negative secondary ion emission from nabf4 : comparison of atomic and polyatomic projectiles at...

JOURNAL OF MASS SPECTROMETRYJ. Mass Spectrom. 34, 554È562 (1999)

Negative Secondary Ion Emission from NaBF4

:Comparison of Atomic and Polyatomic Projectilesat Di†erent Impact Energies

M. J. Van Stipdonk, V. Santiago and E. A. Schweikert*Center for Chemical Characterization and Analysis, Department of Chemistry, Texas A&M University, P.O. Box 30012,College Station, Texas 77843-3012, USA

(n = 0–2) and projectiles were used to bombard a sodium tetraÑuoroborate target at(CsI)nCs‘ C

60‘ (NaBF

4)

energies ranging from 18 to 28 keV. The objective of these experiments was to monitor the emission of two seriesof secondary ions following atomic and polyatomic projectile impacts. One series is based on and its incorp-BF

4—

oration into larger polyatomic ions that reÑect the stoichiometry of the original solid. The other series is based onrepeating (NaF) units, and presumably represents artifact ions created by primary ion impact induced recombi-nation/rearrangement reactions. The relative yields of several secondary ions representing both types of ion forma-tion were measured as a function of the primary ion velocity (velocity is proportional to the kinetic energy per massunit). When normalized to the number of projectile constituents, the secondary ion yields follow distinct trends. Thenon-linear increase in the yield of (NaF)F— demonstrates greater sensitivity to the number of projectile constitu-ents than intact, analytically useful ions such as and Copyright 1999 John Wiley & Sons,BF

4— (NaBF

4)BF

4—. (

Ltd.

KEYWORDS: cluster formation ; ion yields ; secondary ion mass spectrometry ; polyatomic projectiles

INTRODUCTION

Secondary ion mass spectrometry (SIMS) is a widelyapplied and valuable surface analytical tool, especiallyfor the characterization of metal, semiconductor andinorganic solids. Work in our laboratory and others hasdemonstrated that signiÐcantly higher secondary ionyields are gained when polyatomic primary ions areused in SIMS.1h7 The yield improvement is highest forsputtered polyatomic (in the case of inorganic materials)and organic molecule ions. While the enhanced ionyields may beneÐt molecular surface analysis byimproving the sensitivity and limits of detection ofSIMS, recent work in our laboratory has shown thatpolyatomic projectiles are prone to produce dispro-portionately higher yields of damage and artifactpeaks.8h11

In these experiments, Cs`, (n \ 1, 2) and(CsI)nCs`

were used to bombard an sample withC60` NaBF4impact energies ranging from 18 to 28 keV. is aNaBF4model target for studies of ion formation following pro-jectile impact because distinct composition trends forsubstrate-speciÐc negative secondary ions are observed.

* Correspondence to : A. Center for Chemical Char-E. Schweikert,acterization and Analysis, Department of Chemistry, Texas A&MUniversity, P.O. Box 30012, College Station, Texas 77843-3012, USA.

Contract/grant sponsor : National Science Foundation ; Contract/grant number : CHE-9727474.

For example, one involves the emission of aloneBF4~,or incorporated into larger polyatomic anions. An

cluster series is also observed, which reÑects(NaF)nF~

neither the composition nor structure of the originalsolid. In a recent paper, we described the emission andyield of negative secondary ions from producedNaBF4by projectiles at 20 keV impact energy.10 The(CsI)

nCs`

yield of ions, when normalized to the mass of(NaF)nF~

the projectiles, was sensitive to the number of atoms inthe bombarding ion.

Polyatomic ion projectiles are known to producenon-linear enhancements in secondary ion yield. A yieldenhancement occurs when the yield produced by a pro-jectile containing n constituents is greater than the sumof the yields produced by n atomic projectiles. To date,non-linear yield enhancements have been measured foratomic secondary ions such as Au~, inorganic clusterions such as and and organic mol-(CsI)

nCs` (CsI)

nI~

ecules of varying molecular weight. The intent has beento show that the efficiency for producing secondary ionsignal, per primary ion impact, is greater using a polya-tomic projectile. Little attention has been paid,however, to a comparison of the enhancement of intactsecondary ion yield (those considered analytically usefulfor chemical characterization) relative to the yield ofartifact or damage ions. The object of the present studywas to expand on our earlier study of ion emission from

and compare the yields of intact and artifactNaBF4ions produced by atomic and polyatomic projectileimpacts in a range of impact energies.

