Secondary ion mass spectrometry (SIMS) of metal halides. IV. The envelopes of secondary cluster ion distributions

Intemationa1 Journal of Mass Spectrometv and Ion Processes, 57 (1984) 103-123 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

SECONDARY ION MASS SPECTROMETRY (SIMS) OF METAL HALIDES. IV. THE ENVELOPES OF SECONDARY CLUSTER DISTRIBUTIONS

JOSEPH

E. CAMPANA

and BRETT

Chemistry

Division,

Naval Research

(Received

21 October 1983)

103

ION

I. DUNLAP

Laboratory,

Washington,

DC 20375 (U.S.A.)

ABSTRACT The relative secondary ion intensities of cluster ions of the type [M(MI),,]’ and [I(MI),]from the fast atom bombardment mass spectrometry (FABMS) of alkali iodides (MI) for n x 100 are presented, and mass-resolved mass spectra of cesium iodide (CsI) clusters exceeding m/z 25 000 are reported. The FABMS spectra are qualitatively consistent with previously reported secondary ion mass spectrometry (SIMS) spectra. The envelopes of the extended mass spectra are fitted to the distributions predicted by a model based on random bond breaking. The bond-breaking model (BBM) distributions fit well on the overall envelope of both the positive and negative ion sodium iodide (NaI, simple cubic) and CsI (body-centered cubic) cluster ion distributions, when Bethe lattices of the appropriate coordination number are used to approximate the true lattices. Ion abundance enhancements at certain “magic numbers” in the envelopes can be predicted by the BBM; however, these enhancements and the reduced ion abundance immediately following the “magic number” are consistent with experimentally observed unimoleeular, or more appropriately, unicluster decompositions. Finally, we contrast secondary cluster ion distributions with those obtained using other cluster sources and we discuss in detail possible origins of the large cluster ions observed in these studies.

INTRODUCTION

Until recently, reports of mass-resolved spectra above mass-to-charge ratio (m/z) 10000 were exceedingly rare. We recently reported [l-3] the production, mass analysis and detection of alkali iodide cluster ions exceeding m/z 18000 using secondary ion mass spectrometry (SIMS). The mass spectrometry of secondary ions [4] encompasses four promising techniques that differ in the identity of the primary particle, which is shown in parentheses. (1) Molecular secondary ion mass spectrometry (keV ions). (2) Fast atom bombardment mass spectrometry (keV neutral species). (3) Plasma desorption mass spectrometry (MeV particles). (4) Laser desorption mass spectrometry (photons). These techniques have great potential for the mass

104

spectrometric analysis of chemically and biologically important “ middle molecules” that are involatile and/or thermally labile [5]. Furthermore, these revolutionary experimental techniques brighten the prospects for fundamental studies of surface and gas-phase phenomena involving large molecules and clusters. This paper focuses on results obtained by molecular SIMS [6] ahd the more recent technique termed fast atom bombardment mass spectrometry (FABMS [7]). Our understanding of the secondary ion emission of atomic ions, based on ionization mechanisms [8,9], interatomic forces [IO], crystal structure [ll] and primary beam energy deposition in the surface region 1121, is quite detailed. However, our understanding of secondary polyatomic ion emission is much less satisfactory because of the historical rarity of experimental data and theoretical concern. This is partly due to the low probability of large cluster ion emission and the large number of internal degrees of freedom in any polyatomic ion. The low probability of large cluster ion emission makes experimental measurements [13] and computer simulation of experiments [lo] difficult because of poor statistics. The large number of internal degrees of freedom introduces questions concerning the nuclear geometry or conformation [14] and the rate of post-ejection unimolecular or unicluster decay [15-X3]. Studies of the alkali halides are being conducted [l-3,14-21] because of their simple electronic structure and their high secondary ion yield. A representative positive secondary ion cluster mass spectrum for [Cs(CsI),]+, n = 1-48, recently obtained by FABMS, is presented in Fig. 1 as a log-linear plot. (Exponentially decaying distributions are linear with negative slope when so plotted.) The focus of this work is to understand two qualitative features of this mass spectrum: the envelope of the secondary ion cluster distribution that deviates from linearity by being concave upwards and the anomalous ion abundance behavior (positive and negative deviations relative to the envelope) near n = 13 and 22. At this point, two techniques for cluster ion production are contrasted. These techniques are the sputter-emission methods, SIMS and FABMS; and the nucleation methods such as ion nucleation in high-pressure mass spectrometry [22] and neutral nucleation in supersonic molecular beams [23,24]. SIMS and FABMS yield both secondary ion and neutral clusters directly, and the secondary ions are subsequently analyzed by mass spectrometry. Thus, the measured ion abundances are a direct result of the particle (ion or neutral species) bombardment process. Molecular SIMS and FABMS spectra of various compounds are similar to spectra obtained by soft ionization methods such as chemical ionization mass spectrometry [25]; hence, bombardment-induced ion emission need not be considered a hard ionization process.

