Adsorption Behaviour of Liquid 4He on Cesium Substrates


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IOV, Valentin, 2004. Adsorption Behaviour of Liquid 4He on Cesium Substrates

@phdthesis{Iov2004Adsor-4803, title={Adsorption Behaviour of Liquid 4He on Cesium Substrates}, year={2004}, author={Iov, Valentin}, address={Konstanz}, school={Universität Konstanz} }

2011-03-24T14:50:27Z application/pdf deposit-license eng Benetzungsverhalten von flüssigem 4He auf Cäsium-Substraten 2011-03-24T14:50:27Z The aim of this thesis is to investigate the wetting properties of 4He on cesium substrates using optical and electrical methods. Due to the fact that the cesium substrates are deposited at low temperatures onto a thin silver underlayer, it is necessary firstly to study and understand the adsorption of helium on silver. The work presented here is structured as follows: some of the fundamental concepts on the theory of physisorbed films, such as van der Waals interaction, adsorption isotherms on ideal and real surfaces, will be introduced in Chapter 1. We will outline theoretical aspects about solid-liquid interfaces, wetting and prewetting phenomena in Chapter 2, which is by no means, meant to be a complete survey on the field of wetting. The status of research concerning the 4He on cesium is briefly reviewed in Chapter 3. A detailed description of the experimental techniques used to carry out the experiments is given in Chapter 4, where both the theory of the methods and some practical hints in connection to the experiment will be specified. This will be linked to the Chapter 5, where the rather complex experimental setup, the relevant components, the sample preparation and the procedures employed in the measurements are described in detail. In Chapter 6, the results of the simulations which are necessary to optimize the substrates used in the experiments are pointed out. The experimental results are presented in Chapter 7, where the main results are emphasized and discussed. The adsorption features of helium, the crossover between the retarded and non-retarded regime and the topography of silver substrates are described. The results on the adsorption of 4He on cesium as revealed by the Surface Plasmon and the complimentary technique of Photoelectron Tunnelling are presented and interpreted. The latter allows us to measure the thin non-wet film of helium on cesium. Some preliminary experiments necessary to calibrate and check our system are included for the sake of completeness. Chapter 8 outlines the concluding remarks and gives suggestions for future experiments.<br />The optical method of Surface Plasmon Spectroscopy was employed to measure the thickness of an adsorbed film on various substrates. It is successfully used to measure the film thickness of the helium adsorbed on Cs and on the underlying substrate of Ag. The Cs films produced by vacuum deposition on very cold substrates (on previously prepared Ag) by quench-condensing method result in a rough, inhomogeneous Cs surfaces which show columnar growth. The main cause of such a surface topography is the suppressed surface diffusion of Cs atoms due to the low temperatures at which these Cs substrates are produced. However, the roughness of the underlying Ag surface may also have an influence on the columnar growth. The method of Photoelectron Tunnelling is successfully employed as a complimentary technique to the SPR and reveals interesting results. It allows to very sensitively resolving the thickness of an adsorbed helium film on the non-wetted columnar patches of Cs from a submonolayer up to about three layers. At low values of the reduced pressure (i.e., in the submonolayer regime) the photocurrent decay obeys an exponential law and the adsorbed helium atoms behave like a 2Dgas. As the system moves towards coexistence, the photocurrent levels off and the surface coverage gets close to the layer completion. Close to saturated vapour pressure, surface coverage increases suddenly, which is interpreted as the transition from one layer to the 2 − 3 layer regime, but the system is still in the non-wetted state. Combining these two methods allows us to understand the topography of the cesium substrate which is responsible for this adsorption behaviour. The regions where the deposited cesium is too thin will show wetting, as revealed by the SPR technique. In contrast, the cesium columns which are thick enough are non-wetted by helium and these patches are the only remaining regions for electrons to leave the surface and contribute to the photocurrent.<br />The experiments carried out in this work points to the importance of the substrate roughness and how this can alter the wetting properties. Adsorption Behaviour of Liquid 4He on Cesium Substrates Iov, Valentin Iov, Valentin 2004

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