CCC 1076È5174/99/050554È09 $17.50 Received 13 November 1998Copyright ( 1999 John Wiley & Sons, Ltd. Accepted 10 February 1999

ARTIFACT ION YIELDS FROM CLUSTER PROJECTILES 555

Figure 1. Secondary ion mass spectra of produced by (a) Cs and (b) (CsI)Cs primary ions at 20 keV impact energy. In the CsNaBF4

primary ion spectrum, several peaks (labeled with an asterisk) are present due to a primary ion multiplicity effect. There is a high probability,due to the high yield of Cs½ from fission fragment impacts, that a polyatomic projectile will be generated along with Cs from the same fissionfragment. All projectiles will strike the target, causing several secondary ion peaks to appear that are not correlated with the impact of Cs½.The uncorrelated peaks are easily identified and are a factor only in the spectrum produced by Cs½. Such secondary ion overlap does notaffect the measurement of ion yields from Cs½ impacts.

EXPERIMENTAL

Mass spectrometry

Event-by-event bombardment and detection, coin-cidence counting and a special dual-time-of-Ñight (TOF)clusterÈSIMS instrument were used. The mass spectro-meter design and operation have been described indetail elsewhere.5 The experimental method allows us toexamine secondary ion emission from multiple primary

projectiles simultaneously at the limit of single ionimpacts.12

Primary ions were produced using 252Cf Ðssion frag-ment impacts on an aluminized polyester foil coatedwith a vapor-deposited layer of CsI or PrimaryC60 .ions were accelerated to energies ranging from 18 to 28keV, separated in a primary TOF region and allowed tostrike the sample surface sequentially. Secondary elec-tron emission was used to register individual primaryion impacts. Secondary ions were accelerated to [5keV and separated in a second TOF region. Massspectra from each primary ion were extracted from atotal secondary ion mass spectrum using a coincidence

Copyright ( 1999 John Wiley & Sons, Ltd. J. Mass Spectrom. 34, 554È562 (1999)

556 M. J. VAN STIPDONK, V. SANTIAGO AND E. A. SCHWEIKERT

Figure 2. Secondary ion mass spectra of produced by (a) and (b) primary ions at 20 keV impact energy.NaBF4

(CsI)2Cs C

60

counting protocol designed in our laboratory. With thecoincidence counting approach, suites of polyatomicions from the same source foil [i.e. from CsI](CsI)

nCs`

can be directly compared in terms of secondary ion pro-duction under the same instrumental, vacuum andtarget surface conditions. To compare projectilesC60with the projectiles, it was necessary to break(CsI)

nCs`

the vacuum to introduce the source foil. AlthoughC60the two primary ion source foils were run using thesame instrumental settings and at similar backgroundpressure [D8 ] 10~7 Torr (1 Torr \ 133.3 Pa)], thetarget surface conditions (e.g. the amount of adsorbedhydrocarbon and pump-oil contaminant) may have dif-fered. We determined that any changes in ion yield dueto exposure to the atmosphere during a source foilchange were within our experimental error and there-fore were not signiÐcant.

Relative secondary ion yields were calculated bydividing the integrated secondary ion peak areas by the

number of incident primary ions (determined by inte-grating the secondary electron peak areas). The ionyields are not corrected for transmission and detectionefficiency and are presented as per cent relative yield.

Sample preparation

Sodium tetraÑuoroborate was purchased from(NaBF4)Aldrich Chemical and used as received. The samplepreparation procedure was the same as that used in ourpreliminary examination of ion formation at constantprimary ion impact energy.10 BrieÑy, was dis-NaBF4solved in a 50 : 50 deionized waterÈmethanol solution. Asmall portion of an aqueous methylcelluloseÈwatersolution was added to the solution to assist inNaBF4wetting the stainless-steel sample support and topromote homogeneous substrate coverage. A 30 llvolume of the solution was applied to theNaBF4



Figure 3. Relative secondary ion yield ratios for (a) (NaF)FÉ and and (b) FÉ and produced by Cs, (CsI)Cs, andBF4

É BF4

É (CsI)2Cs C

60primary ions each incident on the same target at 20 keV impact energy.

sample support and allowed to dry at room tem-perature in a fume hood. As reported previously,10 acoincidence counting method for probing sample micro-heterogeneity was used to check for the formation ofsodium Ñuoride microdomains within the solidNaBF4during sample preparation. A sodium Ñuoride phasewas not detected, and the peaks in the(NaF)

nF~

mass spectrum are considered to be artifactsNaBF4generated by ion impact.

