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EVIDENCE OF POST FIELD IONIZATION AND OBSERVATION OF NOVEL FEATURES IN THE ENERGY DISTRIBUTION OF FIELD EVAPORATED IONS

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HAL Id: jpa-00225892

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EVIDENCE OF POST FIELD IONIZATION AND OBSERVATION OF NOVEL FEATURES IN THE ENERGY DISTRIBUTION OF FIELD EVAPORATED

IONS

T. Tsong

To cite this version:

T. Tsong. EVIDENCE OF POST FIELD IONIZATION AND OBSERVATION OF NOVEL FEA-

TURES IN THE ENERGY DISTRIBUTION OF FIELD EVAPORATED IONS. Journal de Physique

Colloques, 1986, 47 (C7), pp.C7-11-C7-16. �10.1051/jphyscol:1986703�. �jpa-00225892�

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EVIDENCE O F POST FIELD IONIZATION AND OBSERVATION O F NOVEL FEATURES I N T H E ENERGY DISTRIBUTION O F FIELD EVAPORATED IONS

T.T. TSONG

Physics Department, The Pennsylvania State University.

University Park, PA 16802, U.S.A.

Abstract

-

When field evaporation is carried out under the condition where more than one charge states coexist, the energy distribution of higher charge state ions exhibits a low energy tail similar to that of gas ions in field ionization.

Ions in the tail can only be produced by post field ionization. A double peak structure has been found in the energy distribution of M O ~ + where the low energy peak dominates at high field and the high energy peak dominates at low field, the origin of which is not yet understood. High temperature field evaporation produces M O ~ ~ + ; this has interesting implications to Coulomb explosion and theory of field evaporation. The stability of this ion is probably the result of an effective screening of the two positive charges by the electronic charges in the ion.

I. INTRODUCTION

In low temperature field evaporation, the most abundant ion species can be very well predicted from simple calculations using either the image hump model or the charge exchange model based on a single step electronic transition mechanism.' r 2 On the other hand, in ns high voltage pulse field evaporation at extremely high field, highly charged ions such as

w5+

and

w6+

can be observed which are not expected from these simple

calculation^.^

These highly charged ions have been explained to be produced by post field ionization, or a multistep electronic transition me~hanism.~

Recently a series of experiments and theoretical calculations all favor post field ionization as the mechanism of producing multiple charged ions in field evapora- tion.5 This apparent discrepancy can be reconciled by studying the ion energy dis- tributions in field evaporation taken under different conditions as will be

described below. These ion energy distributions are taken with our high resolution pulsed-laser ToF atom-probe which has an accuracy as well as resolution of 5 parts in lo5 in ion energy analysis.6

11. POST FIELD IONIZATION IN FIELD EVAPORATION

The energy distributions of pulsed-laser field evaporated metal ions, taken under the applied field and temperature where only one charge state dominates, have a FWHM of about 0.3 nF to 0.5 nF eV after the consideration of one to two bin spread which

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986703

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C7-12 JOURNAL DE PHYSIQUE

can be expect+ from any histogram plot; n is the charge state and F is the field strength in V/A. The distribution is nearly symmetrically shaped and there is little or no low energy tai;. The FWHM of the spatial zone wher: these ions are formed is only -0.3 to 0.5 A and the full width is less than 1.5A, comparable to the radius of the atom. The onset flight times agree with calculated values based on the critical energy deficit of

to better than (b or 1) ns where b is the bin width of the histogram in ns.

A

is the sublimation energy, Ii is the ith ionization energy of the metal atoms and is the average work function of the flight tube.7 As these ions are formed in such a narrow zone width, the concept of post field ionization is not entirely transpar- ent. In any case, the mechanism based on single step electronic transition can better explain the charge states and the field strengths needed in low temperature field evaporation.' r 2 Thus there is no need to invoke nor is there evidence in ion energy distribution to support the post ionization mechanism in such experimental conditions.

Evidenpe of post field ionization can be most clearly found in high field, between 5.0 V/A to 4.5 V/A, field evaporation of Ir where

-

60% of ions formed are 1r2+ and the rest are 1r3+. Fig. 1 (a) shows an ion energy distribution of 1r2+ taken at 7.5kV ine6x10-10 Torr vacuum. Th: bin width is 1 ns which corresponds to -0.87 eV or -0.09A. The FWHM iz only -0.5A and there is little low energy tail. The full width is less than 1.8A, only slightly larger than the radius of the atoms. These ions are unlikely produced by post field ionization. The 1r3+, on the other hand, shows a very long low energy $ail as seen in Fig. I(b) which is taken at 8.5kV.

The tail extends as far as 10A in space. The ions in the tail can only be produced by post field ionization since no neutral atoms can reach such distance above the surface. We believe this to be the most clear experimental evidence of post field ionization.