105

1

n-

I

I

393

5329

I

I

10525

12604

m/z+ Fig. 1. Positive ion FABMS spectrum of CsI obtained with 2 keV secondary ions in the double-focusing mode of operation. Anomalous ion abundance regions are observed at n =13-16 and n = 22-25. V,, = 2 kV.

Beam methods typically yield neutral clusters that are post-ionized by hard ionization methods such as electron ionization followed by mass spectrometric analysis. In this case, ion production is not inherent in the beam production process. The relationship between the neutral and ion distributions in the beam sources is uncertain due to variable post-ionization efficiencies and energetics; the latter leads to cluster ion rearrangement and fragmentation. The effect of post-ionization of neutral species by electron ionization from beam sources has been discussed elsewhere [26]. In summary, the envelopes of post-ionized water clusters were not affected by varying the ionizing electron energy in the 12-100 eV range, except for the total ion intensity. However, the relative abundance of the stable [H(H,O),,] + species, which has the clathrate structure, is observed to vary dramatically with ionizing energy below 20 eV [26]. Beam experiments on lead clusters show ion abundance variations with the ionizing electron energy 1241, particularly for small clusters. Furthermore, hydrogen cluster ion distributions produced by the expansion of ionized gas in a beam differ from those distributions produced using neutral gas followed by electron ionization [27]. Multiple sequential collisions play a greater role in the nucleation sources; hence, a closer approach to thermodynamic equilibrium is achieved depending on the source conditions. Consequently, the cluster ion distributions from the nucleation sources can vary dramatically with the experimental

106

source conditions that affect the number of particle collisions. For example, the envelopes from beam experiments are changed from pseudexponential (approximately exponential) to pseudogaussian (peaked at some cluster size) by increasing the stagnation pressure [28], which shifts the predominant seed-gas collisions from monomer/cluster to cluster/cluster. The secondary cluster ion distributions - are insensitve to experimental source conditions, and invariably they have a pseudexponential envelope. Anomalously high cluster ion abundances (relative to the envelope) are observed at certain cluster sizes or “magic numbers” [29] in all of the above cluster sources, and these “magic numbers” are the same for all the alkali halides [l-3,30]. However, the dramatic negative deviations immediately following the “magic numbers” have been observed only in our SIMS and FABMS studies on the alkali halides. (Different, more gradual positive and negative deviations from the envelope have been observed in beam experiments [31] on chemical species other than alkali halides.) In this study, the envelopes of the ultrahigh mass FABMS spectra of CsI and NaI positive and negative cluster ions are compared with the ion abundance distributions predicted by the conceptually simple (but non-trivial) bond-breaking model (BBM) [1,32]. We assume that the only important degrees of freedom during the sputtering process are the breaking (and possible reformation) of nearest-neighbor bonds. Furthermore, we assume that these processes can be described statistically; in particular, we will consider the consequences of assuming that each nearest-neighbor bond is broken with a uniform probability, fi. Although this BBM model ignores dynamic effects and effects due to internal energy, it encompasses two different approximations to the sputter emission process. The first is complete thermodynamic equilibrium in which the only active degrees of freedom are assumed to be the breaking and formation of bonds (nearest-neighbor) that occur with a relative probability, (1 + p)-’ = eeEjkT, where E is the energy gained by bond formation. The second picture is that the primary particle instantaneously shatters each bond in the infinite lattice with probability p (hence leaving each bond unbroken with probability 1 - 8). In this approximation, one views the crystal as an infinite collection of balls (atoms) connected by rods (bonds). Another ball (primary particle) impacts the crystal with sufficient energy and momentum to break a large number of rods (bonds). The rods break with probability fi, and the cluster distribution is determined by counting the occurrence of each entity (cluster) containing N balls, broken from the infinite crystal. The BBM differs from steady-state approaches because it is not necessary to consider a representative (highest binding energy) isomer for each N-atom cluster to determine the parameters (e.g. cross sections) entering into the

107

steady-state expressions for the detailed balancing of reaction rates. Although the name bond-breaking may seem to imply that the cluster ions necessarily originate by direct emission, equivalent distributions are obtained under the assumption of complete thermodynamical equilibrium (rates of association and dissociation are equal). This is because these two appoximations are identical in their treatment of the variation of the surface energy of a cluster with its shape. Neither approximation, in its simplest form, treats the selective processes following sputtering, which occur in the selvedge or gas-phase regions. We discuss how these selective processes account for certain features in the experimental ion abundance distributions. EXPERIMENTAL

