Secondary ion mass spectrometry of sodium nitrate: comparison of ReO4− and Cs+ primary ions

The use of perrhenate (rhenium tetroxide, ReO4−) as a bombarding particle was compared with Cs+ for its ability to generate molecular species from sodium nitrate. The purpose of the study was to quantitatively evaluate the enhancement in sputtering to be gained using a heavy, polyatomic primary particle. It was found that ReO4− is three to five times more efficient at generating ions such as Na2NO3+ and Na(NO3)2−. The nitrate-bearing molecular ions were observed to decrease in intensity as primary ion dose increases; at the same time, nitrite-bearing ions were observed to increase. This observation is interpreted in terms of beam damage to the surface of the target. Disappearance cross sections (σ) using ReO4− bombardment were measured as 960 and 690Å2 for Na2NO3+ and Na(NO3)2−, respectively. σ values measured using Cs+ bombardment were slightly larger. These measurements show that for an equivalent area of the sample disrupted, ReO4− is more effective for the production of nitrate-bearing secondary ions, which increases the probability of completing a measurement before extensive beam damage occurs. Secondary ion energies were evaluated and shown to be comparable for the ReO4− and Cs+ bombardment experiments; for this reason, sample charging is not deemed to be a significant factor in these experiments.