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A COMPARISON OF ELECTRONIC SPUTTERING INDUCED BY MOLECULAR AND ATOMIC IONS
A. Hedin, P. Håkansson, B. Sundqvist, R. Johnson
To cite this version:
A. Hedin, P. Håkansson, B. Sundqvist, R. Johnson. A COMPARISON OF ELECTRONIC SPUT-
TERING INDUCED BY MOLECULAR AND ATOMIC IONS. Journal de Physique Colloques, 1989,
50 (C2), pp.C2-15-C2-20. �10.1051/jphyscol:1989203�. �jpa-00229399�
A COMPARISON OF ELECTRONIC SPUTTERING INDUCED BY MOLECULAR AND ATOMIC IONS
A. HEDIN, P. H & A N S S O N , B.U.R. SUNDQVIST and R.E. JOHNSON*
Division of Ion Physics, Department of Radiation Sciences, University
?f Uppsala, Box 535, S-751 21 uppsala, Sweden
Department of Nuclear Engineering Physics, University of Virginia, Charlottesville, VA 22901, U.S.A.
RESUME
Oes ions m o l e c u l a i r e s
" c ~
ont Bte p r o d u i t s a l ' a c c e l e r a t e u r T a n d e m d ' u p p s a l a e t u t i l i s e s dans une experience de s p u t t e r i n g e l e c t r o n i q u e a f i n de cornparer l e s rendements de s p u t t e r i n g par ions atomiques e t ions m o l e c u l a i r e s a l a v i t e s s e de 0 , 4 9 cmlns. Les ions s e c o n d a i r e s p o s i t i f s ernis par des ions p r i m a i r e s rnol6culaires ont 6 t h e t u d i e s pour l a premiere f o i s . U n e i n t e r p r e t a t i o n p r e l i m i n a i r e indique que l e s " e f f e t s c o l l e c t i f s " sdnt f a i b l e s avec quelques e x c e p t i o n s . I1 e s t ddmontre Bgalernent q u e l e s conclusions que l o o n peut t i r e r de l ' e x p e r i e n c e dkpendent beaucoup des v a l e u r s de p e r t e d ' e n e r g l e B l e c t r o n i q u e u t i l i s e e s .ABSTRACT
Fast 12C2 molecular ions have been produced in the Uppsala tandem accelerator and used in an electronic sputtering experiment designed to compare sputtering yields produced by molecular and atomic primary ions of 0.49 cm/ns. Positive secondary ions of biomolecules sputtered with molecular primary ions are studied for the first time. The experiments show that "vicinage effects"
due to sputtering with molecular primary ions vary in strength with the secondary ion species. It is however also demonstrated that the conclusion of the experiment is strongly dependent on the reliability of electronic stopping power data used in the a n a l y s i s .
INTRODUCTION
As energetic ions with velocities above the Bohr velocity, vo ~ 0 . 2 1 8 cmlns, interact with matter, the predominant energy loss is to the target electrons, so called electronic stopping. In insulators, the primary electronic excitations and ionizations lead to dramatic secondary effects, one of which being the ejection of matter from the surface region; electronic sputtering. A fraction of the ejecta is ionized and this phenomenon has successfully been utilized in ion sources for mass spectrometry [I], where time-of-flight technique has been used in the mass analysis of sputtered secondary ions. In particular large organic molecules with weights up to 34000 amu have been analyzed with this method [2].
The nature of the mechanism of electronic sputtering, i.e. the process by which the primary energy deposited is converted to atomic and molecular motion like in sputtering is still debated. To elucidate this process, the secondary ion yield (the number of secondary ions of a certain species ejected per primary ion) has the last several years been measured as a function of a variety of primary ion parameters such as velocity, charge, angle of incidence and electronic stopping power ( d E / d X ) . A detailed list of these studies can be found in reference 3.
