A detailed analysis of vibrational excitations in x-ray photoelectron spectra of adsorbed small hydrocarbons

Abstract

The vibrational fine structure of x-ray photoelectron (XP) spectra of a number of different small hydrocarbon molecules and reaction intermediates adsorbed on Pt(111) and Ni(111) has been investigated in detail. The data for methyl, methylidyne, acetylene, and ethylene can consistently be analyzed within the linear coupling model. The S factor, i.e., the intensity ratio of the first vibrationally excited to the adiabatic transition, is obtained to be 0.17+/-0.02 per C-H bond; for the deuterated species a value of 0.23+/-0.02 is obtained. Therefore, the vibrational fine structure can be used for fingerprinting in the analysis of XP spectra and for identifying unknown reaction intermediates. From the data, Deltar, the change of the minimum in the potential energy curve upon core ionization, is calculated within the linear coupling model using a first order correction. For all adsorbates, including the deuterated ones, a value of Deltar=0.060+/-0.004 A is obtained. Furthermore, from the binding energy of the adiabatic peak and from the energy of the vibrational excitation in the ionic final state some information on the adsorbate/substrate bond and the adsorption site can be derived.

Step enhanced dehydrogenation of ethanol on Rh

“…Refs. [6][7][8][9]. The energy separation of the C-H vibrational satellites is in all cases close to 400 meV and the intensity distribution closely follows a Poisson distribution as expected from a linear coupling model [15].…”

“…[6][7][8][9][25][26][27]. Assigning these components to specific molecular fragments is in general difficult due to the large number of possible C-H containing fragments, the close-lying and even overlapping C1s binding energies, and the possibility that some of the components are not due to chemical shifts but instead to the excitation of C-H vibrations in the photoemission process.…”

“…In identifying that the 283.0 eV component corresponds to methylidyne we make use of the correlation [6][7][8][9]28] between the intensity ratio (the so-called S-factor) of the first vibrational component and the adiabatic C1s peak and the number of H-atoms bonding to the C-atom. For free molecules [28] and for CH 2 and CH 3 groups in molecules chemisorbed on surfaces [6,7,9], the S-factor is found to depend almost linearly on the number of H-atoms bound to the C atom.…”

Azobenzene-functionalized alkanethiols in self-assembled monolayers on gold

“…The FWHM of 0.95 eV of the trifluoromethyl group is attributed to vibrational broadening [34]. In XAS this group can in principle be used as an independent marker to determine the tilt of the molecules, but, as always with fluorinated hydrocarbons, great care has to be taken with respect to X-ray induced beam damage.…”

Ethylene decomposition on Ir(111): initial path to graphene formation

“…Indeed, it is known that, when measured with high-resolution, the C 1s core level spectra of small hydrocarbon molecules can display vibrational fine structure due to the excitation of C-H vibrational motion in the final state. [39,40] For this reason, we performed low-temperature, high-statistics and high-resolution C 1s photoemission experiments after annealing the high-coverage ethylene structure at increasing selected temperatures. The results, when combined with accurate DFT calculated C 1s core level shifts for different chemical species, enabled us to make an accurate interpretation of the different spectral components (reported in the interpretation of CLS core level spectra section)…”