Intensity calibration and sensitivity factors for XPS instruments with monochromatic Ag Lα and Al Kα sources

Abstract

This paper provides a description of the transmission function of an X‐ray photoelectron spectroscopy (XPS) instrument operating with exchangeable Al Kα (1486.6 eV) and Ag Lα (2984.3 eV) sources. Both sources use the same quartz crystal monochromator and illuminate the same area of the sample. The transmission‐function–corrected data from sputter cleaned gold provides a useful reference material to calibrate other instruments of the same type. Sensitivity factors for Ag Lα and Al Kα are calculated from photoionisation cross sections and electron effective attenuation lengths. These compare well with previous experimental values and data acquired from ionic liquids. The intensity of the Ag Lα source is found to be approximately 50 times lower than the Al Kα source. This, coupled with generally lower photoemission efficiencies, results in noisier data or extended acquisition times. However, there are clear advantages to using the Ag Lα source to analyse certain elements where additional core levels can be accessed and for many technologically important elements where interference from Auger electron peaks can be eliminated. The combination of calibrated data from both sources provides direct and easily interpreted insight into the depth distribution of chemical species. This could be particularly important for topographic samples, where angle resolved experiments are not always helpful. We also demonstrate, using thin coatings of chromium and carbon, that the inelastic background in Ag Lα wide‐scan spectra has a significantly increased information depth compared with Al Kα.

A Multi-Technique Approach for a Complete Thin Film Characterisation

“…In contrast to increased surface sensitivity, high energy excitation sources can be used to increase the analysis depth of a material to >15 nm. For the HfO 2 /SiO 2 /Si sample, the use of a monochromated Ag Lα (2984.2 eV) X-rays will probe deeper into the bulk structure [2]. The use of higher energy X-ray sources also allows the excitation of higher binding energy corelevels not normally accessible with a conventional Al Kα excitation source.…”

Practical guide for x-ray photoelectron spectroscopy: Applications to the study of catalysts

“…Whilst higher energy sources have been available for many years [129][130][131][132][133] , their wider use has been somewhat precluded by primarily wide-scale availability, broader linewidths and decreased elemental sensitivity to orbitals conventionally studies with Al/Mg radiation 134 . Many instrument manufacturers now routinely offer monochromatic Ag or Cr sources, their use is still somewhat in their infancy in catalysis for lab-based systems, with many studies still reliant on synchrotron radiation 135,136 .…”

Quantitative HAXPES

“…To ensure that the data can be interpreted using physical theory, it is necessary to have accurate reference spectrum, and this was provided by NPL many years ago [6]. It is not clear how this may be achieved with HAXPES instruments without considerable effort, however for the Ag L source, where the X-ray spot size and geometry is identical to the Al K source in the same instrument, this can be addressed through extension of the NPL method [4].…”

“…Many manufacturers are equipping their instruments with higher-energy X-ray sources, examples include Kratos with an Ag L source [4] which offers twice the X-ray energy (2984.4 eV) of the Al K source and uses the same monochromator. Much higher energies are now available in a laboratory system using an intense Ga Kα metal jet X-ray source at 9.25 keV [5] from Scienta Omicron.…”