Elsevier

Surface Science

Volume 418, Issue 3, 14 December 1998, Pages L89-L93
Surface Science

Surface Science Letters
Reanalysis of the Rh(111)+(2×2)-3NO structure using automated tensor LEED

https://doi.org/10.1016/S0039-6028(98)00782-1Get rights and content

Abstract

The structure of Rh(111)+(2×2)-3NO has been reanalyzed using automated tensor LEED, in view of new results which strongly suggest different molecular adsorption sites to those previously concluded. The reanalysis confirms and further refines the new model, according to which the NO molecules occupy top sites and both kinds of three-fold hollow sites. These findings are at odds with earlier conclusions from vibrational spectroscopy, which assigned the molecules to top and bridge sites. Together with other metal/NO and metal/CO systems described in the literature, the Rh(111)+(2×2)-3NO phase once again demonstrates that great care has to be taken in adsorption site assignment using the C–O and especially the N–O stretch frequencies.

Introduction

A recent study of the adsorption geometry of Rh(111)+(2×2)-3NO, with 3/4 monolayer coverage, concluded that our earlier low-energy electron diffraction (LEED) analysis [1]might be wrong in terms of NO adsorption sites. The new model, derived from X-ray photoelectron diffraction data [2], favors top- and hollow-site adsorption. The NO molecules were earlier assigned to bridge and near-top sites, based on N–O stretch vibrational frequencies in high-resolution electron energy loss spectroscopy (HREELS) [1]. A LEED study published in the same paper [1]in fact found good agreement between theory and experiment for those sites, with reasonable interatomic distances. The LEED analysis did not check other combinations of sites due to computational expense and due to close similarity with a corresponding Rh(111)+(2×2)-3CO system 3, 4.

A rule of thumb based on NO-containing clusters for the relationship between N–O stretch vibration frequencies and adsorption sites is: one-fold coordinated top sites, two-fold coordinated bridge sites and three-fold coordinated hollow sites for frequencies in the respective intervals 1650–2000 cm−1, 1480–1545 cm−1, and below 1400 cm−1. In the Rh(111)+(2×2)-3NO system, one observes the frequencies 1840 cm−1 (“top”) and 1515–1630 cm−1 (“bridge” to “top”).

This situation indeed parallels the very similar and earlier analysis of CO adsorption on Rh(111): based on HREELS, we first assumed that the almost identical Rh(111)+(2×2)-3CO system has bridge- and top-site occupation [5]. A LEED analysis confirmed this result, also without being able to check other site combinations due to computational expense in the presence of many structural fit parameters and in the absence of effective methods to optimize these parameters automatically 3, 4. In the CO case, we were however unable to later refine the structure when allowing for adsorbate-induced substrate relaxations with more powerful methods: we recognized this as a signal that the model might be wrong. Shortly thereafter, a photoelectron diffraction study [6]suggested an alternative model, involving one CO molecule at a top site and two CO molecules at two different three-fold hollow sites. We then confirmed that alternative model with a new, much more exhaustive and complete LEED analysis, including adsorbate-induced substrate relaxations [7]. On this basis alone, it was thus natural also to question our earlier NO structure [8]. In addition, there has been a recent LEED analysis of the very similar Ru(0001)+(2×2)+3NO system [9], in which NO adsorption at top sites and two kinds of hollow sites was also found.

We have therefore reexamined the Rh(111)+(2×2)-3NO structure in the same fashion as our reanalysis of the Rh(111)+(2×2)-3CO system, thus including more modern theoretical methods: this is the object of this paper. We used the same experimental IV data sets as for the earlier LEED analysis [1], taken at a temperature of 40 K at normal incidence. The data set used for the analysis contains 23 symmetrically independent beams, with a total energy range of 1900 eV.

Section snippets

Calculations

The LEED calculations were performed with the Van Hove/Barbieri automated tensor LEED package [10]. For structural refinement, the tensor LEED approximation in combination with the Powell optimization scheme was applied [11]. Eight phase shifts calculated with the Barbieri/Van Hove phase shift package [12]were used for N, O and Rh. The agreement between theory and experiment was quantified by the Pendry R factor [13], while error bars were obtained using Pendry's formula [11].

We first tested

Results and discussion

The optimum geometry for NO in top and threefold hollow sites is shown in Fig. 1 and detailed in Table 1. This configuration is the only one where NO occupies highly symmetric adsorption sites and forms an ideal hexagonal overlayer. All NO molecules stand perpendicular to the surface (at least on average over time, in view of vibrations). There is a height difference of about 0.45 Å between the N atoms over top sites on the one hand, and over hollow sites on the other hand. For the corresponding

Summary

The Rh(111)+(2×2)-3NO structure has been reanalyzed using tensor LEED combined with an automated optimization procedure. In the optimized structure, one NO molecule sits in a top site while the other two molecules reside in three-fold fcc and hcp hollow sites. This result confirms and refines the conclusion of a recent X-ray photoelectron diffraction study.

Acknowledgements

I.Z. acknowledges a postdoctoral fellowship from the Kosciuszko Foundation. This work was supported in part by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the US Department of Energy under Contract No. DE-AC03-76SF00098.

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