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STUDIES ON CHARGE TRANSFER REACTIONS AND ELASTIC COLLISIONS OF SULPHUR
CLUSTERS
M. Abshagen, T. Fischer, J. Kowalski, M. Meyberg, G. zu Putlitz, T. Stehlin, F. Träger, J. Well
To cite this version:
M. Abshagen, T. Fischer, J. Kowalski, M. Meyberg, G. zu Putlitz, et al.. STUDIES ON CHARGE
TRANSFER REACTIONS AND ELASTIC COLLISIONS OF SULPHUR CLUSTERS. Journal de
Physique Colloques, 1989, 50 (C2), pp.C2-169-C2-173. �10.1051/jphyscol:1989229�. �jpa-00229427�
JOURNAL DE PHYSIQUE
Colloque C2, suppl6ment au n02, Tome 50, fbvrier 1989
STUDIES ON CHARGE TRANSFER REACTIONS AND ELASTIC COLLISIONS OF SULPHUR CLUSTERS
M. ABSHAGEN, T. FISCHER, J. KOWALSKI, M. MEYBERG, G. ZU PUTLITZ, T. STEHLIN, F. TRAGER and J. WELL
Physikalisches Institut der Universitdt Heidelberg Philosophenweg 1 2 , 0-6900 Heidelberg 1, F.R.G.
RCsumC: La neutralisation et la diffusion Clastique du cluster sulfurique Sn, 21n115, ont CtC examinees B l'aide de cibles de gaz diverses. Des processus de fragmentation dus B 1'Cchange de charge ont kt6 detect&
2 l'aide de la spectroscopie de translation. En plus, les sections transversales de neutralisation et de diffusion Bastique ont Ct6 dCterminCes.
Abstract: Neutralization and elastic scattering of sulphur cluster Sn with 21n115 have been studied with different target gases. Fragmentation processes due to the charge exchange could be detected by translation spectroscopy. In addition, neutralization cross sections and elastic scattering cross sections have been determined.
In the last few years, the physics of clusters has become the subject of rapidly increasing interest [I]. In many aspects, however, clusters still represent an unknown state of matter intermediate between atoms and molecules on the one hand and the solid state on the other. Of particular importance are theoretical and experimental studies on the "quantum size" effect, i.e. the question, how physical properties of a cluster such as ionization potential, binding energy, electronic and crystallographic structure or catalytic reactivity vary as a function of the particle size. Presently, only few experiments have actually been able to measure the size dependence of cluster properties for a reasonably large mass range. The broad mass distribution of common cluster sources and, in addition, unavoidable fragmentation processes during ionization inhibit an unambiguous determination of the size related variation of cluster properties. The optimum way to overcome these difficulties is certainly the generation of neutral cluster beams, which only contain clusters of a single mass or at least of a narrow mass distribution.
A promising approach to generate a neutral, mass-selected cluster beam is the method of charge transfer, i.e.
electron capture by a mass-selected cluster ion beam in a target gas [2,3]. An important feature of such charge transfer processes is, that they occur as a "long range" effect at large impact parameters and practically without transfer of momentum [4]. The cross-sections for charge exchruige are on the order of 10-15 to 10-14 cm2 and can be a factor of ten or more larger than for gaskinetic scattering. Therefore, highly efficient neutralization of a cluster ion beam is possible without an increase of the beam divergence. First experiments with this method on sulphur clusters were reported by our group [5]. By choosing appropriate collision partners, we have demonstrated that a neutral, monodisperse cluster,beam can be produced. These experiments are designed such that
1) a large number of target gases such as metal vapours, gases etc. can be employed to match the conditions of a particular experiment and to make the charge exchange resonant or almost resonant,
2) mass spectrometry for beam analysis is avoided since it requires ionization and may involve fragmentation, 3) as a result, a continuous, mass-separated (monodisperse) neutral beam with high intensity can be produced for
many different species.
