The use of ultra-soft near-edge X-ray absorption fine structure (NEXAFS) spectroscopy as a tool for determining the molecular orientations, and changes in the orientations and conformations, of thin organic films is presented. Polarisation-dependent NEXAFS measurements made at the carbon K-edge reveal that vapour-grown n-alkanes, deposited onto Si( 11 l), exhibit structures with a high degree of long-range order. The molecular tilt angles, with respect to the substrate surface, for n-alkanes C,H,, + in the carbon chain-length range n = 22-60 are found to be consistent with their bulk crystallographic structures. Temperature dependent measurements carried out using simultaneous electron yield and fluorescence yield techniques reveal evidence of surface freezing in the longer even n-alkanes (n 2 46). Such a process is not evident in the shorter alkanes. The potential causes of this chain-length dependence are discussed.

A molecular modelling study of the (O001) surface of benzil reveals evidence for surface reconstruction associated with a change in molecular conformation effected via a relaxation of n-n intermolecular interactions which are strong in the solid-state but absent on the exposed surface. This is confirmed by an examination of a single crystal sample using ultra-soft polarised X-ray absorption spectroscopy at the C K-edge.Over the past 20 years UHV surface science techniques have made a dramatic and substantial contribution to our understanding of the surface crystallography of metals and semi-conductors and, to a lesser extent, simple AB components such as oxides. However, despite the importance of surface structure in understanding and controlling the processing of organic solids, particle formation, comminution, colloidal dispersion, tabletting etc., our knowledge of the surface chemistry of most molecular solids remains completely unknown at this time. The surfaces of simple elemental compounds are known to reconstruct due to the termination of strong primary bonding at the exposed surface thus leading to relaxation of atoms from their bulk positions, resulting in surface symmetry reduction. Intuitively one would not expect such effects to be as strong in organic solids due to their crystal chemistry being dominated by much weaker van der Waals' forces. Despite this, recent molecular modelling studies by George et af.' have indicated the likelihood of a degree of surface re-ordering in molecular solids.The inherent flexibility in many complex organic molecules would suggest that some molecular rearrangement should take place in cases where packing forces induce a change in molecular conformation with respect to that expected for the free molecule.Benzil (C6H,C==O)2 comprises two phenyl rings joined together by two trans carbonyl groups, with the latter providing a degree of conformational flexibility. It crystallises in a trigonal structure P312 in a tri-molecular unit cell with Q = 8.409 A and c = 13.672 k o m p a r i s o n between the molecular structure derived from semi-empirical quantum chemistry calculations t and that derived from the published crystallographic ~tructure,~ shown in Fig. 1, reveal a substantial change in molecular conformation reflected by the crystallisation process. This change, associated with an increase in the molar enthalpy of formation from 38.3 kcal mol-' (1 cal = 4.184 J) for the isolated molecule to 97.5 kcal mol-' for the same in the bulk crystal structure, reflects the strong intermolecular packing forces in the solid-state. These forces are dominated by an interleaved packing motif between benzil molecules from adjacent molecular layers which are, in turn, dominated by f Calculations carried out using MNDO. Fig. 1 Overlay of optimised (---) and crystallographically derived (-) molecular structures for benzil showing the nature of conformational changes induced by packing in the solid state Un-reconstructed f ' x 12 -x: interactions Fig. 2 Projection of the molecular packing o...

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The lZ9I Mossbauer emission spectra of the compounds (NH4)2129mTeX, and lZ9"TeX, (X = CI, Br, I ) have been reported against a NalZ9I absorber at 4 OK. The spectra give evidence in the case of the (NH,),TeX, compounds for the formation in the radioactive decay of the octahedral ions ICl;, IBr;, and 11;. In the TeX, compounds, the emission spectra are much more complex and indicate that the lZ9I atoms are not found in an environment iso-structural and iso-electronic with that of the parent.The lZ5Te Mossbauer absorption spectra of the parent compounds were studied to aid in their identification, and the parameters obtained are compared with those in the literature. The lZ5Te isomer shifts for the TeXganions are interpreted in terms of pure p-bonding and, in conjunction with data from the emission spectra and data from previously reported n.q.r. work, an isomer shift vs. p-hole density relationship is established for tellurium.Nous rapportons le spectre d'bmission Mossbauer de lZ9I des composes (NH4)21Z9mTeX6, et '29mTeX, (X = C1, Br, I ) en fonction d'un absorbant NalZ9I a 4 OK. Le spectre prouve, dans le cas des composes (NH4),Te&, la formation des ions octaddriques ICI;, IBr;, et 11; durant la desintegration radioactive. Les spectres d'imission des compos~s TeX, sont beaucoup plus complexes et indiquent qu'on ne trouve pas les atomes de lZ9I dans I'environnement iso-structural et isoblectronique oh se trouvent leurs analogues.Nous avons etudit le spectre d'absorption Mossbauer du lZ5Te des composQ analogues afin de faciliter leur identification; nous avons ensuite compare les parametres obtenus avec ceux de la littbrature. Nous interprttons les d6placements d'isomere lZ5Te pour les anions T e z -en termes de liaisons p pures et, en plus des donntes tirees des spectres d'bmission et celles venant d'un travail de n.q.r. rapport6 antirieurement, nous 6tablissons la relation entre le deplacement d'isomere et la densite de trous p pour le tellure.

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