RESULTS AND DISCUSSION

Negative secondary ion mass spectra from pro-NaBF4duced by Cs` and (CsI)Cs` are provided in Fig. 1. Thespectra produced by and are shown in(CsI)2Cs` C60`Fig. 2. In addition to low-mass ions such as H~, O~and OH~, each primary ion produced a secondary ion

peak corresponding to F~ and and the polyato-BF4~,mic primary ions produced polyatomic secondary ionssuch as and The latter ions(NaBF4)F~ (NaBF4)BF4~.resemble intact portions of the solid. The polya-NaBF4tomic projectiles also produced a series of (NaF)

nF~

(n \ 1È3) peaks, which reÑects a recombination/rearrangement reaction occurring after ion impactsimilar to those reported for positive ion formationusing Ar primary ions.13 The intensity of the (NaF)F~peak produced by Cs` was very low and was often diffi-cult to measure above the background. Secondary ionpeaks were also observed at m/z 65 and 107 that may beattributed to species. Peaks at the same m/z(NaF)

nNa~

ratio are also observed in a background spectrum(acquired from a blank sample substrate), prohibitingunambiguous composition assignment. The projec-C60`tiles produced the highest abundance of low-m/z sec-ondary ions attributed to chemical background (i.e.



Figure 4. Relative secondary ion yield ratios for (a) (NaF)FÉ and and (b) and produced by (CsI)Cs,BF4

É (NaBF4)BF

4É BF

4É (CsI)

2Cs

and projectiles shown as a function of the energy per unit mass of the primary ion. The yield ratio produced by Cs½, omitted for figureC60

clarity, is (a) Ä0.01 and (b) Á1.

The carbon and carbonÈhydrogen clusterCn~, C

nH

m~).

ions originate from hydrocarbon contaminants originat-ing from the pumping system. The carbon atoms in the

projectile (each with D333 eV) deposit their energyC60in the surface layers. This leads to a high energy densitydeposited at the surface, and causes the projectile toC60be the most sensitive for adsorbed organic molecules(see discussion below).

In the experiments reported here, the yields of(NaF)F~ and were used as indicators of ion for-BF4~mation by recombination/rearrangement and intactemission, respectively, because they were the mostabundant representatives within each ion series. In aprevious report, projectiles incident on a(CsI)

nCs`

target were compared at 20 keV impact energy.NaBF4The yield of (NaF)F~ increased, when normalized tothe mass of the impacting projectile, as the number ofatoms in the primary ion increased.10 In the presentstudy, Cs`, (CsI)Cs`, and projectiles(CsI)2Cs` C60`were also compared at constant impact energy. Figure 3

shows a plot of the relative yield ratio for (a)and (b) produced by the suite(NaF)F~/BF4~ F~/BF4~

of projectiles incident at 20 keV. The yield of (NaF)F~increases relative to as the complexity of the pro-BF4~jectile increases. The yield of F~ relative to BF4~,however, remains constant for the polyatomic projecti-les. The comparison is made to demonstrateF~/BF4~the behavior of two ions most likely ejected from thesurface via an intact emission mechanism. Cs projectilesproduce higher yields of atomic secondary ions, whichin turn produces an ratio much higher thanF~/BF4~the polyatomic primary ions.

At constant impact energy, the energy deposited perunit volume is sensitive to the number of atoms in theprojectile. As atoms are added to the projectile, theimpact energy per constituent decreases, which in turndecreases the penetration and range of the primary ion.Energy deposited via the collision cascades initiated byeach projectile constituent is concentrated in a spatiallyconÐned region of the surface. The energy density scales



upward with the number of projectile constituents at agiven impact energy. Based on the trends shown in Fig.3, increasing the energy deposited into the surfaceregion per unit volume promotes an increase in

ion formation relative to(NaF)nF~ BF4~.

Figure 4(a) shows the yield ratio as a(NaF)F~/BF4~function of the energy per unit mass of the projectile,which is proportional to the square of the projectilevelocity. For clarity, only the ratios produced by thepolyatomic projectiles are shown: the (NaF)F~/BF4~ratio from Cs` impacts is less than 0.01. The formationof (NaF)F~ relative to increases with both theBF4~energy of the bombarding ion and the number of con-stituents, and is highest overall for the projectile.C60`Owing to the number of low-mass constituents, C60`would deposit the highest energy density per unitvolume at a given impact energy. Figure 4(b) shows the

yield ratio produced by the polya-(NaBF4)BF4~/BF4~

tomic primary ions (the ratio produced by Cs` is D1)as a function of the energy per unit mass of the projecti-le. The trend shown in Fig. 4(b) is reversed comparedwith Fig. 4(a), with projectiles producing the lowestC60`yield ratio. One explanation for this result may be that

is produced with higher internal energy(NaBF4)BF4~by the larger polyatomic ions, causing rapid disso-ciation into Na` and prior to acceleration. It isBF4~apparent from Fig. 4 that the yield ratios produced bythe complex projectiles depend more on the polyatomi-city of the projectile than on the impact energy in theenergy range used in this study.

