Fission of small multiply charged gold clusters

Volume 156, number 1

FISSION

CHEMICAL PHYSICS LETTERS

OF SMALL MULTIPLY

W.A. SAUNDERS

CHARGED

24 March I989

GOLD CLUSTERS

and S. FEDRIGO

Institutde PhysiqueExpWnentale, Ecole Polycechnique Fkdkralede Lausanne,PHB-Emblem, CH-IO15Lauranne, Switzerland Received I February 1989

The results of collision-induced fragmentation measurements on Au:+, Au:*, Au:+, and Au:+ are presented. In all cases, channels involving fission (separation of charged fragments) are observed. Neutral loss channels are not observed. Fission crosssections (of the order of 50-100 A’ with Xe) are much larger than fragmentation cross-sections of singly charged ions and are relatively insensitive to the target gas. The collision-energy dependence of the branching ratios shows that Au:++Au:+ +A$ and Au:+~Au:+ + Au+ are the respective lowest decay channels.

1. Introduction The multiply charged Au clusters produced by a liquid metal ion source (LMIS) group naturally in two regimes: the large clusters (above 14 atoms) and the small clusters [ 1] (from 2 to 5 atoms). Intermediate sizes, if produced at all, are in such low abundance that they are practically unobservable. Multiply charged clusters in the small size range are of particular interest since they lie well below the critical sizes [2,3] predicted by simple models of cluster binding. Recent theoretical models [ 41 which take detailed account of the short-range bonding forces between the atoms show that clusters in the small size range are metastable. Nevertheless, in spite of the special nature of these systems, there are relatively few experimental data on their measurable properties such as lifetimes [ 5 1, photon- and collision-induced fragmentation, and electron affinities

[61. This paper addresses aspects of the collision-induced fragmentation of multiply charged Au clusters in the small size range (Au:+ to Au:+, and Au:+), including the determination of the fragmentation cross-sections and the measurement of the relative branching ratios. In all cases, the dominant decay process is fission (that is, separation of charged fragments), No clear signature of a neutral loss channel is observed. We find the cross-sections (of the order of SO-100 A*) are large compared to those of 14

singly charged ions and are relatively insensitive to the particular target gas employed. Measurement of the collision energy dependence of the fission branching ratios for Au:+ and Au:+ allow determination of the lowest decay channels for the clusters. Final-state energetics and a simple hydrodynamical interpretation of the fission process agree qualitatively with the observation that Au:+ decays preferentially into Au: +A$ at low collision energy. In the case of Au:+, where the channel giving Au:+ + Au+ is preferred, no simple interpretation is applicable.

2. Experimental The clusters are produced by a LMIS of standard construction [ 7 1. An etched tungsten needle (tip radius approximately 5 km) wetted with pure (99.99% MBtaux Prkieux) Au is housed in an alumina tube filled with about 0.1 cm3 Au. The alumina tube is directly heated by external Ta wire heaters. The needle potential is fixed ( 185 V) while the potential of an opposing extractor electrode is varied to give the desired emission current. Typically, an extractor potential of - 5.5 kV gives an emission of about 40 pA with the needle tip located 1 mm from the extractor electrode. The ions from the source are focused Bnd steered into a triple-quadrupole mass spectrometer where

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CHEMICALPHYSICSLETTERS

they are mass-selected by the first quadrupole and allowed to interact with a gas target in the second quadrupole. Gas pressures are measured with an ionization gauge (Balzers IMR-132) and are corrected for sensitivity to different gases following the manufacturer’s recommendation. The resulting fragments are analyzed with the third quadrupole. Details of the quadrupole construction and performance may be found elsewhere [ 81. Individual ions are detected by a high-voltage conversion dynode and secondary-electron multiplier. The resulting pulses are shaped, counted and logged onto a computer for data analysis.

