The alignment of spectrometers and quantitative measurements in X-ray photoelectron spectroscopy

The alignment of the sample in X-ray photoelectron spectrometers is usually made to optimize the spectral intensities. There are two important classes of spectrometer: (i) those in which the analyser acceptance area is independent of the analysed electron kinetic energy; and (ii) those in which this area varies. Model experiments show how an example of a VG ESCALAB II conforms to class (i) whereas an example of an SSI X-probe is of class (ii) and shows an analyser acceptance area which depends approximately inversely on the emitted electron kinetic energy. This latter result means that the SSI X-probe spectrometer must be aligned for the electrons of the highest kinetic energy (smallest analyser acceptance area). A misalignment of 0.1 mm in the sample height can cause a 10% change in the relative intensities between 0 and 1000 eV binding energies. This dependence of the analyser acceptance area with energy is an effect likely to be common in the advanced electron optical systems of modern electron spectrometers and should be understood in order to use such spectrometers effectively. Such dependencies should be determined by analysts for their own instruments in the operating mode that is used for conducting work in which the repeatability of intensity measurements is important.