More recently, other experiments employing mass-spectrometry have also been reported [6]. The present paper describes new studies important for the understanding of charge transfer processes with clusters.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989229
C2-170 JOURNAL DE PHYSIQUE
Charge transfer reactions on atomic or molecular species have been studied extensively in the past [7]. Such processes can be written as:
A + + B
+
A * + B + + A EA is the species to be neutralized by the target gas B, and AE stands for the energy defect of the reaction. It is given by the difference of the ionization potentials and the related internal energy levels. AE = 0 gives rise to
"resonant", AE # 0 to "nonresonant" charge transfer. Maximum cross-section can be expected for resonant or nearly resonant transfer conditions. In this case, a reasonable intensity of neutralized clusters in the beam should be possible.
A vital condition for the generation of a neutral cluster beam of defined mass by charge transfer reactions is the exclusion of fragmentation processes. Fragmentation of the clusters following neutralization should be avoidable for reactions with minimum or no transfer of internal energy, i.e. close to resonant electron capture into the ground state of the neutralized particle. This reaction path can be realized for transfer conditions with equal or similar ionization potentials of the collision partners. In order to find the optimum transfer conditions, a detailed study of fragmentation processes for different collision partners has to be performed.
As mentioned above, mass spectrometry is not very helpful to detect possible fragmentation processes associate with charge exchange. It requires ionization, which again may cause fragmentation. In this case an unambiguous beam analysis is not possible or limited to special cases. To overcome this complication, a procedure known as translational spectroscopy [8] is applied here. If fragmentation occurs, internal energy is transferred into relative kinetic energy of the fragments. Consequently, assuming an isotropical momentum distribution, the fragmentation shows up in an increase of the beam divergence and therefore in a broadening of the beam prof~le of the neutral cluster beam as compared to that of the ion beam. This broadening is measured by a position sensitive detector and interpreted in terms of energy released in the fragmentation process. Using this method of translational spectroscopy, a detailed study of fragmentation processes in the beam can be done. Fragmentation energies as well as fragmentation channels can be extracted. In addition, this technique is generally applicable and does not require ionization.
Our experimental setup is described in detail elsewhere [3,5]. The cluster beam, generated in a gas aggregation source is ionized by electron bombardment, the ions are mass-selected in an rf-quadrupole mass filter and injected into the neutralization cell. After passing a drift region, in which the ions can be separated from the neutral species by an electrical deflection field, the neutral clusters are detected with a channeltron. For the measurements of the beam profile the detector is equipped with a slit the size of which is 10 x 0.5 mm. It can be moved perpendicular to the direction of propagation of the cluster beam over a distance of S 5 mrn. For the experiments with gaseous targets, a newly developed charge transfer cell was applied [9]. Particularly gases or vapors of liquids with high vapor pressure can be used as targets. This allows for a large variety of different collision partners to be examined. The target gas pressure may well exceed 20 Torr, so that charge transfer reactions as.well as elastic collisions can be studied. In addition, absolute cross sections for both processes can be determined.
In our initial experiments, sulphur clusters Sn with 21n58 have been studied with Na and Zn as target gases.
For different target densities the intensity of the cluster ions and of the neutralized clusters have been monitored.
To measure the neutralized species separately, an appropriate elechical deflection field and a deceleration voltage of
+
1 kV have been applied. The energy of the cluster ions approaching the charge transfer cell has been kept constant at 1 keV. During the experiment, the target gas pressure was monitored accurately to extract a value for the target density. To detect fragmentation processes associated with the neutralization, beam.profies have been measured by scanning the detector across the beam of the cluster ions as well as the neutralized clusters. These measurements have been carried out e.g. with Zn and Na as collision partners. Additional experiments have been done with other target materials and with different kinetic energies of the cluster ions.As already mentioned above, fragmentation should be avoidable by choosing reaction partners with appropriate
S6 excited Na 3s 's,,,
state ground state
9.4 eV
Zn 4s' I S , S6 ground stale
ground stale
Figure 1: Schematic representation of charge exchange with Sg+ cluster ions in Na and Zn vapor. For Na the electron is captured preferably in an excited state of the cluster with subsequent fragmentation. For Zn it is transferred preferably into the neutral cluster ground state so that fragmentation can be avoided.
