Al Kα XPS reference spectra of polyethylene for all instrument geometries

Abstract

This paper extends a previous description of XPS survey spectra from low density polyethylene (LDPE), which was specific for a single type of geometry, to all geometries. Instrument geometries are specified by two angles. The first angle, a, is between the sample-to-monochromator vector and the sample-to-analyzer vector. The second angle, b, is the dihedral angle between the anode-monochromator-sample plane and the monochromator-sample-analyzer plane. The second angle is important because of the polarization induced by the monochromator. We show, using theory, that the XPS spectrum can be decomposed into a “magic angle” reference spectrum, I1, and an anisotropy correction spectrum, f. The geometry for LDPE at which photoemission intensity is equivalent to isotropic emission is shown to be a function of a and b with extreme values for a of 64.6° (b = 0 or 180°) and 57.5° (b = 90°). The deviation of these angles from the “magic angle” a = 54.7° is due to a combination of x-ray polarization and nondipole effects in photoemission. Intensity-calibrated data from a number of instruments with two geometries with b = 180°, one set with a = 60° and the other set with a = 45° are used to determine I1 and f, and these are fitted with simple functions to allow the reproduction of LDPE reference spectra for any instrument geometry. The spectra are taken from the Versailles Project on Advanced Materials and Standards, Technical Working Area 2: Surface Chemical Analysis study A27 and are traceable to the National Physical Laboratory, UK intensity calibration spectra for argon ion sputter-cleaned gold. The functions in this paper may be used in the calibration of XPS instruments with quartz-crystal-monochromated Al Kα x-rays by the comparison of the calculated reference spectrum to data from clean LDPE.