24 March 1989

10°

lo-’

1Ci2

If3

PRESSURE

3. Results and discussion The mass spectrum of clusters produced by the LMIS is shown in fig. 1. The most intense multiply charged cluster is Au:+. Au:+ and Au:+, which are present in the beam [ 61, are overlapped by the singly charged clusters with identical M/z (mass-tocharge ratio). Au:+ and Au:+ are present in low abundance. The peak next to Au:+ is a contaminant peak, being due to AuFe+ or AuMn+. The results of fragmentation probability measurements as a function of target gas pressure for Au:+ are shown in fig. 2. The measurements were carried out at a fixed laboratory energy of 26 eV. Unlike charge-exchange cross-sections [ 6 ], which show a pronounced dependence on the target-gas ionization potential, the fragmentation cross-sections show only a slight variation (less than a factor of 3) with gas type. Taking into account the experimental geome-

10s 4 5 103 s c lo* 8 10’ 1

2

3

4

5

M/z (MA,/tel) Fig. 1. A mass spectrum

(mbor)

Fig. 2, The target-gas dependence ofthe fragmentation probability of Au:+. Taking into account experimental parameters, the fragmentation cross-section with Xe is 80 A”.

of the small Au clusters produced

by the

LMIS. The doubly chargedclusters with even numbers of atoms are hidden behind more intense singly charged cluster peaks.

try, the fragmentation cross-section (reliable to a factor of two) with Xe is 80 A’. The fragmentation cross-section of Au: with Xe is 5.8 A2 measured under identical experimental conditions. In the absence of a target gas, spontaneous fragmentation of Au:+ corresponding to a cluster lifetime of approximately 200 ms is observed. Because the degree of vibrational excitation of the cluster is unknown, the fragmentation cross-section and lifetime of Au:+ represent upper and lower bounds, respectively. The fragmentation cross-section for Au:+ is much larger than the fragmentation cross-sections measured for the Au,+ in the same experiment, although is of the same order of magnitude as the rigid-sphere collision cross-section #I. The magnitude of the crosssections is typical for barrierless exothermic reaction cross-sections [ lo]. Normally, such large cross-sections result from a crossing of long-range repulsive [ 111 or attractive [ 12 ] Coulomb interaction potentials. The fragmentation mechanism in this case, however, does not appear to be related to charge transfer in view of the relative insensitivity to the target ionization potential. The fragmentation crosssections with the noble gases correlate well with their XLThe rigid-sphere cross-section can be estimated from bulk data (see, for example, ref. [9] ). The cluster radius is R,= where r,=3.0 is the Wigner-Seitz radius for Au, a, is r&N”, the Bohr radius, and N= 3 is the number of atoms in the cluster, and the radius of Xe is R,,= 2.17 A. The estimated crosssectionisthusr(R,+R,,)*=62A’. 15

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CHEMICAL PHYSICS LETTERS

polarizabilities [ 13 1, But, when the center-of-mass velocity dependence expected from a Langevin [ 141 (induced-dipole) interaction is included the correlation disappears. Further information about the fragmentation mechanism is available by studying the energy dependence of the cross-sections. It is found for each of the multiply charged clusters studied that one fragmentation channel has a threshold which, taking account of experimental resolution, is indistinguishable from a step function at zero collision energy, while other channels show more gradual collision energy dependence. The “zero threshold” is characterized by the detected fragment intensity rising from zero and reaching its maximum value for kinetic energies within the energy spread of the parent cluster beam. The zero-threshold behavior of the clusters is probably the result of several combined influences, most important among these being the initial vibrational excitation and weak binding of the clusters. The LMIS needle is operated at a temperature above 1400 K (i.e. above the melting temperature of Au) and the cluster ions are extracted in an only partially understood but certainly violent process. Subsequent to formation, the clusters cool by unimolecular decay on a time scale of the order of the observation time ( * 100 ps). There seems to be no a priori justification in assuming the average excitation of the clusters is below that which gives a lifetime much greater than the observation time. Thus, the addition of even a small amount of energy should lead to some observable dissociation. Another factor which may contribute to the zerothreshold behavior and large cross-sections is the weak binding of the clusters. Recent non-relativistic calculations [ 151 for Au:+ find a metastable well of about 0.8 eV in depth. The shallowness of the well may imply the cluster binding is sensitive to weak perturbations induced by the passage of a gas atom. In spite of the difficulty in obtaining quantitative information about the decay barriers of the (metastable) clusters, useful qualitative information about the relative branching ratios can be obtained for clusters with more than one independently observable fragmentation channel. Although Au:+ has two fission channels, the symmetric channel is not observable since the parent cluster and the product have 16

