SIMS studies of hydrogen interaction with the TiFe alloy surface: hydrogen influence on secondary ion yields

Secondary ion mass spectrometry is a powerful analytical tool that offers great capabilities for studying hydrogen interaction with metallic materials. Yet its utilization for the in situ studies of hydrogen interaction with a sample has not been well established. In this research we study the influence of hydrogen partial pressure (5 x 10(-9) - 5 x 10(-5) Torr) in the sample chamber, the sample temperature (305-900K), and primary ion current density on the emission intensity of various secondary ions for the hydrogen-storage TiFe alloy sample bombarded with 12 or 20 keV Ar+ primary ions. Presence of chemisorbed hydrogen on the alloy surface results in the presence of a variety of positive and negative hydrogen-containing secondary ions in mass spectra. Analysis of the influence of the varied experimental parameters has shown that the secondary ion yield changes reflect only changes of hydrogen concentration regardless of which varied parameter induced a concentration change. The yield dependences on concentration in general are different for different ions and tend to be more substantial for the ions containing a larger number of hydrogen atoms. The ions containing one hydrogen atom have similar and likely linear yield dependences when the concentration is low. At higher concentrations, the yield dependences for these ions diverge: their relation to concentration is discussed. For not hydrogen-containing secondary ions, hydrogen saturation of the surface resulted in an 80-400% increase of the yields of Ti+ and Ti-containing positive secondary ions, no yield change for other positive ions, and 2-3-times decrease of yields of negative ions. As for hydrogen interaction processes with the alloy surface, the obtained dependence of the ion yields only on hydrogen concentration indicates that the alloy surface is rather homogeneous i.e. without large parts that have different characteristics of interaction with hydrogen and different ratios of secondary ion yields. The results of the sample temperature influence indicate that the chemisorption probability of hydrogen molecules is temperature independent at least in the similar to 300-500 K range. The substantial decrease in concentration at temperatures above 450 K is a result of associative desorption of hydrogen, although a possible contribution of other processes is not excluded by the presented results.