Trapped holes in magic number ion clusters

Volume

113. number

4

CHEMICAL PHYSICS LITIWIERS

TRAPPED HOLES IN MAGIC NUMBER ION CLUSTERS AJ. STACE

Received 6 October 1984

Observations on the relative intensities of Arn.I* and Ar,-I*, clusters suggest that stable mn cluster configurations w from the presence of low ionization potential sites which trap the post&z charge

to the present discussion, the most refevant ealeulaExperimental evidence for the existence of %mgic’* number atom configurations in xenon clusters was

first reported by Sattler et al. [I]. Subsequently a number of atomic and molecular examples have been observed f24], and irr each case the evldenco has c5me from intensity variations present in the mass spectrum of an ionized cfuster beam. J.n&&y rt was suggested that the presence of Iugh intensity ions reflected the existence of particufarIy stabIe neutral clusters. However, there is consrderable support for the proposal that ion clusters undergo quite extensive fragmentation as the result of electron impact mnrzation [S-g], and it is now thought very unlikely that an intensity fluctuation present in a neutral cluster beam could survive to appear in an ion cluster mass spectrum [9,1 O] In the case of the Arfg “magrc” number configuration, the ion cluster fragmentation pattern has actually been used to confirm *&at it 1s a stable ion rather than a stable neutral cluster which is responsible for the features present in the mass spectntm of argon clusters [6]_ Most of the observed “magic” number patterns appear to be consistent witlr the formatron of icosahedron-like structures. Theoretical evidence for the existence of “magic’* number atom configurations has come mainly in the form of mole&r dynamic and Monte Carlo simulations on clusters of (neutral) atoms bound by Lr?n.nardJones or similar pair potentials [l&12]. By comparison, there have been few detailed theoretical studies on the structures of large ion flusters. With reference

0 009-26 14/&X,&03.30 0 Elsevier Science Publishers B-V_ (North-Holland Physics Pubhsbing Division)

tions are those by Etters et al- [l&14] on the lowtemperature properties ofArm K+ clusters. Of partrcular interest was therr abservation that for ArrZ-K* the most stable structure is an icosahedron with the K’ at its centre. At a more qualitative level, Haberland f9] has discussed possible structures for Arrs, and concluded that the comparairvely stabfe dimer ion, Ar; @, = 1.33 ev), may act as a nucfeation centre around which an addifsonal 17 argon atoms could form a double icosahedron. What both the latter studies have in common is that the central ion in each structure, either K” or A$, traps the positive charge by virtue of the fact that it has a lower ionization potential than the surrounding atoms. It woufd appear that the presence of a trapped hole or posztivc charge creates a sin@e stable structure, as opposed to a mobde hole which could lead to a very large number of degenerate or near-degenerate configurations. In this paper we wish to develop this Idea by riiscussmg the results from some experiments where we have introduced a low ionization potential site into an ion cluster with the intention of trapptig the hole or positive charge, The two ion cluster systems we have studied are Aru -I* and Ar,, -I$_ We shalf show that because the ionization potentials of both I and I2 (IO.@ and 9.31 eV, respectively) are much lower than those of either an argon atorn (15.79 ev) or an argon cluster (14.25 eV [ 15]), their presence results in the observation of atom/ion combinat.rous which correspond to the formation of particularly stable chrster configurations.

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CHEMICAL PHYSIC3 LET’TERS

Volume 113. number 4 2. Experimental

Neutral clusters were generated by the adiabatic expansion of a gas nuxture through a pulsed nozzle operatmg at approximately 20 Hz Following collimation through a OS mm diameter skimmer positioned 2 cm from the nozzle, the modulated cluster beam was ionized by electron Impact and mass analysed on a modified AEI MS 12 mass spectrometer_ As in previous expenments

involving the formation

of mixed

ion clusters [ 163, it was found necessary

to maintain the rodlne concentratron below 100 ppm; any higher than this and the mass spectrum became dominated by iodine clusters. The iodine vapour was introduced into a stream of argon through a heated inlet. Ail the results presented were obtained using an electron impact energy of 70 eV and an ion source potential of 6 kV.

3. Results and discussion

By carefully adJusting the iodine/argon

ratio it is possible to produce a mass spectrum which contains

25 Januari 1985

Ar~,Arn-I‘CandAr,-I.~ ~l~terso~y_Suchaspect~m is shown in fig. 1. Given that it requires approxirnateIy 2.7 eV of energy to break the 1; bond, the presence of Am -I’ clusters in the mass spectrum is quite unexpected. However, this reaction is one of a series of ion cluster fragmentation processes wluch have been observed recently, and a detailed account of the mechanism responsrble for thus observation has been given elsewhere [I 6]_ In this paper we shall concentrate on

the presence of intensity fluctuations

and discuss their

significance in terms of the formation of stable Ion clusters_ The ions in fig. 1 which have intensities that are very much higher than their nelghbours are. Ar12 I+, ArL8 -If and Ar17 - p;2 other moderately intense ions are: Ar2g-I$ and Ar24-I$. The discussion here will be mainly concerned with the first group_ To begin with it should be noted that the total numbers of atoms involved in this group are either 13 or 19, which would suggest that they are members of a “magic” number sequence [ 121. Starting with the 1 g-atom combina-

tion Ar17 -G; th ere is an obvious snmlarity between this ion and the Arl, -Ari configuratron proposed for the Arfg cluster [9]. The most appropriate structure

