Secondary Neutral Mass Spectrometry Using Three-Colour Resonance Ionization: Osmium Detection at the p.p.b. Level and Iron Detection in Silicon at the < 200 p.p.t. Level

Abstract

Among the many uses of resonantly enhanced multiphoton ionization (REMPI) spectroscopy, secondary neutral mass spectrometry (SNMS) is both one of the most demanding and one of the most important. Recently, we have demonstrated that the selectivity of REMPI, and thus the sensitivity of SNMS, can be greatly enhanced by using resonant excitation schemes involving multiple resonant processes. Of particular interest is the use of autoionizing resonances, resonances with energies in excess of the ionization potential of the atom, in the REMPI process. The use of autoionizing resonances can reduce the laser intensity required to saturate the ionization process by more than an order of magnitude. This reduction can strongly reduce non-resonant ionization of background constituents, enhancing the signal to noise of the SNMS measurement. Although this approach to laser ionization SNMS is generally applicable, the three-colour ionization method has been demonstrated using two widely disparate yet important systems. Iron impurity atoms form deep level traps in Si, changing bulk electrical properties even at concentrations approaching 1 p.p.t. In this case, normal mass spectrometry of impurity atoms is complicated by the isobaric interference of ⁵⁶Fe and ²⁸Si₂ molecule. The required mass resolution of greater than 10⁴ exceeds the capabilities of most mass spectrometers. Even for instruments with sufficient mass resolution, the concomitant reduction in useful yield limits detection sensitivity in the near-surface region to 1 p.p.m. REMPI has now been successfully used to separately ionize the impurity atoms of interest. Three-colour REMPI dramatically reduces the residual non-resonant ionization of the isobarically interfering ion, allowing SNMS measurements at levels below 100 p.p.t. This result is accomplished without significant reduction in the fraction of contaminant atoms ionized and, therefore, in useful yield. Mass spectrometric analysis for Os and Re has long been recognized as an important analytical goal in geochemistry. The concentrations and isotopic compositions of these elements must be determined in order to utilize the radioactive decay of ¹⁸⁷Re to ¹⁸⁷Os (t_1/2 = 4.23 × 10¹⁰ years) as a tracer of geochemical processes. Three problems have limited the analysis of Os and Re. First, the concentrations of Os and Re in naturally occurring samples are extremely low, ranging from 1 p.p.b. to 60 p.p.m. Second, the high-ionization potentials (IP) of Os (8.7 eV) and Re (7.9 eV) have precluded the use of thermal ionization and limited the sensitivity of secondary ion mass spectrometry (SIMS) measurement to parts per million. Finally, the ¹⁸⁷Re/¹⁸⁷Os mass difference is less than 1 p.p.m., making conventional mass analysis (without complex chemical separation) impossible. We have demonstrated the ability of three-colour resonant SNMS to detect and separate Os and Re in Ni samples at the 4 p.p.b. level. In situ analysis of Os in iron meteorites demonstrates an elemental selectivity of Os over Re of greater than 10³ without prior chemical separation. Measurements on a suite of samples with Os concentration varying from 10 per thousand down to 100 p.p.b. show a linear correlation between concentration and signal with a precision of better than ± 13% and a useful yield in excess of 1%. These results demonstrate the potential for three-colour resonant ionization to detect and selectively ionize most high IP elements, including all of the Pt group elements, with good sensitivity, accuracy, and precision.

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