The mass spectra were obtained with a VG Analytical Ltd., ZAB-HF mass spectrometer. This instrument is a double-focusing instrument of reverse geometry (magnetic sector precedes electrostatic sector)_ This reverse-geometry instrument allows operation of the mass spectrometer in the single-focus mode (low resolution), or double-focus mode (high resolution) where an ion detection system can be operated at the focal points of the magnetic sector or electrostatic sector. Each of the two detectors consists of a stainless-steel conversion dynode and a l7-stage Venetian blind type copper-beryllium electron multiplier placed opposite each other and perpendicular to the ion beam. The ion beam is postaccelerated to 5 keV into the conversion dynode and the secondary electrons emitted are accelerated into the first stage of the electron multiplier (operated at 2 kV, - 4 X lo3 gain). This dynode/multiplier arrangement allows transmission of the ion beam into the second sector when the voltages are absent from the first detector arrangement. The general features of the basic instrument are described in ref. 33. This instrument used a high-field magnet (2.3T) allowing an ultimate mass of approximately m/z 3250 to be analyzed at an 8 kV acceleration potential. (Mass range is inversely proportional to the acceleration potential). The instrument was fitted with a fast atom bombardment ion source (saddle-field gun; Ion Tech Ltd., Teddington, Middlesex, TW11 OLT, Gt. ,Britain [34]), which was operated at 10 kV and approximately 3.0 mA with xenon as the discharge gas. A primary ion deflector was placed at the exit of the saddle-field gun to assure that only fast neutral species would impinge on the sample. The primary atom beam angle of incidence was about 60° and the secondary ions were extracted at loo; both angles are relative to the sample normal. These non-standard bombardment/extraction angles were a result of physical limitations imposed by retrofitting the gun to a standard mass spectrometer. However, the angles are optimal in terms of cluster ion

108

extraction (determined by [K( KI)] + and [K(KI),]+ abundances) for the fixed atom beam angle, and an adjustable sample angle 1~ (Fig. 2). This 10” extraction angle is preferred over the larger angles typically used in other standard mass spectrometers retrofit for FABMS [35] because of the expected pseudocosine distribution of secondary ions [36,37]. Practically, this smaller extraction angle may permit better secondary ion collection efficiency due to the larger values of the cosine function and a more symmetrical secondary ion extraction field. The reagent grade alkali halides (CsI, Nal) were dissolved in water, the sample holder (NiCr) dipped in the aqueous solution, and the water evaporated in vacua. The salts were not placed in a liquid matrix (e.g. glycerol) as in the usual FABMS experiments [7]. Nevertheless, these experiments will still be referred to as FABMS (neutral bombardment) throughout, because, in general, FABMS only differs from SIMS (ion bombardment) by the charge state of the bombarding species. The mass spectra were collected at sufficiently slow scan speeds so that the signal-to-noise ratio was always greater than five, and the ion signal was detected with conventional analog electronics. The spectra were recorded VERTICAL SAMPLE

EXTRACTION ANGLE t AND SAMPLE ANGLE u (DEGREES) I

70

II 60 50

I 40

I 30

I

I

I

I

I

I

al

10

0

10

20

30

ANGLE OF INCIDENCE 8 [DEGREESI

Fig. 2. Relative ion abundance of KI cluster ions vs. the angle of incidence of the primary particle, the extraction angle and the sample angle. The angle of the primary beam relative to the vertical axis is fixed at 20” due to the retrofitting of the gun to a conventional mass spectrometer. Changing the sample angle (cw) relative to the vertical axis also changes the primary particle angle of incidence (8) and the extraction angle (E) measured relative to the sample normal.

109

with an oscillographic recorder, and the ion abundances were measured directly. The general features (envelope) of our FABMS spectra were similar to those obtained in our laboratory on another double-focusing SIMS instrument [l-3,19]. THE BBM AND,COMPARISON

WITH EXPERIMENT

BBM is conceptually simple for the alkali halides because all nearestneighbor bonds are equivalent in the bulk crystal. Nevertheless, the BBM cluster distributions are impossible to obtain exactly even for this simple system because it is impossible to enumerate all isomers of each N-atom cluster that result from shattering an infinite three-dimensional lattice [38]. Generally, for any number of atoms, N, a few compact isomers (having higher binding energy) and many more extended (lower binding energy) isomers are possible. The BBM can be solved exactly in one dimension, and the resultant cluster ion abundance distribution is the geometric distribution. Specifically, given some probability j3 that any bond in the linear chain of atoms is broken, and the probability 1 - /3 that it is not, the probability of finding an N-atom chain, Pi(p), is given by The