Figure 5 compares the relative yields of (a) (NaF)F~and (b) produced by and projecti-BF4~ C60` (CsI)

nCs`

les as a function of the energy per mass unit of the bom-barding ion. The two ion formation pathways exhibitdistinct trends. The yield of (NaF)F~ increases grad-ually with projectile velocity and complexity. The yield

Figure 5. Relative secondary ion yields of (a) (NaF)FÉ and (b) from Cs, (CsI)Cs, and projectiles plotted as a functionBF4

É (CsI)2Cs C

60of the energy per unit mass of the primary ion. The yield from Cs½ impacts for both ions is Ä0.1%.



Figure 6. Relative secondary ion yields of (a) (NaF)FÉ and (b) from Cs, (CsI)Cs and projectiles plotted as a function of theBF4

É (CsI)2Cs

energy per unit mass of the primary ion. In this plot, the relative yield is divided by the number of atoms in the primary ion to demonstrate theyield per projectile constituent. The yield from Cs½ impacts for both ions is Ä0.1%.

increase for however, is highest progressing fromBF4~,Cs` to (CsI)Cs` with the projectile produc-(CsI)2Cs`ing a slightly higher yield than C60` .

Non-linear yield enhancements have been reported insputtering experiments using andAu

n` 2 (CsI)

nCs` 1

projectiles. The yield enhancements are also attributedto the high energy density deposited into the solid bypolyatomic projectiles, a consequence of multiple over-lapping collision cascades that act in concert to ejectpolyatomic entities.14,15 To probe the enhancement in(NaF)F~ and ion yields as a function of the pro-BF4~jectile polyatomicity, the yields produced by the

primary ions were divided by the number of(CsI)nCs

atoms in the bombarding projectile and plotted in Fig.6. Assuming that the yield trend produced by Cs`remains linear to low velocity, it is clear from Fig. 6 thatthe yield of both (NaF)F~ and per projectile con-BF4~

stituent increases non-linearly from Cs` to (CsI)2Cs`.As shown in Fig. 6(a), the yield increase for (NaF)F~,per projectile constituent, is nearly equal progressingfrom Cs` to (CsI)Cs` and from (CsI)Cs` to (CsI)2Cs`.The yield increase for [Fig. 6(b)], however, isBF4~large using (CsI)Cs` over Cs`, but modest using

over (CsI)Cs`.(CsI)2Cs`The yield of divided by the number of(NaBF4)BF4~,

projectile constituents, is plotted in Fig. 7 as a functionof the energy per unit mass of the primary ion. Hereagain there is a large increase in yield per projectile con-stituent between Cs` and (CsI)Cs`, but the yield perconstituent is nearly equal for (CsI)Cs` and (CsI)2Cs`.The yield trends displayed by andBF4~ (NaBF4)BF4~,when shown as a function of the number of atoms in theprimary ion, are reminiscent of those exhibited by intactorganic molecule anions, such as [M[ H]~ from



Figure 7. Relative secondary ion yield of from Cs, (CsI)Cs and projectiles plotted as a function of the energy per(NaBF4)BF

4É (CsI)

2Cs

unit mass of the primary ion. The yield from Cs½ impacts for both ions is Ä0.1%.

phenylalanine.1,2 Our results indicate that the forma-tion of intact organic ions and the presumably intactunits sputtered from are ejected via a similarNaBF4mechanism (i.e. via intact or “chunkÏ emission) and fromthe periphery of the ion impact zone where the energydensity is lower.

