Formation and stability of singly and doubly charged MgArN clusters

10 June 1994

CHEMICAL PHYSICS LETTERS

ChemicalPhysicsLetters223 (1994) 139-142

Formation and stability of singly and doubly charged MgArN clusters M. Velegrakis, Ch. Liider Foundationfor Researchand Technology- Hellas(FORTH), Instituteof ElectronicStructureand L.aser, P.O. Box 1527, Her&ion 71110, Crete, Greece

Received 8 February 1994;in fina form 26 March i 994

Abstract

Clusters of the type MgArjj (z= 1,2) are produced using a combination of pulsed nozzle and laser ablation techniques. The measured time-of-flight mass spectra of singly charged clusters show enhanced intensity at total number of atoms N+ 1 = 13, 19, 23, 26, 29, 32, 46, 55. The observation of these ‘magic’ numbers can be interpreted in terms of an icosahedral close-packing structure for the cluster. Doubly charged species of small size, which appear to exhibit structure other than icosahedral are also observed.

1. Introduction Following the successful development of experimental techniques for the production and detection of free clusters in molecular beams, the stability and structure of these small aggregates have become a topic of fundamental research interest. The most obvious manifestation of cluster stability is the observation of intensity irregularities in their mass spectra. The peaks dominating the spectrum are thought to correspond to the most stable cluster ion structures and the respective cluster sizes (number of involved atoms) are denoted as ‘magic’ numbers. The indicated structure and stability of the clusters arise from the nature of the interatomic interactions in the cluster. In van der Waals systems, the valence electrons are localized on the constituent atoms, which are kept together by short-range electrostatic forces. Therefore, the cluster structure is determined exclusively by geometrical factors (shells of atoms as closepacked spheres). In contrast, in many pure metallic clusters, a different situation occurs, namely the de-

localized electrons are not influenced by the relative position of the atomic cores (which act as a positive background) and arrange themselves in electronic shells. In this Letter, we report on the production and stability pattern of MgArN cluster ions. Such metaldoped rare gas clusters are appropriate systems for investigating the influence of charge localization on the cluster structure. Since the ionization potential of the metal atom is lower than that of the rare gas atoms, the charge is expected to reside on the metal atom. Recent experimental [ l-3 ] and theoretical [ 41 results on similar heteroclusters provide further insight into the physics of such systems.

2. Experimental method and apparatus In this work MgAr, cluster ions are produced directly using a combination of a free-jet expansion and laser ablation of a Mg target. Ablated Mg ions are ejected perpendicularly into the Ar-flow direction and

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serve as condensation nuclei for the growth of the MgArN cluster ions. As this ion neutral encounter takes place at an early stage of the adiabatic expansion, it is expected that the clusters undergo sufficient collisions to cool down. Under such formation conditions, it is likely that Custer ions are produced by the successive growth of smaller species rather than by fragmentation of larger ones [ 5 1. Our experiments have been performed in a recently constructed molecular beam time-of-flight (TOF) apparatus which will be described elsewhere. In brief, the apparatus consists of three successive differentially pumped vacuum chambers. Argon is introduced into the first chamber from a room temperature pulsed (5 Hz, 700 us pulse width) nozzle at a backing pressure of about 5 bar. About l-5 mm downstream and at an angle of 45 ‘, the focused light of a KrF (248 nm) excimer laser (30-100 mJ) produces plasma from a rotating Mg rod. Mg ions contained in the plasma are mixed with neutral Ar atoms in the high density region of the expansion. Clusters growing around Mg ions are stabilized through collisions and form a cluster beam. A few centimeters further down, the beam enters through a conical skimmer into the second chamber, which houses a pulsed three grid acceleration unit (850 V) for the TOF mass analysis. The distance from the ablation point to the middle of the acceleration field is 18 cm. The accelerated cluster ions enter finally via a collimator into the third chamber and are detected by a two stage microchannel plates detector. The pulsed nozzle, the ablation laser tiring and the triggering of the TOF are synchronized through coupled delay generators. These delays are sequentially optimized for maximum time-of-flight signal. Typical operating pressures are 10m4mbar in the souree chamber, 5 x 1Om6 mbar in the acceleration chamber and 5 x 1O-’ mbar in the detector chamber.