FLIGHT TIME (ns)

Fig. l ( a )

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11. DOUBLE PEAK STRUCTURES I N THE ENERGY DISTRIBUTION OF &-IONS

The i o p energy d i s t r i b u t i o n s of a l l t h e charge s t a t e s i n f i e l d evaporation of a l l t h e metals we have s t u d i e d s o f a r show s i n g l e peak s t r u c t u r e , sometimes with a low energy t a i l a s discussed i n t h e l a s t s e c t i o n . To our g r e a t s u r p r i s e Mo2+ i o n s show a very w e l l separated double peak s t r u c t u r e i n t h e E.D., and Mo3+ i o n s a l s o g i v e some i n d i c a t i o n of having a double peak s t r u c t u r e . Fig. 2 shows t h e ~ o ~ + ~ o r t i o n

FLIGHT TIME (ns)

Fig. 2

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C7-14 JOURNAL

DE

PHYSIQUE

of a mass spectrum of Mo taken between -4.8 to 4.3 v/L

.

Each of the mass lines exhibits clearly a double peak structure. Statistically more reliable ion energy distributions can be obtained by combining all the seven isotope lines together by proper shifts of their flight times according to their mass differences. Such ion E.D. are shown in Figs. 3 (a) and (b) for Mo2+ and M O ~ + . Both E.D. are obtained in vacuum in the middle l!-l0 T o n . In Fig. 3(a), the black area fs data collected between "4.7 to 4.8 V/A, thp shaded area between"4.5 to 4.7 V/A, and the non-shaded area between 4.3 to 4.5 V/A. It is interesting to note that at high field, the ions formed have lower energy which peaks at -25 eV below tbat expected from the critical energy deficit of the ions, or as far away as -2.6A from the critical distance of field evaporation xc. At low field the high energy peak, or the peak observed in normal field evaporation then dominates. We do not believe that the low energy peak is produced by post field ionization since no MO+ ions are observed within this field range. If MO+ ions are completely post field ionized, then one would expect the post ionization to occur closer to the surface at higher field than at lower field, in contradiction to the experimental observation. The E.D. of Mo3+

also show a doublepeak-like low energy tail structure. This tail disappears also at low field. We have also found that the onset energy of Mo3+ E.D. to be too large by almost 30 eV. Mo3+ ions are most abundant when Mo2+ ions formed are mostly in the low energy peak. At the present moment, we have no satisfactory explanation for these anomolous features.

F1. T i m e Diff. < n s > F1. T i m e D l f f . < n s >

Fig. 3

IV. FORMATION OF ~02~' IONS AND STABILITY OF DOUBLY CHARGED DIATOMIC CLUSTER IONS In the course of a study of ion E.D. in pulsed-laser stimulated field evaporation of Mo, we have observed M O * ~ + . As we believe this to be the first time stable or metastable doubly charged diatomic cluster ions of a metal have been observed and the stability of multiple charged cluster ions of smallest size is a subject of great current interest, we have carefully established the existence of this ion species. I n a pulsed-laser stimulated field evaporation experiment of a Mo tip, cooled down to 45K in $he middle 10-lo Torr, when the dc holding field is in the range of 4.8 to 3.5 V/A, the only ion species found are Mo2+ and Mo3+. As the tip is gradually blunted by the field evaporation and the laser power density is appro- priately increased to maintain a constant rate of field evaporation, a few M O ~ ~ + ions can be detected. A new ion species, presumably MO+, start to appear. Around

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Fig. 4

On a closer examination we find this ion species to be M O ~ ~ + rather than MO+ since there are 15 mass lines of mass-to-charge ratios ranging from 92 to lOOu separated each by 0.5u, with 92.5 and 99.5 u absent. These 15 mass lines are a result of a random combination of the 7 isotopes of Mo. This does not exclude MO+ ions since they will fall into 7 of these 15 lines. The fractional abundances of MO+ and can be found by a multinanial expansion coefficient analysis. If we represent the fractional abundances of the 7 Mo isotopes by a, b, c, d, e, f and g, then the fractional abundances of the 15 mass lines of stable ~ 0 2 ~ ' can be calculated. The calculated values and the experimental values are listed in Table 1. Fran the excellent agreement between these values, we conclude that few, if any, MO+ ions are formed.

Table 1. Fractional Abundances o f Ion Species i n Mo:+

M/n Abundance Th.-% Exp.-%

1

M/n Abundance Th.-% Exp.-%

There are at least two interesting implications of this finding. First, the present theory of field evaporation deals with the formation of atomic ions. Al- though some discussions of cluster ion formation have recently been presented,* no formation of doubly charged diatomic cluster ions prior to the formation of singly charged atomic ions has been anticipated. It is worthwhile to investigate theoret- ically the favorable conditions for forming cluster ions. Another implication is the critical sizes of doubly charged ions. Our observation shows that this number