Fig. 1. Maa spectrumof the At:, eC P and AI,-K$ clusters.PC1AC, -I+;(+) Ar,-q:; selectedvaluesof n arealso given.Each arrow indicatesa changein recorder senstmty

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CHEMICAL PHYSICS LE’ITERS

for either cluster is a double icosahedron with the central site being occupied by the dimer ion. In both cases, stability of the latter would serve to trap the positive charge. It is interesting to note that our experiment provides no evidence to suggest that the combination Arll - I+2 IS particularly stable. However, it is not possible for the 13-atom lcosahedron to accommodate a stable dimer without signi~can~ distortion [Q]; thrs would aIso account for the absence of a stable ArTB cluster because formation of the dimer ion Arz appears to be of pnme importance following ionizat1on [9,10]. The stable 13-atom combination we do observe is Ar12-I+, wluch suggests a central positive charge localized on the iodine atom and surrounded by a shell of argon atoms. There is obviously a strong similarity between Ar,-I’ and the Ar,-K+ series studied by Etters et al. [13,14], and although their results favour the formation of an icosahedron it should also be recognized that both facecentred-cubic and hexagonalclose-packed configurations could be suitable as structures for Arl,-I*. In the case of the 19-atom combination A=,,-I+, th e presence of a central atomic ion reduced the choice of stable configurations. The double icosahedron is no longer appropriate which leaves either a face-centred-cublc or a hexagonai-close-packed arrangement. Because the most stable 19.atom close packed structure arises from the addition of six atoms to the square faces of the 13-atom structure [17], we would suggest that ArIB -I? is face-centred-cubic rather than hexagonal-close-packed because the former configuration allows for a larger spatial displacement between each of the square faces

25 Januari 1985

tion of tlus fact allows us to propose some quatitative conditions regarding the atomic or molecular configuration of smd stable ion clusters: if the ten tral nucleating species is atomic 3r approximately spherrcal, i.e. S<, we would expect the observed “magic” number sequence to begin at 13, with the 19-member ion cluster hating octahedral symmetry; if a dlmer ion is the nucleating species then we would expect the sequence to begin at 19, i.e. Ar&, and this ion cluster will most probably have icosahedral symmetry. Given these two conditions, it is interesting to note that xenon is the only inert gas to show evidence of having a particularly stable 13-atom ion cluster, and of all the inert gas dlmers, Xez is the least stable. We have restricted these conditions to snzall ion clusters for two reasons. First, the mfluence of the positive charge can be expected to decline quite rapidly beyond the first atomic or molecular shell, and the Interactions between outer neutrals in large ion clusters may force a change of symmetry. Secondly, members of the icosahedral coordination group do not transform into densely packed three-dimensional structures in the buIk 1171: hence, at some stage of growth it must be necessary for an icosahedral cluster to change its symmetry. However, if the initial building block of the cluster is a dimer ion, the geometric influence of such a nucleus could extend well beyond the first atomic of molecular shell.

Acknowledgement The author would iike to acknowIedge financial support from the SERC and to thank Dr. A. Pidcock for h~s comments on these results.

4. Conclusion The introduction of a low ionization potential site into an ion cluster has led to the formation of at least three species which appear to rely for stability on the presence of a trapped hole or positive charge. For each example we have been able to ratIonaltie our observatlons in terms of a structure with either cuboctahedral (face-centred-cubic and hexagonal-close-packed) or icosahedral symmetry; these structures are also members of a “magic” number sequence. The nature of the nucleating species, either r’ or G, appears to dictate the symmetry of the stable cluster, and recogni-

References 111 0. Echt. K. Sattler and E. Recknagel. Phys- Rev_ Letters

47 (1981) 1121. E&t. A. Reyes Flotte, M. Knapp, K. Sattler and E. Recknagel, Ber. Bunsenges.Physrk Chem. 86~(i982) 860. PI A. Ding and J. Hershch, Chem. Phys. Letters 94 (1983) 54. [41 P.W_ Stephens and J.G King, Phys. Rev. L.etters 5 1 (1963) 1538. is1 AJ. Stnce and AK ShukIa, Intern J. Mass Spectrom. Ion Fhys. 36 (1980) 119.

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[61 A.J. State and C. Moore.Chem. Phys. Letters 96 (1983) 80. VI U. Buck and H. Meyer. Phys. Rei. Letters 52 (1984) 109. Ml 0. Echt, D. Kreisle. M. Knapp and E. Recknagei. Chem. Phys. Letters 108 (1984) 40L PI H. Haberland, 13th International Conference of Eiectronic and Ionic CoBisions, Berlin 1983, Book of Jnviled Lectures, edr J. Eichier. 1. Herb1 and N. Stolterfobt (North-Holland, Amsterdam, 1984). 1101 J.M. Soler, J.J. Saeru. N. Garcia and 0. Echt. Chem. Phyr Letters 109 (1984) 71.

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[ll) M.R.‘Hoare and P. Pal, Advan: Phys. .24.(1975) 645. [,l2] MR. Hoar+Advan. Chem. Phys. 40 (197?),49. [ 131 R.D. Etters. R. Dantioriricz and JDugan. J. Chem. Phys. 67 (1977) 1570. (141 R.-P. Pan and R.D:Etiers. J. Chem. Phyi 72 (1980) 1741. [IS] P-M: Dehmer and ST. Pratt; J. Chem. Phys.76 (1982) 843. [ 161. A.J. Srace. J. Am. Chem. Sot. 106 (1984) 4380. [17] A.F. Wells, Structural inorganic chemistry, 5th Ed. (Clareridon Press, Oxford, 1984) ch. 4.