3. Results and discussion Fig. 1 displays a typical time-of-flight spectrum generated under the above-mentioned working conditions and for a time delay between ablation laser firing and TOF triggering of 270 us. The spectrum is dominated by a strong series, which is identified as MgAr,$. The smaller peaks between the MgArz are

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Fig. 1. Time-of-flight mass spectrum of MgAr$ clusters averaged over 2500 shots. The most intense peaks are numbered according to the total number (iV+ 1) of atoms in the cluster.

due to ArN and to Mg ( HZ0 ) At-, cluster ions (the latter resulting from water impurities present in the source chamber). No pure Mg clusters are observed. Similar results are also obtained using either the fundamental ( 1064 nm) or the second harmonic (532 nm) of a Nd: YAG laser instead of the excimer laser. From the arrival time of the ions to the acceleration region we calculate the mean velocity of the cluster beam to be 670 m/s. This is comparable to the expected isentropic velocity of the Ar beam (560 m/s) indicating efficient thermalization of the plasma plume. The MgAr$ series exhibits pronounced peaks at sizes N+ I= 13, 19, 23, 26, 29, 32, 46, 55. Less pronounced peaks at sizes 39,49,61,64 and 71 are also evident in the spectrum. Larger clusters could not be resolved, because of the limited resolution of our mass spectrometer in this mass range. In order to check the reproducibility of these ‘magic numbers’ we varied the source conditions such as the Ar backing pressure, nozzle-ablation point distance, laser fluence, and delay between nozzle pulse and laser firing. Although these variations affect the mass distribution by shifting it either to smaller or to larger masses, the same ‘magic’ numbers are always observed. The largest mass-range spectra are obtained by tiring the laser in the middle of the gas pulse and employing moderate laser fluences below 10” W/cm*. Most of the ‘magic’ numbers detected here coincide with those reported previously for pure Ar$ clusters [ 61 and AlAi- clusters [ 71 both grown as ions, and those of postionized neutral AlAr, clusters [ 3 1. The observed sequence of intense MgAr$ peaks

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is identical with that predicted from the model of close-packing of spheres [ 6,8,9]. The numbers 13 and 55 correspond to the completion of the first and the second icosahedral shell of atoms (Mackay icosahedra), respectively. Harris et al. [ 61 have developed a growth model for Ar$ clusters presuming an icosahedral building unit consisting of a central atom (in our case may be Mg+) surrounded by twelve neutral Ar atoms. Placing atoms at neighbor faces of the icosahedron and counting the number of their near neighbors results in maxima at N= 19, 23, 26, 29 and 32. In a similar manner, removing atoms from the filled second shell (N= 55), the number of the bonds maximizes at N=49,46 and 39. The geometric structures that are obtained in this way have icosahedral symmetry. Assuming that the total binding energy contribution of every new added atom is proportional to the number of near-neighbor bonds, these maxima are expected to reflect binding energy differences of consecutive clusters, i.e. clusters with these total number of atoms should be more stable than others. The fact that in our spectra exactly this predicted sequence of ‘magic’ numbers appears constitutes strong evidence for icosahedral close-packing in the MgAr$ clusters. The comparison of these MgArs results to the previously known, XeArj$ [ 2,4] and AlAr$ [ 31, enables us to assess the tolerance of the icosahedral geometry of close-packed hard spheres in accommodating a central atom of smaller size. In these systems, the ratio of the central atom diameter to that of Ar (3.76 A) varies from 0.77 for Al+-Ar [ 31, to 0.73 for Xe+-Ar [4] and to 0.55 for Mg+-Ar [lo]. This comparison indicates that a 45% decrease in the size of the central sphere (Mg+ atom) does not disrupt the essential icosahedral symmetry. Electronic considerations are not explicitly taken into account in this crude estimation. Fig. 2 shows another mass spectrum obtained for shorter delays ( z 200 us) between the ablation laser pulse and the starting pulse for the time-of-flight. The mean size of the clusters is now shifted to lighter masses. A new series of intense peaks appears in the spectrum in addition to the MgAr$ one. This series is reproducible and independent of the employed ablation laser. This is identified as the MgAr$+ clusters. The peaks accompanying the MgArn,ions are due to Ar,$, Mg(HzO)Ar$ and Mg(H,O)A$. We

