We compare the results of ab initio calculations with measured reflection anisotropy spectra and show that strongly bound surface-state excitons occur on the clean diamond (100) surface. These excitons are found to have a binding energy close to 1 eV, the strongest ever observed at a semiconductor surface. Important electron-hole interaction effects on the line shape of the optical transitions above the surface-state gap are also found.

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a b s t r a c t 24 Superspreading is the phenomenon where a drop of trisiloxane surfactant solution rapidly spreads on a 25 moderately hydrophobic substrate over a very large area. However the mechanism of superspreading is 26 unknown to date. In this work Reflection Anisotropy Spectroscopy (RAS) is used for the first time in order 27 to investigate the causes of superspreading. An aqueous solution of the superspreader Evonik S240 (0.1%) 28 is compared with the chemically similar but non superspreading solution of Evonik S233 (0.1%), on a 29 polycarbonate (pcp) substrate. This study is aimed at finding evidence of organised structures, such as 30 micelles, at the contact line of the receding film which could be part of the mechanism of superspreading. 31 The RA signal is monitored as the solutions recede from vertically aligned pcp substrates. The results are 32 then analysed for evidence of anisotropy on the surface arising from any micelle structures located at the 33 contact line of the film. 34 Ó 2016 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND 35 license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 36 37 38 Introduction 39 When a drop of water is put on a moderately hydrophobic sur-40 face, such as a polycarbonate sheet, it takes the shape of a spherical 41 cap with hardly any wetting of the surface; however, when small 42 amounts of certain trisiloxane surfactants are added to water, 43 spreading is greatly enhanced, which is commonly referred to as 44 ''superspreading". To date, the mechanism governing this phe-45 nomenon is poorly understood; in particular, there is little direct 46 experimental evidence for the state of the superspreader/substrate 47 interface at the molecular scale. Models have been proposed which 48 try to explain the different behaviours of non superspreading and 49 superspreading surfactants, even though they may have similar 50 chemical structures. These models relate micelle crystallisation 51 and mono/bi layer formation to their effect on superspreading 52 [1,2]. Currently there is no experimental evidence for these models, 53 although macromolecular structures near a moving contact line 54 have been shown to affect the wetting behaviour of systems such 55 as dilute solutions of flexible polymers [3]. 56 Here, Reflection Anisotropy Spectroscopy (RAS) is used to inves-57 tigate whether superspreaders generate ordered macromolecular 58 structures (e.g. micelles or bi-layers) on surfaces, which could help 59 to understand the mechanism of superspreading. RAS is a non-60 destructive optical technique where reflected light yields data on 61 the anisotropy of a surface, or a process occurring at a surface. 62 RAS [4,5] measures the difference in reflectance (Dr) of normal 63 incidence plane-polarised light between two orthogonal directions 64 in the surface plane (x, y), normalised to the mean reflectance (r): 65 Dr r ¼ ðr x À r y Þ ðr x þ r y Þ=2 ð1Þ 67 67 68 where Dr/r represents the measured complex reflectance aniso-69 tropy...