Concerning molecular primary ions, Thomas in 1985 used H$ clusters in sputtering experiments on CsI targets [4]. Recently, Salehpour et al. have used molecular .heavy primary ions in electronic sputtering
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989203
C2-16 JOURNAL DE PHYSIQUE
of biomolecules [5,6,7]. Molecular ions such as 12C2, 12C3, 12C,1H, and 12C19F3 were shown t o give large negative secondary ion yields of the amino acid valine.
In this paper we present data of positive secondary ion yields of biomolecule targets bombarded with 12C2. The main purpose of our experiment is to quantitatively compare electronic sputtering induced by molecular and atomic ions. In order t o do this, the secondary ion yield dependence on electronic stopping power of atomic ions is determined by bombarding with 12C, 160, 32S and 79Br primary ions of equal velocity. The secondary ion yield at bombardment with 12C2 of the same velocity is then determined and compared to the yield expected with twice the stopping power of l2C1. In this way the molecular ion yield data are "fitted" to the atomic ion data using the electronic stopping power dependence.
Furthermore, a general problem concerning molecular ions has been addressed: When determining desorption yields as functions of electronic stopping power the primary ions should be charge equilibri- ated as they hit the sample in order for the stopping power to be constant throughout the region which contributes to electronic sputtering, i.e. to depths of the order of 10'
A
[8]. For an atomic ion this is readily achieved by letting the ion pass through a thin foil (typically 40 pg/cm2 12C) prior to hitting the sample. This procedure would however cause break-up of a molecular ion. Furthermore, the lifetime of such ions produced in particle accelerators decreases rapidly with charge state and one can hence not solve the problem by choosing a charge state close to equilibrium. It has however been argued that charge equilibrium a t velocities above but close to vo (v=0.49 cm/ns in our case) is reached already at distances short compared t o typical desorption depths (see e.g. discussion by Salehpour et al. [7].) In order t o ver- ify this the charge state dependence of the sputtering yield is determined for 0.49 cm/ns 160 and 79Br ions.EXPERIMENTAL
Targets of the amino acid valine (MW 117 amu) were prepared by spin-depositing [9] a droplet of 8 pg/pl tri fluoro acetic acid solution of the target material onto a 130 pg/cm2 Al foil. CsI targets were prepared by evaporating 20 pg/cm2 CsI layers on 130 pg/cm2 A1 foils. Thin targets are essential so that the primary ion can be detected having penetrated the sample.
Primary ions were produced in a Cs sputter ion source and accelerated by the Uppsala E N tandem machine. In this way 1.5 MeV 12C+, 3 MeV
12c;,
2 MeV 160+, 1603+, 1604+, 4 MeV 32S2+ and 9.9 MeV 79Br4+, 7 9 ~ r 6 + , 7 9 ~ r 8 + were obtained, all of velocity 0.49 cm/ns. We have thus demonstrated that this is a useful way of producing molecular fast heavy ions of"c?.
Both gas and foil strippers could be used for the molecular ions. The positive and negative secondary ion yields are determined in a standard time-of-flight mass spectrometer[lo].
By keeping the velocity constant in the experiment, the energy density around the ion path is increased in going to a heavier ion without altering the dimensions of the tracks in which the energy is deposited.
This gives a more 'clear-cut' physical situation than if the stopping power is varied by changing the speed of a single ion species.
RESULTS AND DISCUSSION
All yield data to be discussed below are reproducible t o within 5% when different measurements on the same sample are compared.
A. Yield d e p e n d e n c e o n electronic s t o p p i n g p o w e r f o r a t o m i c ions.
In a discussion of yield dependencies on electronic stopping power, dE/dX, it is necessary to be aware of-the discrepancies between different stopping power data available. Fig. 1 shows a comparison of dE/
dX values from three different sources: tables by Northcliffe&Schilling (N.S.) [ll], the Lindhard-Scharff approximation (L.S.) (121 and a simple ZZfl-dependence using Z. = ~ ( 1 - e - ~ I ~ ~ ~ ~ ' ~ ) [13]. The data are normalized at Z=6, the lowest Z used in thls experiment. The absolute values are not interesting for our purposes.
