M.J. Van Stipdonk

TIME-OF-FLIGHT-SECONDARY ION MASS SPECTROMETRY OF NABF4 : A COMPARISON OF ATOMIC AND POLYATOMIC PRIMARY IONS AT CONSTANT IMPACT ENERGY

(CsI)nCs+ projectiles, n=0–3, were used to bombard a NaBF4 (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)nF−, 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.

Coincidence measurements in mass spectrometry

The detection of coincidental emissions (electron-ion, ion-ion, photon-ion) can enhance the amount of information available in desorption time-of-flight mass spectrometry (TOF-MS) by identifying physical, chemical and/or spatial correlations. This paper outlines the conditions for coincidence measurements and the methodology for identifying correlations. Applications of coincidence-correlation mass spectrometry include the study of the composition and structure of polyatomic ions, the process involved in ion production from solids and the chemical Microhomogeneity of surfaces.

High energy chemistry caused by fast ion-solid interactions

Abstract Plasma desorption induced by the impact of 252 Cf fission fragments was used to study the emission of positive cluster ions from a lanthanum nitrate, La(NO 3 ) 3 · 6H 2 O, substrate. Two large series of peaks were observed. One corresponded to the addition of LaO + to a repeating base unit of La 2 O 3 . A less intense series contained the same base unit, with H + attachment. Neither series contained clusters that reflect the structure or stoichiometry of the original sample. We propose that the cluster ions are produced by high temperature and pressure chemical reactions inside the fast ion track.

Secondary ion emission from keV energy atomic and polyatomic projectile impacts on sodium iodate

Sodium iodate and sodium iodide are inorganic solids in which iodine exists in different chemical environments. In sodium iodate, the iodine atom is bonded to oxygen to make the trigonal pyramidal IO3 anion, which in turn is incorporated with sodium into an ionic crystal. In sodium iodide, however, the iodide anion and sodium cation are ionically bound in a crystal lattice. Nearly half of the negative secondary ion yield generated from keV energy polyatomic ion impacts on a sodium iodate surface is characteristic of ion emission expected from sodium iodide (i.e. (NaI) nI 2 ), yet x-ray photoelectron spectroscopy data indicate that the solid material resulting from aliquots of aqueous sodium iodate dried on stainless steel contains no more than 2% sodium iodide. To determine how the number of atoms in the primary ion influences the amount of iodide type ion formation from sodium iodate, secondary ion yield measurements were performed using Cs, (CsI)Cs, and (CsI)2Cs projectiles incident at energies ranging from 10 to 25 keV. The experiments were run on an event-by-event basis at the level of single ion impacts. The yields of iodate (composed of Na, I, and O) and iodide type secondary ions increase with the energy of the projectile and the number of constituent atoms. When compared on a per-incident atom basis, however, we found that the yields of secondary ions characteristic of iodate saturate at three total projectile atoms, but continue to increase nonlinearly for iodide species (i.e. the yield per impacting atom increases). (Int J Mass Spectrom 197 (2000) 149 ‐161)

Ion–neutral correlations from the dissociation of metal oxide cluster ions in a reflectron time-of-flight mass spectrometer

A time-of-flight mass spectrometer with an electrostatic mirror was used to probe the composition and structure of polyatomic secondary ions via their metastable dissociation following the impact of MeV energy primary ions. The experimental approach combines event-by-event bombardment and detection and coincidence counting, allowing the correlation of neutral and ion fragments originating from the same precursor molecule or cluster ion. To evaluate the ion-neutral correlation method, the dissociation pathways for metal oxide cluster ions sputtered from bismuth [(Bi 2 O 3 ) n BiO + ], lanthanum [(La 2 O 3 ) n LaO + ] and calcium nitrate [(CaO) n H + , (CaO) n Ca + ] were determined. The metal oxide clusters dissociate by shedding stoichiometric units. For instance, (La 2 O 3 ) n LaO + cluster ions dissociate by evaporating a single La 2 O 3 unit. Similar results were obtained for the bismuth oxide cluster ions. The (CaO) n H + , cluster ions from calcium nitrate dissociate by evaporating one or more CaO units, and show preferential decay into fragment ions with favored face-centered-cubic structures such as (CaO) 2 H + and (CaO) 4 H + .

