This study was to investigate the chemical composition of typical laser mirror coatings to locate and identify absorbing impurities in ThF4 thin films. Analysis by scanning Auger Electron Spectroscopy, X-Ray Photoelectron Spectroscopy and Secondary Ion Mass Spectroscopy showed an abnormally high oxygen content in the films which indicates the presence of water and may eliminate ThF4 as a potential coating material for HF (2.75 micron) laser systems.
Ronald L. Lusk,
"Surface Microanalysis Characterization Of ThF4 Thin Films", Proc. SPIE 0346, Thin Film Technologies and Special Applications, (15 September 1982); doi: 10.1117/12.933790; https://doi.org/10.1117/12.933790