F,:(P) =

(1

- py-’

(1)

As in the experimental data presentation, the normalization is P,‘(p) = 1. A log-linear plot of this distribution is exactly linear with (negative) slope ln(1 - j3) for uniform p. One might argue that this uniform fl approximation is not appropriate for predicting secondary cluster ion distributions, because each primary particle impacts only a single point on the lattice; in other words, far removed from the impact zone, the probability of breaking any bond must virtually disappear because of the rapid decay of energy density with distance from the point of primary particle impact [39]. This effect can be modelled by a fi that decreases exponentially in distance from the impact zone. This exponential /3 model can be solved in one dimension, and the results are experimentally indistinguishable from those obtained in the uniform fi approximation (Eq. 1 [32]). (In the latter case, the only significant change is the meaning of 1 - j?.) Since these two distributions are experimentally indistinguishable, the spatial dependence of the bond-breaking probability J? is neglected in the following discussion. The uniform /3 BBM cannot be solved exactly in two or more dimensions. However, all the isomers of a cluster containing a small number of atoms, N, can be enumerated. Some resultant two-dimensional abundance distributions are presented in Figs. 3-5 for the honeycomb, square and triangular lattices.

1

2

4

CLUSTER

6 SIZE

I

I

I

1

I

2

8

I

I

4

CLUSTER

(N)

1

I

I

6

8

SIZE

(N)

Fig. 3. BBM distributions on a honeycomb lattice. Note the anomaly at N = 6 caused by the high binding energy (compact) hexagon. Fig. 4. BBM corresponding

distributions on the square lattice. to the square and the double square.

Note

the peaks at N = 4 ana

N = 6,

Note two features of these plots that are given for the small bond-breaking probabilities of O.Oi and 0.001. The first feature is that the envelope of these distributions deviates from linearity by being concave upwards. This is due to the extreme rapidity with which the number of isomers grow with increasing N. The second, which is most noticeable for small /!I, is the enhanced abundance at certain “magic numbers.” These are due to the existence of certain high binding energy (compact) clusters, that is, clusters having a high ratio of unbroken bonds to N. An abnormally high binding energy cluster for the honeycomb lattice exists in the six-membered hexagonal ring (N = 6), and the most stable clusters for the square lattice are the square ( N = 4) and double square ( N = 6). The presence of high-stability clusters at consecutive N values results in the observed breaks in the BBM distributions on the triangular lattice. Here, a break occurs at N = 2, caused by the N = 3 triangle; however, the intensity does not fall at N = 4 due to the compact rhombus (two triangles joined along a common edge). Similar compact trestle-like structures occur for N = 5-7. A break occurs at N = 6

111

0.

-40.

1

I 2 CLUSTER

6

4 SIZE

(N)

Fig. 5. BBM distributions on the triangular lattice. Note the breaks in the curve following iV = 2 corresponding to the triangle (N = 3) and rhombus (N = 4) and following N = 6 corresponding to a hexagon with a central atom (IV = 7). Fig. 6. A portion of the infinite Bethe lattice of connectivity u + 1= 3, which approximates the honeycomb lattice.

u = 2 or of coordination

number

because the N = 7 cluster has a second even more compact structure, a hexagon with a central atom. The total number of N-atom isomers in three dimensions can be determined for a few small N values, but this number grows rapidly [40]. Thus, in the related percolation problem, Monte-Carlo methods are usually used when three-dimensional lattices are considered 1411. Alternately, one finds that the analytically solvable Bethe lattices can provide an approximate solution of the three-dimensional BBM. Bethe lattices can have only two nodes connected by one path. Figure 6 shows a small portion of the infinite Bethe lattice of connectivity u = 2 or coordination number u + 1 = 3, where each atom is bonded to three other atoms. This Bethe lattice of u = 2 approximates the honeycomb lattice and is appropriate when considering a single basal plane of graphite. Bethe lattices do not contain closed loops (compact clusters); therefore, Fig. 6 depicts two atoms near points A, B and C. instead of one as in the honeycomb lattice.

112

-a-

l-

10

20

CLUSTER

30

40

SlZE (NJ

I

10

I

,

20 CLUSTER

I

I

I

I

30

I

40

SIZE

(N)

Fig. 7. BBM distributions for a Bethe lattice u = 2. Fig. 8. BBM approximating

distributions for a Bethe lattice u = 5 of coordination the three-dimensional cubic lattice.