As seen in Fig. 1, N.S. and L.S. differ significantly for both valine and CsI for the primary ions of interest. The L.S. approximation is good if v is well below v o ~ 2 / 3 which is questionable in our case where
~ ~ ~ ~ / ~ = 0 . 7 2 cm/ns for 12C and v=0.49 cm/ns. The validity of the N.S. data in this velocity regime has also been debated.
in
the N.S. values are equat for valine and CsZ targets.
Valine [M+H] + [M-COOH]+
[2M-tH]+
[M-HI - CN- CsI c s + CsINa+
csz1+
Cs212Na+
1-
Northcliffek Schilling
Q R
Lindhard- Scharff
a R
Table 1. The a- and R-values obtained with Northcliffeb Schilling and Lindhard-Scharff stopping powers.
C2-18 JOURNAL DE PHYSIQUE
In determining the yield dependencies on dE/dX we have made least-squares fits of expressions of the form Y=k(dE/dX)a to our data with k and a as free parameters. The results (a-values) for a multitude of secondary ions are given in Table 1 for both N.S. and L.S. The fit for valine [M-HI- using N.S. is given in Fig. 2. In general these expressions can be well fitted to the data. The linear behavior of the valine [M+H]+ ion ( a = l . l ) , using N.S. and the quadratic (a=1.9), using N.S. for the [M-HI- are in good agreement with the literature [3] for this and similar targets and for velocity ranges where stopping power theory is more established. The fast scaling of the ions from the CsI target (e.g. a=3.1 for Csf) and of CN- (a=3.4) from valine are also compatible with other measurements. The values obtained using L.S. data differ considerably from established results for other primary ion velocities. For this reason our discussion will be focussed on the results obtained with the Northcliffe&Schilling tables. We do this although there is not yet an established theory indicating that the scaling power is constant over a wide range of velocities.
In varying the charge state of 160 from 1 to 4 and that of 79Br from 4 to 8, the yield variations correspond, in the above fits, to variations in dE/dX of less than 5% for 160 and less than 7% for 79Br for all secondary ions. The equilibrium charge states of 160 and 79Br ions at 0.49 cm/ns are 3.4 and 6.6 respectively. These errors are well included in the f 10% error bars assigned to the stopping power values in Fig. 2.
B. Molecular primary ions
The yield of a certain secondary ion obtained with
12c;
primary ions corresponds t o a certain dE/dX- value in the fits described above. This dE/dX-value is expected to be close t o twice the stopping power of12cf
and the deviation is measured by the parameter R, following Brandt et al. [14],also given in Table 1. In the case of the valine positive molecular ion, we find R = . 9 l f 0.1 whereas for the negative molecular ion R=1.2f 0.1. Salehpour et al. found R d . 4 for the latter species when 12Cz and 12C1 yields were compared assuming a = 2 in the scaling [5,7]. We also find an R-value differing significantly from R = l for CN- (R=1.2f 0.1). The R-values of the L.S. data are in many cases significantly lower than one which gives us a further reason t o reject the L.S. approximation in this study.
An R-value higher than one in this experiment indicates that "vicinage effects" play a role in the energy loss of the molecular primary ion. Such vicinage effects have been found in energy loss experiments where R=1.4 was found for 'Hz in good agreement with theory [14]. In a picture first suggested by Bohr [15,16], the stopping power may be divided into parts due to close direct and distant resonant collisions with interaction ranges limited by rcl,,, = h/2mv and rdi,t = v/wo where m is the electron mass and wo is the plasmon resonance frequency of the medium. If these distances are compared to the internuclear separation ro=1.23
A
in the l2C; molecule it is found that rdiat is comparable to ro whereas rcl,,, << ro.This implies that the two nuclei partly act as a united charge in distant collisions giving rise to vicinage effects since the interaction depends roughly quadratically on the charge.
