Experimental and model study of the Rh/Al system by means of EPES

Abstract

The Rh/Al system has drawn significant attention as a model heterogeneous catalyst due to its unique adsorption properties. In this study, a set of samples consisting of Al substrates made of polished and etched pure metal sheets and deposited in situ with ultrathin Rh overlayers of different thickness was prepared. During the preparation of the samples, in-depth diffusion of Rh into the surface region of Al substrates obviously took place. To quantify this process, i.e. by non-destructively ascertaining the resulting depth profile of the samples, elastic peak electron spectroscopy (EPES) was employed. An EPES measurement was performed for energies of 500, 750 and 1000 eV with the initial Rh overlayer thickness in the range 3-20Å and using pure Rh and Al standards. The measurement was carried out on a precision spherical retarding-field analyser (RFA). The resulting experimental EPES intensities, as a function of the primary energy and the initial thickness of the Rh overlayer, were compared and fitted with the intensities simulated by the Monte Carlo (MC) method. An MC model of the sample surface consisting of three regions of different inelastic mean free path (IMFP) and Rh concentration were assumed: the first -Rh on the substrate top, the second -Rh/Al with exponential decay of Rh in Al and the third -pure Al. The best fit and best consistency for different primary energies were achieved under the assumption that no Rh was left on Al substrate top, but an exponentially decaying profile of Rh in Al was created at the surface.

Interface effect for EPES sampling depth for overlayer/substrate systems

“…Step length and scattering angles after each collision are the main features of the MC algorithm and are described by probability distribution functions [3,4,21,23]. As Eq.…”

“…ð2Þ where x k o;t and x k s;t are the total lengths of the kth trajectory travelled in overlayer and substrate, respectively, and k 0 and k s are the corresponding IMFPs [8,23].…”

“…We applied here MC simulation because it is a convenient method for determining the ID for different materials, electron energies and measurement conditions. The MC model of the overlayer/substrate system with sharp interface has been described in some details in [21,23]. As in [21] we present results of the MC calculations for selected energies and for typical EPES configuration consisting of cylindrical mirror analyser (CMA) containing a coaxial electron gun and with normal incidence of the beam on a sample (a i = 0 and a 0 = 42°± 6°).…”

Investigations of Au Segregation in AuNi Alloys by the Line Shape Analysis of Photoelectron Spectra

“…Au/Ni and Ni/Au, as well as their interfaces, the surface composition, the electronic structure, and the surface and interface processes have already been investigated in numerous experimental and theoretical studies [1][2][3][4][5][6][7][8][9][10][11][12]. These systems have been frequently used for numerous commercial applications.…”

“…The experimental studies on AuNi systems surface are based on electron spectroscopy methods, like the X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) [6,7], the elastic peak electron spectroscopy (EPES) [7,[9][10][11][12], impact collision ion scattering spectroscopy [5] and the X-ray diffraction analysis (XRD) [8]. For quantitative analysis with the XPS and AES spectroscopy methods, as well as for determining the mean escape depth (MED) [13][14][15] of photoelectrons and Auger electrons, the electron transport parameter, i.e.…”