The cluster distributions expressed [42] as

cm)

(u + l)(Nu)!

= N![N(u-

l)+l]!

number

for Bethe lattices of all connectivities,

(1 _ p)~~1~w-l)(u-l1)

u + 1 = 6,

u, can be

(2)

This equation reduces to Eq. (1) when u = 1. There exists a critical bondbreaking probability &[O < & = (u - l)/ u -C l] for all Bethe lattices, where all large clusters become equally probable. The envelope for any other value of /3 falls off more rapidly with increasing value of IV than for PC_Figures 7 and 8 show envelopes for & and another p value for the u = 2 lattice (approximation to the honeycomb lattice) and for the u = 5 lattice (approximation to the three-dimensional cubic lattice). Two characteristics of these Bethe lattice cluster distributions are emphasized. The first is the lack of “magic numbers”, resulting from the inherent lack of closed structures in the Bethe lattices. The second is that the Bethe lattice cluster envelopes deviate from linearity by being concave

I13

-\

I

I

I

I

10

I

20

I1

I

30

40

I

I‘I

I

50

60

Fig. 9. NaI positive ion abundance distribution fitted with the BBM distribution for a Bethe lattice (I = 5, corresponding to coordination number 0 + 1 = 6. The experimental data were obtained with 3 keV secondary ions in the double-focusing mode. The BBM distribution was fitted with a bond-breaking probability of p = 0.711; the critical bond-breaking probability is /3, = 0.8. - - - - - -, Theoretical; -, experimental data.

II

11

10

20

I

I

30

II

40

11

50

1

I

60

n+ Fig, 10. NaI negative ion abundance distribution fitted with the BBM distribution as in Fig. 9. The best fit yields a bond-breaking probability of /3 = 0.725. Is, = 0.8; - - - - - -, theoretical; experimental data. -3

114

0

-5

-6

I

I

1

20

I

I

40

I

I

60

I

I

J

100

60

n+ Fig. 11. CsI positiveion abundance distribution fitted with the BBM distribution for a Bethe lattice u = 7 corresponding to coordination number u + 1 = 8. The experimental data were obtained with 1 keV secondary ions in the single-focusing mode. The best fit uses the critical bond-breaking probability & = 0.857. - - - - - -, Theoretical; -, experimental data.

upwards. This latter effect is believed dimensions greater than one. We have fitted the envelopes of the cluster ion distributions of NaI (cubic) the B3M model on Bethe lattices (Figs.

-6'

I

I

20

I

I

I

I

60

40

I

I

60

I

to be characteristic

of the BBM in

positive and negative ion secondary and CsI (body-centered cubic) using 9-12) of the coordination numbers

I

loo

n'

Fig. 12. CsI negative ion abundance distribution fitted with the BBM distribution 11. The experimental and theoretical parameters are the same as in Fig. 11.

as in Fig.

115

(u + 1) six and eight. The positive ion spectrum of NaI is fitted best with fl= 0.711, and the NaI negative ion spectrum gives the best fit with p = 0.725. These bond-breaking probabilities are slightly less than the critical bond-breaking probability, j3, = 0.8; thus, these envelopes have almost the largest curvature allowed in the BBM. The positive ion spectrum of CsI gives the best fit with p = /3,= 0.857. The CsI negative ion spectrum has an envelope slightly different from the others. Although we cannot explain this somewhat minor difference, the distribution is shown for j3,.

DISCUSSION

These FABMS results show some interesting trends when compared with previous results [l--3] using a SIMS instrument. The characteristic features of the alkali halide secondary cluster ion intensity distributions previously reported [l-3] are listed. (1) The secondary cluster ion distributions decrease pseudexponentially. (2) The ion abundance of the n = 13 species is more intense than the n = 12 species, and the ion abundance of the n = 14 and 15 species are dramatically absent or of decreased abundance relative to the n = 13 and 16 species. (3) The latter characteristic apparently repeats itself at n = 22-24, n = 37-39, and n = 62-64. These features have been observed in the SIMS spectra of all the alkali halides [19]; however, only the highly emissive (high secondary ion yields) alkali iodides gave adequate ion signals to be recorded beyond about n = 22. The ion abundance enhancements at n = 13, n = 22, n = 37, and n = 62 correspond to particularly stable “cubic-like” atomic arrangements of 3 x 3 X 3, 3 x 3 x 5, 3 X 5 X 5, and 5 X 5 X 5, respectively, where the numbers refer to the number of atomic species on the edge of a cube (NaI) or rhombohedron (CsI). The ion abundance of the n = 4 species (1 x 3 x 3) is enhanced for certain alkali iodides [19]. (The n values where these enhancements occur are referred to as “magic numbers”.) The magnitude of the ion abundance anomalies occurring in the CsI spectra over the regions n = 13-16, n = 22-25, n = 37-40, and n = 62-67 are decreased or absent in the secondry cluster ion distributions reported here. This difference is related to the different mass analyzer experimental conditions (shorter ion flight time from ion formation to detection); it is not a result of neutral bombardment (FABMS) [17]. This phenomenon, a result of unicluster decay at rates comparable to the ion flight time, has been briefly discussed [15-171, and it has been observed and studied independently by time-of-flight mass spectrometry [18]. The time-of-flight data gives the effective CsI secondary cluster ion distribition within 200 ns after the emission process, and this distribution is identical (within experimental error) to our double-focusing