24 March 1989

identical M/z. It is significant that while the fission product Au: is observed when M/z= 2 (in units of MA”/ 1~1) is selected by the first quadrupole (verifying the existence of Au:+ ) , no evidence is found for the neutral loss channel to form Au:+. This agrees with our intuitive expectation that fragmentation channels involving loss of neutral particles lie well above the fission channels for multiply charged clusters in the small size range. The smallest doubly charged cluster which has two independently observable fragmentation channels is Au:+, these being Au+Au,++Au+

(1)

and Au:+ -+A$

.

+Au:

(2)

The fragmentation channel (2) shows the characteristic zero threshold mentioned above. Channel ( 1 ), by comparison, shows only a small intensity at low center-of-mass collision energy, but rises to a maximum value at around 10 eV. These results are summarized in fig. 3, which shows both the distribution of kinetic energies of the Au:+ clusters and the relative intensity of the Au: fragment, defined as

I(Au:) ’

Z(Au,+) +I(Au:)

where I( Au: ) is the measured intensity of the fragment Au:. The observed branching ratios can be partly understood in terms of the energetics of the final

o.ot

0

2

4

6 Energy

8

10

12

14

10.00

(eV)

Fig. 3. The center-of-mass collision energy dependence of the relative fraction of Au: product formation ( 0 ) from the fission of Au:+, The measured kinetic energy distribution of Au:+ is also shown (0).

Volume 156. number 1

states. Taking calculations [ 161 for Na cluster ions as indicative for s-electron bound metals, the final state of reaction (2) is expected to be favored energetically over reaction ( I), which is consistent with the measured relative branching probability. Considering, however, that the point of scission is far from the equilibrium geometries of both the parent and the separated fragments, it is not obvious how the ground-state energies of the products can directly influence the fission process. It is also of interest to note that classically the instability of a charged liquid drop is greatest for the surface modes of lowest order [ 171. The correspondence to the observation of a roughly symmetric fission channel (2 ) (as opposed to the asymmetric channel ( 1) ) is interesting from the standpoint of a dynamical picture of cluster fission. While this hydrodynamical model is certainly oversimplified in the case of such a small cluster, it may represent a starting point for a microscopic understanding of the decay processes of multiply charged metal clusters. Two products with M/z, 4/3 are observed in collisions of Au:+ with Ar, these being AU; and Au:+. The fragment Au:+ shows the characteristic zero threshold with a cross-section of 60 A2 measured at 26 eV laboratory energy. It is not possible to say with absolute certainty whether the signal at M/z=2 is due to the charge-exchange product Au:* or the fragment Au:, though indirect evidence [ 61 suggests the latter. In any case, the important point for the following discussion is that the measured signal at M/z=2 places an upper limit on the probability of fragmentation into Au;. The relative fraction of the M/z=2 signal for collisions with Ar is shown in fig. 4. For Au:+, in contrast to Au:+, the asymmetric decay channel Au:+ -+A$+ +-Au+ is more probable

than both the symmetric

Au:+ +Au$+ + Au:

(4) channel (5)

and Au:+-+At$+Au++Au+,

24 March I989

CHEMICAL PHYSICS LElTERS

(6)

Abiding by the assumption that Au]+ is metastable, reaction (6) is favored energetically over (4) by at least the binding energy of Au,+. We deduce that fl-

0.0

’

I1,,,,,I,L,,,1,

0

10 Energy

20 eV)

Fig. 4. The center-of-mass collision energy dependence of the relative M/r=2 fragment signal from the fission ofAu:*.

nal state energetics alone do not determine the fragmentation branching ratios and therefore that microscopic (kinematic) influences must be important. The loss of a single Au+ apparently removes suff~cient excess energy to stabilize the remaining Au:+. In the absence of accurate binding energies for the products of reactions (4) and (5 ), we cannot say which reaction has the lower final-state energy. That reaction (4) has the lower threshold may partly account for the large abundance of Au]+ in the beam. This is of particular interest with regard to the model of LMIS cluster formation proposed by van de Walle and Joyes [ 181 in which highly excited, highly charged droplets emitted from the needle tip rapidly undergo decay by fission to form the species observed in the mass spectrum.

