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Vibrational Spectroscopy of Surface Adsorbates on Metal Surfaces. Experiments and Calculations

Ásmundsson, Ragnar K LU (1999)
Abstract
This work deals with a detailed analysis of the vibrational spectra of methoxy (CH<sub>3</sub>O-) and ethoxy (CH<sub>3</sub>CH<sub>2</sub>O-) on W(110) and Cu(100) single crystal surfaces. By using theoretical <i>ab initio</i> quantum mechanical modeling, it is demonstrated that an unprecedented quantitative understanding of the vibrational frequencies of a surface adsorbate can be obtained. A qualitative understanding of the intensities is also obtained. This was achieved using a simple model in the calculations, where the metal surface was presented with one molybdenum metal atom only. The model was shown to be able to determine small isotopic shifts on both metal surfaces studied, dispite... (More)
This work deals with a detailed analysis of the vibrational spectra of methoxy (CH<sub>3</sub>O-) and ethoxy (CH<sub>3</sub>CH<sub>2</sub>O-) on W(110) and Cu(100) single crystal surfaces. By using theoretical <i>ab initio</i> quantum mechanical modeling, it is demonstrated that an unprecedented quantitative understanding of the vibrational frequencies of a surface adsorbate can be obtained. A qualitative understanding of the intensities is also obtained. This was achieved using a simple model in the calculations, where the metal surface was presented with one molybdenum metal atom only. The model was shown to be able to determine small isotopic shifts on both metal surfaces studied, dispite both material and structural differences in the metal substrates. The isotopic shifts were obtained through <sup>13</sup>C isotopic labeling in ethoxy, using <sup>13</sup>CH<sub>3</sub>CH<sub>2</sub>O-, CH<sub>3</sub><sup>13</sup>CH<sub>2</sub>O- and <sup>13</sup>CH<sub>3</sub><sup>13</sup>CH<sub>2</sub>O-. Once the accuracy of the calculated shifts had been established, it was possible to assign previously undetected vibrational modes in ethoxy-Cu(100) in the C-H stretch region, where the spectra are complicated due to bend overtone interactions with the stretch fundamentals. In addition, the interaction between fundamentals and overtone modes in methoxy, Fermi resonances, is treated using a phenomenological model. Fermi resonances occur when a near accidental degeneracy exists between a fundamental and overtones having the same symmetry, causing the modes to shift away from each other and share intensity. An effective Hamiltonian was used to calculate the Fermi resonance coupling constants through a least-square fitting procedure. This latter treatment reproduces qualitatively the observed intensity redistribution caused by the coupling.



The technique used to record the vibrational spectra was Fourier transform reflection-absorption infrared spectroscopy in the mid-infrared spectral region. This technique does not disturb the surface chemistry and gives the resolution required to be able to resolve the small isotopic vibrational shifts induced by the <sup>13</sup>C labeling. All experiments were performed in a stainless steel ultra high vacuum chamber with a base pressure below 7*10<sup>-11</sup> Torr. (Less)
Please use this url to cite or link to this publication:
author
opponent
  • Prof Friend, Cynthia M., Harvard University
organization
publishing date
type
Thesis
publication status
published
subject
keywords
ab initio electronic structure calculations, Fermi resonance, vibrational spectroscopy, Reflection absorption infrared spectroscopy, Fourier transform infrared spectroscopy, Cu(100) and W(110) single crystal surfaces, Physics, Fysik
pages
45 pages
publisher
Ragnar K. Ásmundsson, Chemical Physics, Lund University
defense location
Getingevägen 60, hall D
defense date
1999-09-10 10:15
external identifiers
  • Other:ISRN: LUNKDL/NKKF--99/1008--SE
language
English
LU publication?
yes
id
efdd3b44-1b12-48d4-a591-58f7a8dcffab (old id 39827)
date added to LUP
2007-10-14 17:11:35
date last changed
2016-09-19 08:45:07
@misc{efdd3b44-1b12-48d4-a591-58f7a8dcffab,
  abstract     = {This work deals with a detailed analysis of the vibrational spectra of methoxy (CH&lt;sub&gt;3&lt;/sub&gt;O-) and ethoxy (CH&lt;sub&gt;3&lt;/sub&gt;CH&lt;sub&gt;2&lt;/sub&gt;O-) on W(110) and Cu(100) single crystal surfaces. By using theoretical &lt;i&gt;ab initio&lt;/i&gt; quantum mechanical modeling, it is demonstrated that an unprecedented quantitative understanding of the vibrational frequencies of a surface adsorbate can be obtained. A qualitative understanding of the intensities is also obtained. This was achieved using a simple model in the calculations, where the metal surface was presented with one molybdenum metal atom only. The model was shown to be able to determine small isotopic shifts on both metal surfaces studied, dispite both material and structural differences in the metal substrates. The isotopic shifts were obtained through &lt;sup&gt;13&lt;/sup&gt;C isotopic labeling in ethoxy, using &lt;sup&gt;13&lt;/sup&gt;CH&lt;sub&gt;3&lt;/sub&gt;CH&lt;sub&gt;2&lt;/sub&gt;O-, CH&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;13&lt;/sup&gt;CH&lt;sub&gt;2&lt;/sub&gt;O- and &lt;sup&gt;13&lt;/sup&gt;CH&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;13&lt;/sup&gt;CH&lt;sub&gt;2&lt;/sub&gt;O-. Once the accuracy of the calculated shifts had been established, it was possible to assign previously undetected vibrational modes in ethoxy-Cu(100) in the C-H stretch region, where the spectra are complicated due to bend overtone interactions with the stretch fundamentals. In addition, the interaction between fundamentals and overtone modes in methoxy, Fermi resonances, is treated using a phenomenological model. Fermi resonances occur when a near accidental degeneracy exists between a fundamental and overtones having the same symmetry, causing the modes to shift away from each other and share intensity. An effective Hamiltonian was used to calculate the Fermi resonance coupling constants through a least-square fitting procedure. This latter treatment reproduces qualitatively the observed intensity redistribution caused by the coupling.<br/><br>
<br/><br>
The technique used to record the vibrational spectra was Fourier transform reflection-absorption infrared spectroscopy in the mid-infrared spectral region. This technique does not disturb the surface chemistry and gives the resolution required to be able to resolve the small isotopic vibrational shifts induced by the &lt;sup&gt;13&lt;/sup&gt;C labeling. All experiments were performed in a stainless steel ultra high vacuum chamber with a base pressure below 7*10&lt;sup&gt;-11&lt;/sup&gt; Torr.},
  author       = {Ásmundsson, Ragnar K},
  keyword      = {ab initio electronic structure calculations,Fermi resonance,vibrational spectroscopy,Reflection absorption infrared spectroscopy,Fourier transform infrared spectroscopy,Cu(100) and W(110) single crystal surfaces,Physics,Fysik},
  language     = {eng},
  pages        = {45},
  publisher    = {ARRAY(0xba19a80)},
  title        = {Vibrational Spectroscopy of Surface Adsorbates on Metal Surfaces. Experiments and Calculations},
  year         = {1999},
}