116

SIMS results apart from the absence of “magic numbers” in the time-of-flight data. The double-focus secondary cluster ion distribution results for [Cs(CsI), j+ (Fig. 1) fall off slightly more rapidly than the single-focus secondary cluster ion distribution (Fig. Il), due to u&luster decay during the different flight times (different ion acceleration voltages and ion flight paths) [D-17]. Consequently, the fits for the double-focus experiments yield a somewhat smaller 8. This ion transit time factor may explain why the NaI (double-focus) data give a better fit with a /3 smaller than &. Furthermore, we observe experimentally that the anomalous regions begin to diminish as the ion transit time becomes shorter. We believe from these observations that as the ion analysis time approaches zero, the experimental distribution approaches the instantaneous distribution given by the BBM. The somewhat smaller /3 (relative to &) used in some of these fits can be attributed to unicluster decay phenomena and experimental error. The p values used do, however, allow almost the maximum curvature allowable with fi = &. (There is no a priori reason to expect p = &.) Implicit in these fits is the absence of ion detector mass-discrimination effects. The detector ion-to-electron conversion’ efficiencies would be expected to affect the measured cluster ion abundances. The kinetic and potential emission models for secondary electron emission are inappropriate because the cluster ions are moving at velocities below the threshold for kinetic emission (5.5 x lo4 m s-l), and the ionization energies of the cluster ions are presumably too low relative to the work functions of the dynodes (stainless steel, copper-beryllium, and channeltron-lead oxide), used in these and our other studies, to allow Auger processes [2,37]. It is most likely that these large ionic clusters are detected via electron emission following decay of long-lived electronic excitations in the clusters (which are prevalent and of high energy in the alkali halides), or through their break-up upon impact with the detector 121. The low-mass NaI cluster ion envelopes fall off more rapidly than the high-mass CsI cluster ion envelopes, which indicates a true chemical effect. This observation precludes detector discrimination effects as being the dominant cause for the shape of the envelopes. The alkali halides may not show discrimination effects due to an unusual mechanism of secondary electron emission. While the precise values of p obtained in these fits are somewhat uncertain, we believe that the quality of the fits suggests that the approximations of the BBM are not unreasonable. We have also obtained for the first time high-mass negative ion secondary cluster ion distributions of CsI and NaI for n > 4. The negative ion CsI spectrum gives similar anomalous ion abundance regions (n = 13-16 and n = 22-25) as the positive ion spectra of other alkali halides [l-3,19];

117

however, the NaI negative ion spectra do not show strongly anomalous ion intensity regions. This may indicate that negative ion NaI clusters at n = 14 and 15 are less likely to decay to more compact (stable) clusters or, in other words, that their rates of unicluster decay vary less dramatically with n than the corresponding positive ion rates. As previously discussed, the BBM distribution could be fitted to the NaI negative cluster ion abundance distribution; however, the negative ion CsI data did not fit as perfectly. The decrease in the ion abundance following the extremely stable n values can be rationalized by consideration of cluster surface energies [I] of the stable species, for example, n = 13, and the subsequent species in the series resulting from the addition of one or more salt molecules. The two cluster species immediately following the stable one would be expected to have high surface energy regions as a result of incomplete cubic faces. However, when the third molecule is added, the relative surface energy of the cluster will decrease because a face of the three atomic species edge cluster is 2/3 filled forming a step on the cube. This is not true for the 5 x 5 x 5 “cubic-like” structure that requires five molecules to be added to form a step on one of the faces. This can be seen from the ion abundance variation at n = 62-66 in previously reported data [1,3]. This consideration supports our belief that “cubic-like” rather than other isomers, for example spherical, dominate at each value of n. COMPARISON