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WFlWl Fig. 2. Mass spectrum showing singly and doubly charged MgAr, clusters. For clarity reasons the peaks corresponding to the MgAr$+, and MgAr$ clusters are connected by solid and dotted lines, respectively. Furthermore some dominant species are indicated by dots.

could also observe weak signals corresponding to MgAr*+, MgAt$+ and MgAt$+ in the vicinity of the very intense peak of Ar+ (which is about four times larger than e.g. the MgA$ peak). The whole series of MgA$ could be resolved up to N= 35. The reason that doubly charged species arrive, on average, earlier to the acceleration electrodes can be probably attributed to space charge effects in the cluster formation region. Thus ions with the same mass but carrying double charge, experience higher acceleration and leave this region with higher velocities. Interestingly, the stability pattern that is exhibited by the doubly charged clusters differs from that of the singly charged. Specifically, there are no distinct prominent irregularities in the spectrum of Fig. 2, except for a peak at N+ 1 = 7 and a dip after N+ 1 = 15. No clear evidence for icosahedral packing exists in this case. Patil [ 111 has analyzed the interaction energies and geometries of small MgA&+ (N= l-6) clusters using perturbation theory. He predicted that the maximum number of Ar atoms needed to saturate the strong Mg’+-Ar interaction is four. Any additional atom experiences weaker polarization, resulting in a decreasing stability of the larger clusters. This prediction is in variance with our results, since it appears that MgA<+ is considerably stronger than MgAr$+. At the moment, we can only speculate about the stability behavior of the doubly charged MgArN clusters. Trying to find a growth sequence in this case, one can begin with the assumption that the most probable geometry of MgA$ is an octahedron,

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where the small Mg2+ (ionic radius = 0.66 8, [ 12 ] ) is fixed at the center. Placing additional Ar atoms at the faces of the octahedron, a completed (eight atoms) second layer is reached at total number of atoms N+ I= 15. Thus the observed stability of M&a could reflect this high symmetry, which now has fee geometry. Finally, as the ionization potential of Ar, for the range N= 2- 10 is about 14.4 eV [ 13 ] and thus is close to the second ionization potential of Mg ( 15 eV), it is reasonable to consider also the possibility of charge transfer in a MgA$ cluster. As concluded by Patil [ 111, one-electron transfer is energetically not possible. This would lead to Coulomb fission in contrast to our observation of small MgA&+ clusters. The possibility, however, for partial charge transfer still remains. The occurrence of such effects in pure rare gas clusters has been proposed previously [ 141. The charge may be then distributed over a larger unit in the cluster, such as dimer, trimer, etc. Furthermore, as has recently been pointed out by Garcia et al. [ 15 1, the extent of charge delocalization depends on the total number of atoms in the cluster. In this complicated situation, structures other than the icosahedral ones can be more favorable. In conclusion, we think that the simple picture of a localized positive charge placed in the center of the cluster and surrounded by neutral atoms fully accounts for our experimental results on MgAr+, clusters, which exhibit icosahedral geometries. However, in the case of the doubly charged ions other symmetries seem to be more favorable. Further work, both theoretical and experimental, is required in order to establish the geometry of these clusters. Additional experiments which address the optical absorption of mass-selected MgAr, ions and the possible metastability of large clusters are in progress.

Acknowledgement We wish to thank Professor K.H. Meiwes-Broer for stimulating discussions at the initial stage of this work. Support was provided by the Ultraviolet Laser Facility (project No. G/89 100086GEPP) at FORTH. Financial support by the Volkswagen&Stung (project No. I/67 73 1) and the EC Human Capital and Mobility program (project No. ERB4001 GT92 1580) is also gratefully acknowledged.

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