Negative secondary ion emission from NaBF4 : comparison of atomic and polyatomic projectiles at different impact energies

(CsI) n Cs + (n = 0-2) and C + 60 projectiles were used to bombard a sodium tetrafluoroborate (NaBF 4 ) target at energies ranging from 18 to 28 keV. The objective of these experiments was to monitor the emission of two series of secondary ions following atomic and polyatomic projectile impacts. One series is based on BF 4 - and its incorporation into larger polyatomic ions that reflect the stoichiometry of the original solid. The other series is based on repeating (NaF) units, and presumably represents artifact ions created by primary ion impact induced recombination/rearrangement reactions. The relative yields of several secondary ions representing both types of ion formation were measured as a function of the primary ion velocity (velocity is proportional to the kinetic energy per mass unit). When normalized to the number of projectile constituents, the secondary ion yields follow distinct trends. The non-linear increase in the yield of (NaF)F- demonstrates greater sensitivity to the number of projectile constituents than intact, analytically useful ions such as BF 4 - and (NaBF 4 )BF 4 - .

Determination of the metastable dissociation pathways for chromium/oxygen cluster ions sputtered from potassium chromate and dichromate using the ion-neutral correlation method

Abstract Event-by-event bombardment and detection, coincidence counting, and the ion-neutral correlation method were used to study the dissociation of chromium/oxygen and chromium/oxygen/potassium cluster ions in the drift region of a reflectron time-of-flight (TOF) mass spectrometer. The cluster ions were sputtered from potassium chromate and dichromate by the impacts of 252 Cf fission fragments. The dissociation channel observed for the fragmentation of negative cluster ions was the emission of CrO 3 − . The positive secondary cluster ions decayed by shedding K + . Decay fraction measurements were also performed to gain information about the relative stability to secondary ion dissociation and/or neutralization.

The use of coincidence counting mass spectrometry to study the emission and metastable dissociation of cluster ions

Abstract Coincidence counting and time-of-flight mass spectrometry were used to study the coincidental emission of ions from single fission fragment, impacts from an inorganic solid. Removal of interfering peaks is possible by observing those ions in coincidence with a mass peak unambiguously representative of the sample of interest. The use of coincidence also allows a direct study of the correlation of ions and neutrals formed due to the metastable dissociation of ions while in the drift region of the spectrometer. Such studies allow insight into the structure of cluster ions, which can aid in relating the cluster ion composition and structure back to that of the original solid.

A mass spectrometric method for probing surface structure

Abstract Coincidence counting mass spectrometry (CCMS) has been used to study the negative and positive cluster ion formation from two inorganic compounds, NaBF 4 and αZrP. Coincidence spectra are obtained from conventional mass spectra by collecting only those desorption events that include an ion of interest. Direct comparison of the cluster ion compositions to the stoichiometry and structure of the original solid were made. Negative ions reflect the original solid composition and structure while the positive cluster ions do not. Also, new evidence supports an intact emission for negative cluster ions and a recombination mechanism for positive cluster ions. Changing the crystallinity of the solid affects the negative spectrum, with larger cluster ions observed in the more crystalline samples. No difference is observed with the positive clusters, consistent with a gas phase formation process. In the negative ion mode CCMS can be an effective tool for studying the composition and the nanoscale order of a solid surface.

Coincidence experiments in desorption mass spectrometry

Abstract The detection of coincidental signals can enhance the amount of information available in desorption time-of-flight mass spectrometry (TOF-MS) by identifying physical, chemical and/or spatial correlations between secondary ions. Detection of coincidental emissions requires that the target surface be bombarded with individual primary ions (keV or MeV), each resolved in time and space. This paper will discuss the application of coincidence counting to TOF-MS to: extract the secondary ion mass spectrum and secondary ion yields from an organic target produced by a single primary ion type when multiple primary ions simultaneously impact the sample; examine the metastable dissociation pathways and decay fractions of organic secondary ions using an ion-neutral correlation method; and study the chemical microhomogeneity (on the sub-μm scale) of a surface composed of two chemically distinct species.