WITH BEAM RESULTS

The following is a detailed comparison of secondary cluster ion distributions with beam distributions on the alkali halides [24,30]. The envelopes from the two types of experiments are quite different; nevertheless, all the alkali halide cluster ion abundance distributions show strong ion abundance enhancements at the “magic numbers,” corresponding to the “cubic-like” structures, previously discussed. The first “magic number” observed in both experiments is n = 4, corresponding to the 1 X 3 X 3 planar cluster structure. The ion abundance enhancement of this cluster is correlated with anion size (the sodium halide series) or the interatomic distance (all alkali halides) of the crystal [19]. The relative ion abundance of the n = 4 cluster increases with decreasing interatomic distance in both types of experiments [19,24]. Intermediate to the “cubic-like” ion abundance enhancements, other less abundant “ magic numbers” are observed in the beam results. These intermediate “magic numbers” correspond to the low surface energy clusters formed by the addition of three alkali halide molecules to the “cubic-like” structure; i.e. the series n = 4, 7, 10, 13, 16, 19, 22,. . ., where the underlined ” magic numbers” (corresponding tothe “cubicle” struture) are observed in our study. Stacked rings of alkali halides have been shown to be a stable

118

form of neutral cluster [14], and such cationized stacked hexagonal ring structures have been attributed to “magic numbers” at 6, 9, 12,. . . observed in our experiments on CsI (2, 3, 19). The intermediate “magic numbers” in the beam results have been interpreted to result from the decay of ionized stacked ring structures to compact “cubic-like” structures. For example (MX),i

+ e + (MX):

* + [M(MX),,_,]

+ + MX,

where r’is a positive integer. The intermediate “magic numbers,” 7 (1 x 3 x 5) and 10 (1 x 3 X 7), have been observed in our SIMS experiments for a few sodium halides [19], and, as with the n = 4 species, they are strongly dependent on ionic radii effects. We do not .believe that~ these stable “cubic-like” species arise from the decay of the ring structures in the sputter emission methods as could be the case in the beam (nucleation) studies. We believe these intermediate “cubic-like” cluster ions are an inherent result of bombardment-induced emission from the crystalline cubic lattice. ORIGINS

OF SECONDARY

CLUSTER

IONS

In view of the new secondary cluster ion distributions that indicate significant unicluster decay [15-181, it is enlightening to reexamine two extreme views of the origin of the large cluster ions observed in this study. The cluster ion abundance distribution can arise from either of two two-step mechanisms. The first mechanism involves direct emission. (i) Direct emission of both stable and unstable species yields a cluster ion abundance distribution that is pseudexponential, and has no anomalies. This distribution, possibly with enhancements, can be predicted by the BBM and has been observed experimentally [18]. (ii) The unstable or metastable species (high surface energy or low binding energy cluster ions) decay by unicluster mechanisms to form stable species (low surface energy or high binding energy cluster ions), for example n = 14,15 + n = 13. The preference for unicluster decay to compact clusters is due to a greater density of such final states. This results in the experimentally observed anomalous ion abundance regions reported in our experiments. The second mechanism involves recombination in the selvedge region [43]. (i) The emission of small species followed by their recombination yields larger unstable and stable species. The corresponding secondary cluster ion abundance distribution is also pseudexponential but should be derivable from a growth statistic, neglecting u&luster decay [44]. (ii) This step is the same as the second step of the first mechanism. First, consider general evidence for recombination. The most compelling theoretical evidence comes from molecular dynamics (MD) calculations [lo].

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It has been shown through simulation studies that, when crystalline metals are bombarded, small clusters can be formed that are primarily composed of atomic species from non-contiguous sites. Because of the relative rarity of large cluster emission in these simulations, the evidence is statistically most significant for dimers and trimers; however, a computer simulation of an event resulting in the ejection of a 25atom argon cluster has been reported [45]. Experimental evidence for recombination comes from the SIMS of ternary salts [46]. Some secondary ions from the ion bombardment of NaBF, were observed at m/z values that can correspond only to rearrangement of lattice constituents. While the intensity of these ion signals were later shown to increase with bombardment-induced damage, they are still present, although to a minor degree in the static SIMS of freshly pressed powder samples [47]. Second, consider general evidence in favor of direct emission. It is well-established that very large organic molecules, weakly bound to surfaces, can be ejected intact even when energetic (MeV) primary particles are used as the bombarding species [48]. The MD procedure has been used to model the desorption of organic molecules from metal surfaces due to particle bombardment and considerable molecular desorption is observed [49]. A shearing or “peeling-off” of contiguous atoms in potassium iodide has been observed in MD studies [50]. Such emission events (peeling or shearing) become more prevalent as the angle of incidence of the primary particle beam is increased [49,50]. As is typical in SIMS, a large angle of incidence (60°) was used in our SIMS and FABMS experiments such that a high degree of peeling or shearing might be expected. Other theoretical evidence favoring direct emission suggests that the growth of negative ions by recombination would be impeded by electron autodetachment [51]. This decay mode differentiates between negative and positive ion clusters; therefore, if recombination dominated, one would expect different positive and negative ion intensity distributions, which are not observed (within the experimental error) in the present studies on alkali halides. (Although electron autodetachment is less probable in alkali halides than in covalent materials, it is energetically allowed and will occur after some negative ion recombinations.) Experimentally, SIMS positive ion spectra of complex solids such as catalysts [52] and mica [53] show that a majority of identifiable secondary ions can be correlated to urn-eat-ranged fragments of the original lattice. Ternary salts with covalently bonded anions, for example, [ClO,]- and [SO,]-, show intact anion emission in their SIMS spectra 1541. The SIMS spectra of frozen molecular solids indicate that cluster ions of unrearranged components are emitted from binary mixtures of organic [55] and inorganic [56] molecules. For example, a mixture of CHi60H and Hi80 does not show incorporation of 180 in the methanol clusters.[55], and the secondary cluster ion spectrum

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of a 1: 1 solid mixture of N, and 0, is different from the spectrum of solid NO [%I. The question of direct emission vs recombination becomes moot when emission of large clusters from alkali halides is considered. Simple considerations of interaction ranges, ejection times, and the fact that the distributions are invariant over six orders of magnitude of primary ion flux [l--3,18] show that each primary impact can be considered to be independent. Therefore, it is hard to: imagine how a cluster containing approximately 200 atomic species could grow completely by recombination of quasi-isolated atomic, dimeric or trimeric species during the emission process. The direct emission of preformed species weakly bound to the surface (analogous to organic molecules physisorbed on metal [39]) would not require a high local energy density. Such preformed species could arise from radiation damage (bombardment-induced) or sample preparation effects. If the emission of preformed precursors to the large cluster ions is the dominant large cluster source, then we would expect to see time-dependent and sample preparation effects. Such effects have not been observed in our studies 121, although single crystals have not been studied due to experimental difficulties [57]. Alternatively, the ejection of a compact cluster from a perfect crystal face (direct emission) can occur, but it involves the breaking of a large number of bonds. Furthermore, the finite energy of the primary particle alone limits the size of clusters that can be ejected [SS]; thus “chunk” sputtering (micron-sized particles) phenomenon can only occur using the energy stored in defects and surface topography. Time-dependent (radiation dose) effects [59] and surface preparation effects (60) have been observed for “chunk” sputtering. However, the primary particle could supply the energy necessary to eject < 0.1 pm [58]. The even smaller clusters observed in our “minichunks”, studies can be ejected from perfect crystals in a single event. Thus, we do not believe that the secondary cluster ion distributions from single-crystal alkali halides would deviate significantly from our results until much larger clusters or “chunks” are considered. In any event, experimental and theoretical determination of the relative roles of direct emission and recombination in SIMS and FABMS remains open. CONCLUSIONS

We have qualitatively interpreted the secondary cluster ion distributions of alkali halides using the BBM. The envelopes of the experimental ion abundance distributions are almost linear on a log-linear plot with the deviation from linearity being concave upwards. This shape, unvoidable in the BBM, is due to the rapid increase in the number of different configura-

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tions that N atoms can assume with increasing N. The BBM can partially explain anomolous ion abundance variations near, for example, n = 13. The enhanced ion abundance at n = 13 is due to the occurrence of. the compact 3 x 3 x 3 (N = 27) “cubic-like” structure [l-3]. However, the low ion abundances immediately following n = 13 cannot be explained by a BBM. These are due to the preferential unicluster decay of the larger clusters to the more stable cubic cluster [15-181, also contributing to the enhancement of, n = 13, after the cluster decay. The results reported in this paper also indicate that state-of-the-art mass spectrometers can produce, mass analyze, and detect ultra-high mass ions [17]. This implies that the current limitations of middle-molecule mass spectrometry [5] may be in the production of high mass ions rather than their mass analysis and detection. ACKNOWLEDGEMENTS

Many thanks to Robert H. Bateman and Brian N. Green of V.G. Analytical Ltd. for their assistance in obtaining the FABMS mass spectra [17] and their permission to use the sample angle vs. ion intensity data. We are also indebted to our colleagues at the Naval Research Laboratory, Richard J. Colton and Jeffrey R. Wyatt for extensive discussions. We thank Professor G. St6ckli.n (KFA, Julich) for discussions concerning nuclear sputtering phenomena. REFERENCES

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