means of a Debye-Waller factor. For the system NH3 on Ni{111} it is shown that the parallel vibrations have only a minor influence on the N 1s photoelectron diffraction spectrum in normal emission, whereas the intensity modulations in off-normal emission directions are damped considerably. A comparison between theory and experiment for the polar angles theta = 0-degrees, 25-degrees and 35-degrees, gives a value of 0.04 angstrom 2 for the mean-square displacement parallel to the surface, which corresponds to a tilt angle of 6-degrees.

Local site identification for NO on Ni(111) in vibrationally distinct adsorption states

Vibrational spectroscopy of NO on Ni(111) shows two distinct N–O stretching bands for adsorption on the clean surface at different coverages which have been attributed to tilted and perpendicular bridging species at low and high coverage, respectively, while an even higher vibrational energy seen when NO is coadsorbed with oxygen has been attributed to atop NO. We have investigated the local adsorption geometry of these species with scanned energy mode photoelectron diffraction from the N and O 1s states, a method previously found to be very sensitive to site changes and capable of quantitative site identification. We find essentially identical photoelectron diffraction spectra from all three species, indicating that there are no site changes to within a few hundredths of an Angstrom unit. Moreover, a quantitative analysis using double scattering model calculations indicates that in all cases the adsorption site is actually a threefold coordinated hollow and not bridge or atop.

Another Look at Controversial Issues and the Curriculum.

Over recent years, particularly as a result of the Humanities Curriculum Projects recommendations, there has been more than a little support for the view that the curriculum should cater for controversial issues. There has, however, been some disagreement about what controversial issues are. Charles Bailey, for example, wants to account for controversial issues in behavioural terms. R. F. Dearden, on the other hand, rejects Baileys approach and advances an epistemic account. My own view, which I will develop in this paper, is that both of these approaches are mistaken, and that the employment of Deardens account could lead to unfortunate consequences. It could, I believe, lead to pupils acquiring a mistaken view about what is involved in taking up a particular position on a controversial issue or to pupils thinking that in the realm of the controversial, and even at the frontiers of knowledge, we must trust to subjective preference and cannot trust to reason. Yet, what I think is more important than finding an account of the controversial that avoids these consequences is that we identify certain issues which might, some would say should, be catered for in the curriculum, issues which illuminate certain features of the nature of human inquiry and which may all too easily be overlooked or discarded given many of the ways in which school subjects are organized and presented. According to Bailey, That an issue is controversial is, of course, a matter of social fact. That is, an issue is controversial if numbers of people are observed to disagree about statements and assertions made in connection with the issue. As far as Dearden is concerned, to support Baileys account is to favour a behavioural criterion, and the adoption of such a criterion has at least two unfortunate consequences. First, it would allow for disagreements which result from either simple ignorance o r . . . mere undisciplined assertiveness to rate as controversial even when there exists a clear decision procedure and, possibly, a publicly known and available answer to settle the dispute. Here we may agree with Dearden that there is more to something being controversial than the social fact of disagreement. People may disagree about the result of a race or a football match, but we do not normally speak of these matters as controversial. Furthermore, issues do not become more controversial the more people drink, though disagreements may increase with intoxication. Deardens second argument may be a little more difficult to follow. It is that a behavioural criterion of the kind advanced by Bailey could give undeserved encouragement to relativism. In support of this claim Dearden reasons:

The origin of inverse absorption bands observed in the far-infrared RAIRS spectra of SnCl4 and SnBr4 adsorbed on thin-film SnO2 surfaces

The adsorption of SnCl4 and SnBr4 on polycrystalline SnO2 has been studied using synchrotron radiation based far-infrared reflection absorption infrared spectroscopy FIR-RAIRS. In order to exploit the sensitivity advantages of the buried metal layer method, the SnO2 is in the form of a thin film deposited on a tungsten foil substrate. Adsorption of SnCl4 and SnBr4 on an oxygen sputtered surface at 120 K results in spectra characteristic of condensed multilayers. In addition, both spectra exhibit an inverse absorption band centred at 355 cm(-1). Modified 4-layer, wavelength-dependent, Greenler calculations show that this inverse absorption band is induced by the presence of the adsorbate but is characteristic of the SnO2 layer. The lack of any frequency shift upon changing the adsorbate from SnCl4 to SnBr4 rules out the possibility that the inverse absorption band is due to a dipole-forbidden parallel mode of the molecule excited via the interaction with free electron oscillations in the metal, resulting from the radiation induced oscillating electric field just below the surface

Deciding in Democracies: A Role for Thinking Skills?

In societies that respect our right to decide many things for ourselves, exercising that right can be a source of anxiety. We want to make the right decisions, which is difficult when we are confronted with complex issues that are usually the preserve of specialists. But is help at hand? Are thinking skills the very things that non-specialists need when faced with those difficult decisions and is this why such skills should be taught in schools? These are the questions this article examines.