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Abstract
Little is known about the temperature dependence of electron transfer occurring at real metal surfaces. For iron surfaces scratched in seven environments, we report Arrhenius activation energies obtained from the data of photoelectron emission (PE) and X-ray photoelectron spectroscopy (XPS). The environments were air, benzene, cyclohexane, water, methanol, ethanol, and acetone. PE was measured using a modified Geiger counter during repeated temperature scans in the 25–339 °C range under 210-nm-wavelength light irradiation and during light wavelength scans in the range 300 to 200 nm at 25, 200, and 339 °C. The standard XPS measurement of Fe 2p, Fe 3p, O 1s, and C 1s spectra was conducted after wavelength scan. The total number of electrons counted in the XPS measurement of the core spectra, which was called XPS intensity, strongly depended on the environments. The PE quantum yields during the temperature scan increased with temperature, and its activation energies (ΔEaUp1) strongly depended on the environment, being in the range of 0.212 to 0.035 eV. The electron photoemission probability (αA) obtained from the PE during the wavelength scan increased with temperature, and its activation energies (ΔEαA) were almost independent of the environments, being in the range of 0.113–0.074 eV. The environment dependence of the PE behavior obtained from temperature and wavelength scans was closely related to that of the XPS characteristics, in particular, the XPS intensities of O 1s and the O2− component of the O 1s spectrum, the acid–base interaction between the environment molecule and Fe–OH, and the growth of non-stoichiometric FexO. Furthermore, the origin of the αA was attributed to the escape depth of hot electrons across the overlayer.
Highlights
Much attention is given to studying charge transfer and electronic excitation in the overlayers of real metallic surfaces in relation to various problems in tribology, lubrication, adhesion, corrosion, andAppl
The present study reveals the effects of temperature, photon energy, and surface overlayer on photoelectron emission (PE) from scratched iron surfaces
Information known about the temperature dependence of the transfer of electrons photo-excited from the metal base through superficial layers is limited, owing to the difficulties related to experimental measurements of such electrons
Summary
Much attention is given to studying charge transfer and electronic excitation in the overlayers of real metallic surfaces in relation to various problems in tribology, lubrication, adhesion, corrosion, and. For the above-scratched iron samples, we obtained the activation energies from the glow curves of PE quantum yield (emitted electrons per incident photon), Y, during four temperature scans under 210-nm-wavelength light irradiation and reported their relationship with the acid–base interaction between molecules of the liquid environments and. The PE experiments and results for the wavelength scan were summarized as follows [7]: (1) the wavelength was scanned in the 300–200 nm range at 25, 200, and 339 ◦ C; (2) using the electron emission intensity, from which PE quantum yields can be estimated, as a function of wavelength of incident light, the values of electron photoemission probability, αA, and photothreshold, φ, were determined Both αA and φ values increased with increasing temperature. (2) the acid–base interaction of the molecule of the environments with Fe–OH, producing an electric dipole; (3) the presence of Fex O with a p-type semiconductor characteristic [12], which is considered based on the ratio of XPS intensity of the O2− component of the O 1s spectrum to the XPS intensity of Fe 2p and Fe 3p at 25, 200, and 339 ◦ C, and which is thought to grow with temperature in the neighborhood of the interface between the base metal; (4) the presentation of the band model of the energy diagram for Fex O
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