Density functional theory study of the near edge X-ray absorption fine structure and infrared spectroscopy of acetylene and benzene on group IV semiconductor surfaces Frans A. Asmuruf, Nicholas A. Besley * School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK article info Article history: Received 10 July 2008 Accepted for publication 29 October 2008 Available online 9 November 2008 Keywords: Infrared spectroscopy NEXAFS DFT Acetylene Benzene abstract The near edge X-ray absorption fine structure and infrared spectroscopy of acetylene and benzene adsorbed on C(1 0 0)-2  1, Si(1 0 0)-2  1 and Ge(1 0 0)-2  1 surfaces is studied with density functional theory calculations. Time dependent density functional theory calculations of the near edge X-ray absorption fine structure with a modified exchange-correlation functional agree well with experiment, and show that the spectral features arise from excitation to p  , r  CAH and r  XAC orbitals, where X represents C, Si or Ge. The r  XAC excitation energies are dependent on the surface, and for acetylene, the location of the p  band also varies with the surface. Calculations of the vibrational modes show the CAH stretching frequencies for carbon atoms bonded directly to the surface vary significantly between the three surfaces, while those for carbon atoms not bonded to the surface do not change significantly. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction The study of hydrocarbon chemistry on semiconductor surfaces has been the focus of considerable attention in recent years. This interest is in part motivated by the importance of these systems in emerging technologies [1]. Problems that have been addressed include the reaction of organic molecules with the surface, and the structure of the resulting adsorbed molecule. Most of this work has focused on the Si(1 0 0)-2  1 surface. Detailed studies have been reported for a wide range of molecules, from small molecules such as butadiene [2–4] to much larger systems such as C 60 [5–7]. In comparison to Si(1 0 0)-2  1, the related Ge(1 0 0)-2  1 and C(1 0 0)-2  1 surfaces have received much less attention. How- ever, several studies have been reported, and much of this work has been reviewed elsewhere [1,8,9]. Hydrocarbons adsorbed on Ge(1 0 0)-2  1 reveal similar chemistry to the reactions on Si(1 0 0)-2  1, but the products are less strongly bound to due to the weaker CAGe bond [10,11]. The structure of acetylene on Si(1 0 0)-2  1 has been studied by a number of groups [12–14]. Re- cent work has used multireference wavefunctions with dynamic correlation [15]. This work found the most stable binding site to correspond to acetylene bonded to a single surface dimer, with an acetylene carbon–carbon bond length of 1.36 Å. A comprehen- sive investigation of benzene adsorbed on the Si(1 0 0)-2  1 sur- face has been reported, and showed that the favoured binding site to correspond to the butterfly structure arising from a 4 + 2 addition [16]. Density functional theory (DFT) calculations have been used to study the adsorption of acetylene and benzene on the Ge(1 0 0)-2  1 surface [17]. These calculations were consistent with the observation that there is a weaker binding to the Ge(1 0 0)-2  1 surface. Theoretical studies have also investigated the adsorption of several small molecules, such as acetylene, the methyl radical and carbon dimer on diamond surfaces [18–22]. Spectroscopic measurements, often in concert with theoretical calculations, can allow the structure of the adsorbed species to be determined, and infrared (IR) and near edge X-ray adsorption fine structure (NEXAFS) spectroscopies have been used extensively to probe molecules adsorbed on the Si(1 0 0)-2  1 surface. In this paper, the sensitivity of the spectroscopy of an adsorbed unsatu- rated organic molecule to the nature of the underlying surface is studied. Acetylene and benzene are used as model systems, and their NEXAFS and IR spectroscopy on the C(1 0 0)-2  1, Si(1 0 0)- 2  1 and Ge(1 0 0)-2  1 surfaces is computed using DFT. NEXAFS measures the excitation of core electrons to semi- stable states below the ionization threshold. Several groups have reported NEXAFS studies of acetylene and benzene on Si(1 0 0)- 2  1. In a study of acetylene on the Si(1 0 0)-2  1 surface, Matsui and co-workers observed peaks at 284.7, 286.0 and 287.6 eV, which were assigned to excitations from the carbon 1s orbitals to p  CAC , r  SiAC and r  CAH orbitals, respectively [23,24]. Above threshold, a broad peak at 300 eV was observed and assigned to the r  CAC orbital. In a later study, Pietzsch et al. reported a fully polarization resolved NEXAFS investigation of acetylene on the surface [25]. 0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.susc.2008.10.043 * Corresponding author. Tel.: +44 115 951 3474; fax: +44 115 951 3562. E-mail address: nick.besley@nottingham.ac.uk (N.A. Besley). Surface Science 603 (2009) 158–164 Contents lists available at ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc