Structure of an unusual tilted state of CO on Fe(001) from x-ray photoelectron diffraction

Abstract

The highly tilted a 3 state of CO on Fe(OOl) has been studied using combined polar and azimuthal xray photoelectron diffraction. A more accurate structural picture is derived, with the molecule being tilted at an angle of 55° ± 2° with respect to the surface normal along <100> azimuths and probably occupying fourfold hollows. An estimate is also made of the vertical C distance above the Fe surface. The utility of combined 0/0 x-ray photoelectron diffraction measurements for such unusual adsorbate structures is demonstrated.The existence of an "abnormal" or "unusual" type of surface bonding state of CO that involves a tilted or lying-down geometry and a considerably lowered CO stretch frequency has been discussed recently for several different transition-metal surfaces including Fe(OOl), 1 " 3 Cr(llO), 4,5 and Mo(OOl). 6 This state is of considerable interest because it may represent a precursor to dissociation with a weakened CO bond due to additional electronic interactions with the metal substrate. x However, little is known about the exact geometric structure of the CO for any of these cases. For the case of a^ CO on Fe(001) that we will consider here, electron-energy-loss (EELS) measurements show a very low CO stretch frequency of 1210 cm -1 . 1 Near-edge x-ray-absorption fine-structure (NEXAFS) measurements on this state have been used to deduce an elongation of the CO bond by 0.07 ± 0.02 A from a shift in the position of the a resonance. 3 From the relative lack of a change in the absolute and relative intensities of the n and a resonances with a change in radiation polarization, an estimated tilt angle with respect to normal of 45° ± 10° has been deduced. 3 However, the precision of this determination is not high, and it also was not possible in this study to specify whether there is any preferred azimuthal orientation of the tilt. Measurements of electron-stimulateddesorption ion angular distributions (ESDIAD) are consistent with such a high tilt in showing a very weak signal for 0 + , but no bond angle information could be derived, probably due to the strong image forces which deflect and attenuate the desorbing ions. 2 We have thus applied x-ray photoelectron diffraction (XPD) for the first time to this intriguing adsorption state in an attempt to better understand its structure, combining both polar and azimuthal scans of core-level intensity in a way that has not been attempted before for such adsorbate structure studies. This method makes use of the dominance of highly-peaked forward scattering in core-level emission at energies of greater than approximately 500 eV. 7 " 10 As these peaks have typical FWHM values of only about 20°, the precision of bond tilt determinations should be considerably higher than in measurements such as NEXAFS where the dependence on variable polarization goes as a more slowly varying cos 2 function. 3,810 Also, a simple single scattering cluster (SSC) model has been shown in prior studies to model very well this kind of intramolecular scattering, 7 " 10 although useful str...

The Study of Surface Structures by Photoelectron Diffraction and Auger Electron Diffraction

“…In Figs. 9a and 9b, we show both polar and azimuthal C Is data obtained by Saiki et al 86 from CO adsorbed at room temperature on Fe (001) so as to form predominantly the so-called £l'3 state. This rather unusual species has been the subject of prior studies by several techniques, including EELS, ESDIAD, and NEXAFS.…”

Improving metal dusting resistance of. transition‐metals and Ni‐Cu alloys

“…It is interesting to note that the adsorption of sulfur atoms on Fe(100) surfaces has also been experimentally detected to prevent the dissociative adsorption of CO [29]. This is because those adsorption sites of CO [30], C, O [31], and S [32] on Fe(100) surfaces are identical with the four-fold hollow sites, and CO molecules lose their dissociative reaction fields with a sulfur monolayer. These experimental results suggest that an understanding of atomistic properties of CO adsorption on transition-metal surfaces is the key ingredient for metal dusting suppression technologies.…”

“…On bcc-Fe(100) surface, a CO molecule is experimentally known to occupy the hour-fold hollow site, where the molecular axis is tilted along the vertical direction of the surface [30]. In the present work, however, the electronic structures of non-tilted states are calculated for simplicity.…”

Effect of Hydrogen in Adsorption and Direct Dissociation of CO on Fe (100) Surface: A DFT Study

“…In the same way, Erley [11] proposed an upright OT adsorption of CO at 0.25 ML and a tilted and displaced OT adsorption at 0.5 ML. Saiki et al [8] reported a tilting angle of 55˚ ± 2˚ using XPD and Dwyer et al [9] found an angle of 54.7˚ using NEXAFS. The α3 state has been proposed as a precursor for the dissociation of CO molecule on Fe(100), and the activation energy calculated from TPD has been suggested to be the apparent activation energy of dissociation of CO molecule instead of the activation energy of desorption of the CO molecules in the α3 state.…”

“…Several experimental techniques have been used to investigate the adsorption of CO on Fe (100). These techniques include Temperature-Programmed Desorption (TPD) [4] [5], Ultraviolet Photoelectron Spectroscopy (UPS) [5], X-ray Photoelectron Spectroscopy (XPS) [6], ElectronEnergy Loss Spectroscopy (EELS) [7], X-ray Photoelectron Diffraction (XPD) [8], and Near-Edge X-ray Absorption fine structure (NEXAFS) [9] [10]. Experimentally, it is known that CO adsorbs molecularly on Fe (100) at low temperatures (at all coverages) giving rise to three distinctive TPD peaks, which are labeled as α1 (220 K), α2 (305 K), and α3 (440 K) corresponding to three molecular desorption states [4] [5].…”