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C7-16 JOURNAL

DE

PHYSIQUE

i s 2 f o r MO,~+ i o n s . Although t h e r e have been r e p o r t s of doubly charged diatomic c l u s t e r i o n s of non-metallic elements by a charge s t r i p p i n g method, no mass s p e c t r a of such i o n s have been published.9 Our spectrum is t h e only one a v a i l a b l e f o r doubly charged m e t a l d i a t o m i c c l u s t e r ions. The s t a b i l i t y of M O ~ ~ + i s r e f l e c t e d i n t h e l a c k of low energy t a i l s i n t h e i o n energy d i s t r i b u t i o n s . An a n a l y s i s shows t h a t t h e FWHM of t h e s e l i n e s a r e only 3 b i n widtheand t h e f u l l width i s aboyt 8 b i n s , o r a zone of i o n formation of FWHM of -0.4% and a f u l l width of -2.3A. T h i s f a c t shows t h a t ~ 0 h a s a l i f e time a t l e a s t s e v e r a l t i m e s l a r g e r t h a n 15ns, t h e ~ ~ '

f l i g h t time of t h e s e i o s i n t h e a c c e l e r a t i o n s e c t i o n of t h e atom-probe. A s t h e s e

Y

i o n s a r e produced a t t h e s u r f a c e a t a temperature of --1000K, we b e l i e v e t h e s e M O ~ ~ +

i o n s a r e completely s t a b l e . T h i s assumption, however, h a s y e t t o b e proven.

The b i n d i n g energy i n a n e u t r a l M02 c l u s t e r has been e s t i m a t e d t o b e about 4.2 e ~ . ~ I n d i s c u s s i n g t h e s t a b i l i t y of m u l t i p l y charged c l u s t e r i o n s , many people u s e t h e

o v e r - s i m p l i f i e d p i c t u r e of Coulomb r e p u l s i v e i n t e r a c t i o n of t h e s e p o s i t i v e charges w i t h o u t c o n s i d e r i n g t h e s c r e e n i n g e f f e c t of t h e atomic e l e c t r o n s . We w i l l show how erroneous such a n argument c a n be. Such a simple argument w i i l r e q u i r e t h $ two p o s i t i v e charge c e n t e r s t o b e s e p a r a t e d 2y more t h a n 13.6 eV.A/4.2eV = 3.2A i n M O ~ ~ + which i s much l a r g e r t h a n t h e 1.94A bond l e n g t h of Mo2 c l u s t e r s . 1 0 I n r e a l i t y t h e s e p o s i t i v e c h a r g e s w i l l b e v e r y e f f e c t l y s h i e l d e d by e l e c t r o n i c charges. L e t u s c o n s i d e r t h e p o t e n t i a l energy of t h r e e p o i n t c h a r g e s w i t h t h e c o n f i g u r a t i o n : two n u c l e a r c h a r g e s of +Ze each i s s e p a r a t e d by a d i s t a n c e r, and an e l e c t r o n i c c h a r g e of - ( 2 z - 2 ) e is l o c a t e d between t h e two n u c l e a r charges. The p o t e n t i a l energy i s

Assuming t h a t t h e k i n e t i c energy of t h e e l e c t r o n i c c h a r g e s i s one h a l f of U ( r ) a s i n a Bohr atom, t h e t o t a l energy of t h e system is t h e n

E ( r )

<

0 f o r any r i f Z

>

8/7. Thus t h i s v e r y simple s c r e e n i n g model p r e d i c t s t h e e x i s t e n c e of doubly charged d i a t o m i c c l u s t e r i o n s of any element e x c e p t hydrogen.

Of c o u r s e a more r e a l i s t i c quantum mechanical c a l c u l a t i o n i n c l u d i n g s c r e e n i n g e f f e c t w i l l be needed f o r p r e d i c t i n g t h e s t a b i l i t y of m u l t i p l e charged c l u s t e r ions. I t is i n t e r e s t i n g t o n o t e t h a t ~ e h a s been claimed t o have been d e t e c t e d although a ~ ~ + mass spectrum h a s n o t been o b t a i n e d . l l I n any c a s e Fig. 4 i s t h e f i r s t time a mass and energy spectrum of a doubly charged d i a t o m i c c l u s t e r i o n s p e c i e s of an element h a s been obtained.

REFERENCES

D. G. Brandon, S u r f a c e S c i . 3, 1 (1965).

T. T. Tsong, S u r f a c e S c i .

6

102 11968);

10,

221 (1978).

E. W. Muller and S. V. Krishnaswarny, Phys. Rev. L e t t .

x,

1011 (1976).

R. Haylock and D. R. Kingham, Phys. Rev. L e t t .

44,

1520 (1980); S u r f a c e S c i . 104, L194 ( 1 9 8 1 ) .

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N. E r n s t and Th. J e n t s c h , Phys. Rev.

el

6234 (1981); G. L. Kellogg, S u r f a c e S c i .

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319 (1982); M. Konishi, M. Wada and 0. Nishikawa, S u r f a c e S c i .

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T. T. Tsong, S. B. McLane and T. J. Kinkus, Rev. S c i . Instrum. 5 3 , 1442 (1982);

T. T. Tsong, Y. Liou and S. B. McLane, Rev. S c i . Instrum.

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T. T. Tsong, Phys. RW.

m,

4946 (1984).

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K. A. Gingerich, Curr. Top. i n Mater. S c i .

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J. B. Hopkins, P. R. R. Langridge-Smith, M. D. Morse and R. E. Smally, J.

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