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 (sigma) using ReO4- bombardment were measured as 960 and 690 Angstrom(2) for Na2NO3+ and Na(NO3)(2)(-), respectively. sigma 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. (C) 1997 Elsevier Science B.V.