4. Summary and conclusions In summary, we have presented the results of dissociation measurements on multiply charged Au clusters. We find that Au:+ and the other multiply charged clusters have fragmentation cross-sections which are much larger than those of the corresponding singly charged species. Measured collision energy dependences show the small multiply charged Au clusters produced by the LMIS have dissociation thresholds which are indistinguishable from zero. This is probably due to residual vibrational excitation and the weak binding of the clusters. In spite of this difficulty in obtaining quantitative information about the fragmentation barriers, we find distinct preference for one channel over the others for the 17

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CHEMICAL PHYSICS LETTERS

clusters with two or more observable channels. This preference is understandable, in the case of Au:+ both in terms of the final-state energies of the decay and in terms of a classical hydrodynamical model. However, in the case of Au:+, neither the final-state energetics nor the classical model seem to provide an adequate description of the fragmentation behavior. This discrepancy clearly points to the need for a useful microscopic description of the fragmentation processes in small multiply charged clusters.

References [ 11P. Sudraud, C. Colliex and .I.van de Walle, J. Phys. (Paris) 40 ( 1979) L207; A.R. Wraugh, J. Phys. D 13 (1960) L203; W. Drachsel, Th. Jentsch, K.A. Gingerich and J.H. Block, SurfaceSci. 156 (1985) 173. [ 2 ] K. Sattlet, J. Muehlbach, 0. Echt, P. Pfau and E. Recknagel, Phys. Rev. Letters 47 (1981) 160; P. Pfau, K. Sattler, R. Plaum and E. Recknagel, Phys. Letters A 104 (1984) 262; A. Hoareau, P. Melinon, B. Cabaud, D. Rayne, B. Tribollet and M. Broyer, Chem. Phys. Letters 143 (1988) 602. [3] D. Tomlnek, S. Mukhejee and K.H. Bennemann, Phys. Rev. B 28 (1983) 665. [4] G. Durant, J.P. Daudy and J.P. Mahieu, J. Phys. (Paris) 47 (1986) 1335;

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24 March 1989

F. Liu, M.R. Press, S.N. Khanna and P. Jena, Phys. Rev. Letters 59 ( 1987) 2562; S.N. Khanna, F. Reuse and J. Buttet, Phys. Rev. Letters 61 (1988) 535. [ 5) T.T. Tsong, J. Chem. Phys. 85 ( 1986) 639. [ 6 ] W.A. Saunders, to be published. [7]A. Wagner and T.M. Hall, J. Vacuum Sci. Technol. 16 (1979) 1871; S. Fedrigo, DiplBme Thesis, Ecole Polytechnique F&I&ale de Lausanne (1989). [ 8 ] P. Fayet and L. Wiiste, 2. Physik D 3 ( 1986) 177; P. Fayet, Ph.D. Thesis, Ecole Polytechnique F&d&ale de Lausanne (1987). [ 91 N.W. Ashcmft and N.D. Mervin, Solid state physics ( Halt, Rinehart and Winston, New York, 1976). [lo] R.D. Levine and R.B. Bernstein, Molecular reaction dynamics (Oxford Univ. Press, Oxford, 1974) p. 46. [ 111R.A. Mapleton, Theory of charge exchange (Wiley, New York, 1912), [12] R.D. Levine and R.B. Bernstein, Molecular reaction dynamics (Oxford Univ. Press, Oxford, 1974) p, 86. [ 131 R.R. Teachout and R.T Pack, At. Data 3 ( 1971) 195. [ 141 G. Gioumousis and D.P. Stevenson, J. Chem. Phys. 29 (1958) 294. [ IS] Li Yi, S.N. Khanna and P. Jena, private communication. [ 161 J.L. Martins, R. Car and J. Buttet. Phys. Rev. B 31 (1985) 1804. [ 171J.W. Struts and Lord Rayleigh, Theory of sound, Vol. 2 (Dover, New York, 1945) p. 374. [IS] J. van de Walle and P. Joyes, J. Phys. (Paris) 46 (1985) 1223.