ionization potentials. In the case of a large energy defect AJZ, the electron may be captured in an excited state of the cluster with subsequent fragmentation. As an example, the charge exchange for Sg+ clusters is illustrated schematically in Figure 1. In contrast to Na as target gas, charge exchange with Zn gives rise to a near-resonant transfer of the electron into the ground state of the cluster. Indeed, no significant broadening of the neutral beam as compared to the ion beam is observed for charge exchange in Zn. In the case of Na, however, a large broadening of the neutral beam due to fragmentation processes can clearly be detected.
In order to explain the broadening of the profiles quantitatively, Monte Carlo simulations of the trajectories of the fragments have been carried out. Depending on the assumed dissociation channel, values of about 1 eV for the released kinetic energy are obtained. Moreover, the calculations show that a small change of the neutral beam profile in the case of Zn as target cannot be explained by fragmentation, but only by elastic scattering processes 191. This is being supported by studies of neutralized atomic beams, whose profiles can be broadened by gaskinetic collision processes only. The ionization potentials of sulphur clusters Sn with 21nS8 fall into the range of 9 to 10 eV [lo]. Therefore, a near-resonant charge transfer with Zn for several sulphur aggregates could be realized [9]. For other cluster species our method is also applicable. In any case, the target partners have to be chosen carefully to prevent dissociation. However, even if fragmentation associated with charge exchange should occur with a certain probability, the dissociative products will be mainly scattered out of the beam core due to the released kinetic energy. With collimators and apertures, the beam can be "cleaned from fragmentation products.
Our earlier studies were directed at the investigation of charge exchange involving small sulphur clusters. In order to extend the mass range of particles to be neutralized, and examine collision processes at high target densities in more detail, investigations of gaskinetic collisions as well as neutralization of S, with 21nS15 have been carried out, e.g. with C S 2 as target gas. The cross section for gaskinetic collisions can be determined from the decrease of the cluster ion intensity at increasing target gas density. The attenuation of the beam, containing neutral and ionized species, exhibits an exponential behavior as shown in Figure 2. Assuming an equal elastic scattering cross section for the neutrals as well as for the cluster ions, the intensity of the beam can be written as
T(p) = T(O) exp ( -bs P )
where p stands for the target density in particleslcm2 and crs fbr the cross section for elastic scattering. The target gas density has been calculated with appropriate gas flow models, which have been verified accurately by experimental studies on atomic collisions with known elastic scattering cross sections. Figure 3 shows the absolute cross sections for elastic scattering of Sn with 21n<15 and CS2 as target gas. The cross-sections are given classically by the projected areas of the collision partners. Using a N213
-
dependence for the area of theJOURNAL DE PHYSIQUE
' a) b.)
+ c.)
+
0 1 2 3 4 5 6
target density [ 1014 ~ r n - ~ ]
Fimre 2: Intensity of the total cluster rate (a), the neutralized cluster rate (b), and the normalized neutral cluster rate (c) as a function of the CS2 target gas density.
cluster, with N being the number of atoms per particle, the cross-section for elastic scattering can be calculated as a function of the cluster size. The solid line in Figure 3 represents the result of such a calculation. The overall agreement with the experimental data is quite good. This indicates the influence of a geometrical size effect on the scattering process of clusters.