The underlying mechanism for the formation offrom and the reason why polyatomic(NaF)

nF~ NaBF4projectiles are more efficient at generating these species

are not clear. The formation of NaF would be the ther-modynamically favored product from Na and F reac-tants generated from the atomization of solid NaBF4 .Although we do not suggest the existence of thermody-namic equilibrium at the primary ion impact site, it ispossible that the high energy density deposited by thepolyatomic projectiles causes complete disruption of ananodomain (i.e. cubic nanometers) of the solid. Thekinetic energy imparted to substrate atoms wouldpromote more reactive collisions between Na and F inthe activated impact site, leading to an increased abun-dance of these ions in the mass spectrum. Quasi-equilibrium conditions have been postulated for theformation of polyatomic secondary ions in theC

nH

mhot infratrack regions of MeV ion impacts.16h19 Inaddition, recent results have shown that similar highenergy chemistry (bare and hydride-attached carboncluster formation) occurs following 20 keV andC60`MeV energy Ðssion fragment impacts on polymer sub-strates.20

The series may also arise from fast(NaF)nF~

rearrangement reactions of sputtered clusters.(NaBF4)nLoss of stable units would result in the formationBF3of NaF, and with the association of F~ could producethe artifact ion series. The polyatomic projectiles maysputter more units intact but with higher(NaBF4)ninternal excitation (for instance, the larger polyatomicprojectiles produce less relative to(NaBF4)BF4~ BF4~,

as discussed above). Provided that the rearrangementreaction occurs on a picoÈnanosecond time-scale, the

rearrangement ions would appear as discrete(NaF)nF~

peaks in the mass spectrum.In addition, the absence of large negative cluster ions

from metal iodide21 and metal oxide22 solids has beenattributed to electron autodetachment processes. Theyield of clusters sputtered from may(NaF)

nF~ NaBF4also be a†ected by electron detachment reactions. An

alternative explanation for the increase in (NaF)F~yield observed in these experiments is that the numberof ions formed is the same for all projectiles(NaF)

nF

but those produced by polyatomic ions leave the surfacewith less internal excitation and are thus less prone toshed an electron. Recent results in our laboratory havedemonstrated, however, that organic ions, includingthose prone to electron autodetachment, have higherinternal energies when sputtered by polyatomic projecti-les.23 Although similar experiments on inorganic ionshave yet to be performed, a higher internal energywould presumably lead to a higher probability of elec-tron detachment and a lower abundance of (NaF)

nF~-

ions (assuming that the autodetachment reactiontypeoccurs before or during the acceleration of the second-ary ion away from the surface). While the exact forma-tion mechanism is speculative, it is clear from thepresent study that the abundances of the artifact peakssuch as (NaF)F~ are more intimately linked to changesin the polyatomicity and hence energy density depositedby the primary projectile.

CONCLUSIONS

The yields of several negative secondary ions fromwere measured for Cs`, (CsI)Cs`,NaBF4 (CsI)2Cs`



and primary ions incident in the range 18È28 kev.C60`At constant energy, the yield of (NaF)F~, a secondaryion not representative of the sample composition,increases relative to a substrate-speciÐc ion, asBF4~,the number of atoms in the bombarding ion increases.When measured through the range of impact energies,secondary ion yield ratios were more sensitive to thenumber of projectile constituents than the total energydeposited by the primary ion.

The yield increase of each secondary ion produced bypolyatomic primary ion impacts on is non-NaBF4linear : the yield per constituent atom in the projectileincreases as more complex projectiles are used. In thisregard, the present results conÐrm earlier work oncluster ion-induced sputtering. The novel Ðnding inthese experiments, however, is that the yield trend as afunction of projectile velocity of an artifact peak such as(NaF)F~ is distinct from the trends demonstrated byintact units such as and The non-BF4~ (NaBF4)BF4~.linear enhancement created by polyatomic projectileimpacts on is greater for the artifact ions. TheNaBF4

di†erence in yield trends opens up the possibility ofverifying the analytical integrity of polyatomic second-ary ions from an unknown or uncharacterized inorganicsurface. This could be done, for instance, by comparingtheir di†erent slopes or yield enhancements.

In the context of surface analysis, the results present-ed here indicate that the proper choice of projectile isimportant. Among those used in the present study, apolyatomic projectile composed of a few high-mass con-stituents produces high yields of analytically usefulsignal while minimizing the yield of artifact species. Itremains to be determined whether a projectile com-posed of a few low-mass constituents [e.g. (NaF)Na` vs.(CsI)Cs`] will produce comparable results.

Acknowledgement

Financial support for this work was provided by the National ScienceFoundation (grant CHE-9727474).

REFERENCES

1. M. G. Blain, S. Della-Negra, H. Joret, Y. Le Beyec and E. A.Schweikert, Phys.Rev. Lett . 63, 1625 (1989).

2. M. Benguerba, A. Brunelle, S. Della-Negra, J. Depauw, H.Joret, Y. Le Beyec, M. G. Blain, E. A. Schweikert, G. BenAssayag and P. Sudraud, Nucl . Instrum. Methods B 62, 8(1991).