The energy from the soft distant collisions is deposited closer to the ion path than that from the close collisions. This implies that secondary ions requiring high energy for their formation and which thus originate close to the ion path, would obtain relatively higher R-values.
As the particle penetrates the medium, Coulomb break-up occurs and the inter-nuclear separation increases. This increase is however not significant compared to rdi,t over the sputtering depth d-100
A
[141.
We do not intend t o calculate the net result of the interplay between all these factors but note that the internuclear separation is comparable to rdi,t and that of the two secondary ions showing significant R-values, namely CN- and valine [M-H]-, in particular the former requires a high energy density for its formation.
In conclusion, we have demonstrpted that
12c$
molecular ions of MeV energies can be produced with a tandem accelerator. In an electror,~ sputtering experiment aiming at comparing atomic and molecular primary ions, it has been shown that the charge state dependence of the sputtering yield is weak at v-0.5 cm/ns for charge states coresponding to L-shell ionizations. Further, we measure vicinage effects which dif- fer significantly between secondary ions. The magnitude of these effects seem to be compatible with theory, a result which however depends strongly on the stopping power values used to analyze the data, illustrat-ported by the Swedish Natural Science Research Council.
Valine [M-HI
-Y=k(dE/dX)
dE/dX (arb. units)
Pig. 2. The valine [ M - 4 - yields of 1.5 MeV C l , 2 MeV 01,
4
MeV S1 and 9.9 MeV Brl as a function of dE/dX (N.S.). The [ M - 4 - yield of C2 corresponds to a (relative) dE/dX value of 2.4 giving an R-value of 1.2 in this case.REFERENCES
1. D F Torgerson, R P Skowronski and R D ~ a c f a r l a n e Biochem. Biophqls. Res. Comm. 60 (1974) 616 2. A C Craig,
A
EngstrZim, H Bennich and I Kamensky35:th Ann. Conf. of Mass Spectrometry an Allied Topics, Denver 1987, Book of Abstracts, p. 528
3. For a review, see
B Sundqvist in R Behrisch and K Wittmaack (Eds.)
Sputtering by Particle bombardment III, Springer Verlag, (in press 1988) 4. J P Thomas, P E Filpus-Luyckx, M Fallavier and E A Schweikert
Phys Rev Lett 55 (1985) 103
5. M Salehpour, D L Fishel and J E Hunt J Appl Phys 64 (1988) 831
6. M Salehpour, D L Fishel and J E Hunt Iat J Mass Spectrom Ion Proc 84 (1988) R7
C2-20 JOURNAL DE PHYSIQUE
7. M Salehpour, D L Fishel and J E Hunt Phys Rev B 1988, in press
8. G Sawe, P Htikansson, B U R Sundqvist and U Jiinsson Nucl Instr Meth B 2 6 (1987) 579
9. G Sawe, P Htikansson, B U R Sundqvist, U Jiinsson, G Olofsson and M Malmquist Anal Chem 5 9 (1987) 2059
10. B U R Sundqvist and R D Macfarlane Rev. Mass Speetrom. 4 (1985) 421 11. L C Northcliffe and R F Schilling
Nucl. Data Tables 7 (1970) 233 12. J Lindhard and M Scharff
Phys Rev 1 2 4 (1961) 128
13. W Brandt in S Datz, B R Appleton and C D Moak (Eds.) Atomic collisions in solids 1 Plenum (1975) 261
14. W Brandt, A Ratkowski and R Ritchie Phys Rev Lett 33 (1974) 1325
15. N Bohr
Kgl. Dan. Vidensk. Selsk., Mat.-Fys. Medd. 18 (1948) 16. J Lindhard and A Winther
Kgl. Dan. Vidensk. Selsk., Mat.-Fys. Medd. 34 (1964)
