Time-of-flight-secondary ion mass spectrometry of NaBF4: a comparison of atomic and polyatomic primary ions at constant impact energy

Abstract

(CsI) n Cs + projectiles, n = 0-3, were used to bombard a NaBF 4 (sodium tetrafluoroborate) target at the limit of single ion impacts. The relative yields of ions sputtered from the target were measured and are compared as a function of the number of atoms in the primary ion. When normalized to the mass of the primary ion, sputtered ions that do not resemble the original sample composition, (NaF) n F -, increase in yield as the primary ion complexity increases. The yields of atomic species and polyatomic ions emitted presumably as intact units decrease as the projectile complexity increases. © 1997 John Wiley & Sons, Ltd. Received 2 May 1997; Revised 13 August 1997; Accepted 15 August 1997 Rapid. Commun. Mass Spectrom. 11, 1794-1798(1997 In secondary ion mass spectrometry (SIMS), it is well established that enhanced secondary ion (SI) yields are gained using polyatomic primary projectiles.1-9 The enhanced yields are produced when the amount of energy deposited in the surface region of a solid is high. At keV bombarding energy, where nuclear stopping predominates, polyatomic projectiles will penetrate a surface, fragment and deposit energy by recoil cascades in a spatially confined region. The overlap of recoil cascades leads to the formation of a collision or energy spike.10-12 Recent work has suggested that the energy density, the amount of energy deposited into the solid per unit volume, near the surface, as opposed to the energy loss (dE/dx) is the important parameter for the enhanced yield.13 At constant bombarding energy, the energy density deposited can be scaled upward by increasing the number of atoms in the projectile; viz. higher energy density is achieved using projectiles with many atoms. 14 The object of this study was the relationship between the features of a mass spectrum (SI composition and yield) and the number of atoms contained in the primary projectile. There are two general negative ion formation pathways for SI sputtered from sodium tetrafluoroborate, NaBF 4 , observed first in ion formation studies using plasma desorption mass spectrometry. 15 One pathway is the emission of intact BF 4 -alone and its incorporation into larger cluster species; the other is the formation of ions following the series (NaF) n F -. The latter series is assumed to be formed by recombination/rearrangement reactions in the ion formation process. 16 For this reason, NaBF 4 was chosen as a model target for this study. Our goal was to determine whether the yields of different types of SI are sensitive to the energy density deposited into the surface region. The study was performed by monitoring the SI yields induced by the impacts of Cs + and (CsI) n Cs + (n = 1-3) projectiles at constant bombarding energy. The experiment was conducted in the event-by-event bombardment and detection mode at the limit of single ion impacts using coincidence counting techniques. EXPERIMENTALA dual time-of-flight (TOF) mass spectrometer was used to measure simultaneously the electrons and ions emitted from a target during energetic io...

Cluster ions derived from sodium and potassium tetrafluoroborate and their collision induced dissociation in an ion trap mass spectrometer

“…The preferred loss of BF 3 units, in the case of NaBF 4 , is consistent with the disproportionation of NaBF 4 into NaF and BF 3 , which has been observed most dramatically during studies of secondary ion emission from solid NaBF 4 stimulated by the impact of energetic ions 28–32. For example, the positive secondary ion mass spectra generated by MeV energy ion impacts (fission fragments from 252 Cf) on solid targets composed of NaBF 4 were dominated by a series of cluster ions following the trend (NaF) n Na + , including enhanced intensities at several ‘magic number’ values of n such as 4 and 7 31.…”

“…Because of the potentially labile nature of this Lewis acid/base complex, one might expect the use of relatively high energy desorption and/or ionization methods (such as keV or MeV ion impacts and laser radiation) to hinder, to some degree, the generation of intact, gas‐phase oligomers of NaBF 4 or KBF 4 . For example, in previous experiments involving the sputtering of NaBF 4 by keV and MeV energy ions, such as in secondary ion and plasma desorption MS, the negative ion mass spectra have been shown to contain (in proportions that depend on the projectile complexity, mass and energy) F − , the intact BF 4 − anion and a larger cluster ion such as (MBF 4 )BF 4 − , where M denotes the metal cation 28–30. In all cases thus far, however, the positive ion mass spectrum is dominated by a cluster series following the trend (MF) n M + , including intensity maxima at certain ‘magic number’ ions that adopt stable, cuboid‐like structures 28,.…”

Sputtered molecular fluoride anions (M=B, Si, P, K, Ti, In, W, Pb)

“…This appears to be particularly prominent in the case of negatively charged ions; noteworthy is the very abundant formation of anionic fluoride molecules from many samples. Although several studies have examined such fluoride anions in the past [9][10][11][12][13][14][15], their possible formation and ionization processes and relative abundances are still not known in detail. (A comprehensive review on molecular anions can be found in [16]).…”

Comparison of Ga+ and SF5+ primary ions for the molecular speciation of oxysalts in static secondary ion mass spectrometry (S-SIMS)

“…Polyatomic primary ions proved to be more efficient for producing both high and low mass secondary ions. 8 Interestingly, the relative yield enhancement of ions BF 4 À and NaBF 4 Á BF 4 À reached a plateau value at high primary ion mass, whereas that of (NaF) n Á F À continued to increase. This trend pointed to overlapping collision cascades.…”