3. A. D. Appelhans and J. E. Delmore, Anal . Chem. 61, 1087(1989).

4. G. S. Groenewold, J. E. Delmore, J. E. Olson, A. D. Appel-hans, J. C. Ingram and D. A. Dahl, Int . J . Mass Spectrom. IonProcesses 163, 185 (1997).

5. M. J. Van Stipdonk, R. D. Harris and E. A. Schweikert, RapidCommun.Mass Spectrom. 10, 198 (1996).

6. F. Ko� tter, E. Niehuis and A. Benninghoven, in Secondary IonMass Spectrometry , SIMS XI, edited by G. Gillen, R. Lareau,J. Bennett and F. Stevie, p. 459. Wiley, Chichester (1998).

7. B. Hagenhoff, P. L. Cobben, C. Bendel, E. Niehuis, A. Ben-ninghoven, in Secondary Ion Mass Spectrometry , SIMS XI,edited by G. Gillen, R. Lareau, J. Bennett and F. Stevie, p.585. Wiley, Chichester (1998).

8. R. D. English, M. J. Van Stipdonk, R. D. Harris and E. A. Sch-weikert, in Secondary Ion Mass Spectrometry , SIMS XI,edited by G. Gillen, R. Lareau, J. Bennett and F. Stevie, p.589. Wiley, Chichester (1998).

9. C. W. Diehnelt, M. J. Van Stipdonk, R. D. Harris and E. A.Schweikert, in Secondary Ion Mass Spectrometry , SIMS XI,edited by G. Gillen, R. Lareau, J. Bennett and F. Stevie, p.593. Wiley, Chichester (1998).

10. M. J. Van Stipdonk, R. D. Harris and E. A. Schweikert, RapidCommun.Mass Spectrom. 11, 1794 (1997).

11. D. R. Justes, C. Chusuei, M. J. Van Stipdonk, D. W. Goodmanand E. A. Schweikert, in Proceedings of the 46th ASMS Con-

ference on Mass Spectrometry and Allied Topics , Orlando, FL,1998, p. 1225.

12. M. J. Van Stipdonk, E. A. Schweikert and M. A. Park, J. MassSpectrom. 32, 1151 (1997).

13. J. A. Taylor and J. W. Rabalais, Surf . Sci . 29, 742 (1978).14. R. Z— aric� , B. Pearson, K. D. Krantzmann and B. J. Garrison, in

Secondary Ion Mass Spectrometry , SIMS XI, edited by G.Gillen, R. Lareau, J. Bennett and F. Stevie, p. 601. Wiley, Chi-chester (1998).

15. R. Z— aric� , B. Pearson, K. D. Krantzman and B. Garrison, Int . J .Mass Spectrom. Ion Processes 174, 155 (1998).

16. R. L. Betts, E. F. da Silveira and E. A. Schweikert, Int . J . MassSpectrom. Ion Processes 145, 9 (1995).

17. M. J. Van Stipdonk and E. A. Schweikert, Nucl . Instrum.Methods B 96, 530 (1995).

18. R. M. Papale� o, G. Brinkmalm, D. Fenyo� , J. Eriksson, H. F.Kammer, P. Demirev, P. Ha- kansson and B. U. R. Sundqvist,Nucl . Instrum.Methods B 91, 667 (1994).

19. R. M. Papale� o, P. A. Demirev, J. Eriksson, P. Ha- kansson andB. U. R. Sundqvist, Int . J . Mass Spectrom. Ion Processes 152,193 (1996).

20. C. Diehnelt, M. J. Van Stipdonk, E. A. Schweikert, in Pro-ceedings of the 46th ASMS Conference on Mass Spectrom-etry and Allied Topics , Orlando, FL, 1998, p. 1224.

21. J. E. Campana and B. I. Dunlap, Int . J . Mass Spectrom. IonProcesses 57, 103 (1984).

22. J. N. Coles, Surf . Sci . 55, 721 (1976).23. M. J. Van Stipdonk, D. R. Justes, C. W. Diehnelt and E. A.

Schweikert, in Proceedings of the 11th Annual SIMS Work-shop, p. 71. May 10–13, 1998, Austin, TX.


negative secondary ion emission from nabf4 : comparison of atomic and polyatomic projectiles at...

Documents