For neutralization processes, the intensity of the neutralized particles is given by N ( P ) = T ( O ) . o n . P
where On is the cross section for neutralization. Its size can be extracted from the data by calculating normalized beam intensities of the neutral clusters:
N n o r n ( ~ ) = N ( p ) . T ( O ) / T ( p )
N(p) is the intensity of the neutral beam and T(p) stands for the intensity of the beam containing cluster ions and neutral clusters. This definition takes into account the attenuation of the neutrals due to the elastic scattering processes. As can be seen from Figure 2 (curve c), this definition is no longer valid for high target gas densities.
In this pressure range, the normalized intensity saturates below the value for the maximum neutralization efficiency. This strongly supports reaction models, which take into account additional processes like charge exchange from the neutralized cluster to the target gas. Figure 4 shows the cross section for neutralization of sulphur clusters Sn with 21n115 and CS2 as target gas. The cross sections exhibit pronounced jumps as a function of cluster mass indicating distinct variations of the electronic structure.
1201 1
0 2 4 6 8 10 12 14 16
number of atoms per cluster
Fieure 3: Cross-sections for elastic scattering of sulphur clusters Sn+ with 21n115 and CS2 as target gas.
0 2 4 6 8 10 12 14 16 number of atom per cluster
Figure 4: Cross-sections for neutralization of sulphur clusters S,+ with 26n115 and CS2 the target gas.
In conclusion, we have demonstrated that a neutral cluster beam of well defined mass can be generated by neutralization of mass selected cluster ions. Fragmentation processes due to charge transfer could be detected by the method of translation spectroscopy of the neutral beam. Most importantly, however, fragmentation of the clusters can be avoided by choosing appropriate target materials. In addition, the experimental setup allows further studies on neutralization processes as well as elastic scattering at high target densities. Presently, ongoing studies on this subject for other target gases and metal clusters are under work in our laboratory.
This work is supported by the Deutsche Forschungsgemeinschaft.
References:
[I] Metal Clusters, Proceedings of the International Symposium on Metal Cluster, Heidelberg, April 7-1 1, 1986, edited by F. Trager and G. zu Putlitz (Springer Verlag, Berlin) 1986; Microclusters, Proceedings of the First NEC Symposium, Hakone and Kawasaki, Japan, October 1986, in Springer Series in Material Science 4 edited by S. Sugano, Y. Nishina and S. Ohnishi (Springer Verlag, Berlin) 1987; Elemental and Molecular Clusters, edited by G. Benedek, T. P. Martin, and G. Pacchioni (Springer Verlag, Berlin) 1987 [2] Arnold M., Kowalski J., zu Putlitz G., Stehlin T., and Tfiger F.: Surf. Sci. 156, 149 (1985)
[3] Arnold M., Kowalski J., zu Putlitz G., Stehlin T., and Trager F.: Z. Phys. A 322 ,179 (1985), [4] Kaiser E. W., Crowe A., Falconer W. E.: J. Chem. Phys. 61,2720 (1974)
[5] Abshagen M., Kowalski J., Meyberg M., zu Putlitz G., Trager F., Well J.: Europhys. Lett. 5, 13 (1988) 161 Brechignac C., Cahuzac Ph., Leygnier J., Pflaum R., Weiner J.: Phys. Rev. Lett. 61, 314 (1988) [7] Massey H. S. W. and Gilbody H. B., Electronic and Ionic Impact Phenomena, Vol. 4, in Recombination
and Fast Collisions of Heavy Particles (Clarendon Press, Oxford) 1974; Utterbach, N. G.
and Miller G. H.: Rev. Sci. Instrum. 32, 1101 (1961)
[8] De Bruijn D. P. and Los J.: Rev. Sci. Instrum. 53, 1020 (1982); Gellene G. I., Williams B.W. and Porter R. F.: J. Chem. Phys. 74, 5636 (1981)
[9] Abshagen M., Kowalski J., Meyberg M., zu Putlitz G., Trager F., Well J.: to be published [lo] Rosinger W., Grade M., and Hirschwald W.: Ber. Bunsenges. Phys. Chem. 87, 536 (1983)
