Probing Depth of Ultraviolet Photoelectron Yield Spectroscopy in Hydrocarbon Films

Abstract

In order to clarify the probing depth of ultraviolet photoelectron yield spectroscopy, the escape depth (L) of photoelectrons with kinetic energy (Ek) lower than 1.5 eV in hydrocarbon films was determined by using self-assembled monolayers of n-alkylmercaptane adsorbed on gold. The yield of photoelectrons from the gold substrate decreased exponentially as the chain length of n-alkylmercaptane increased. The L was 1.0±0.1 nm for electrons with Ek lower than 0.3 eV, and increased to 3.0±0.9 nm for electrons with the maximum Ek of 1.5 eV. KeywordsPhotoelectron emission, escape depth , self-assembled monolayer, alkylmercaptane, goldUltraviolet photoelectron yield spectroscopy (UPYS) is a useful technique for characterizing the surface electronic properties of materials under atmospheric conditions. i-3 As UPYS employing photoelectrons as a probe, it is expected that information concerning a very thin surface can be obtained from the UPY spectrum. In practice, we have shown that the UPY spectrum of a gold film with only 0.3 nm thickness can be easily obtained. ' However, the probing depth of UPYS has not yet been clarified. Knowledge concerning the escape depth of very low-energy photoelectrons from the sample surface is required for a quantitative analysis of the UPY spectrum. Especially, the escape depth is essential for an analysis of the thickness of layered materials. Nevertheless, there have been few investigations concerning the escape depth of very low-energy (< 3 eV) electrons detected in UPYS in the condensed phases.4-6 n-Alkylmercaptanes spontaneously assemble from solution onto metal surfaces to form homogeneous monolayer films. These films are called self-assembled monolayers (SAMs).? The structures of SAMs of nalkylmercaptanes on a gold substrate have been well characterized by external-reflection infrared spectroscopy.8 The alkyl chain in the film is predominantly trans-extended and canted ca. 27° from the surface normal. Since the incremental chain length per methylene group has been reported to be 0.127 nm9, the increment in thickness of SAM on gold per methylene group is estimated to be 0.11 nm. It is therefore possible to prepare homogeneous hydrocarbon films whose thickness is controlled on the sub-nanometer level scale. These SAMs are suitable for determining the escape depth of photoelectrons. The attenuation length of photoelectrons with a kinetic energy range of 500 to 1500 eV in hydrocarbon films has been successfully determined using SAMs on metal surfaces. i°"In this study, the escape depth of very low-energy electrons (< 1.5 eV) used in UPYS in hydrocarbon films were determined with SAMs of n-alkylmercaptanes (CnH2n+iSH) on a gold surface in order to clarify the surface sensitivity and the probing depth of UPYS. ExperimentalThree types of gold plates (>99.99%, 6 and 7 mm4, 0.2 mm thickness and 4X4 mm, 0.1 mm thickness) were used as substrates. These substrates were polished using alumina (0.05 µm) and ultrasonically rinsed with ultra-pure water several times. The substrates were then dri...

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