J. Phys. Chem. 1993,97, 86-90
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Electronic Shell Structure of Indium-Sodium (In,,Na,) Bimetallic Clusters Examined by Their Ionization Potentials and Mass Distributions Atsushi Nakajima, Kuniyoshi Hoshino, Tsuneyoshi Sugioka, Takashi Naganuma, Tetsuya Taguwa, Yoshiyuki Yamada, Katsura Watanabe, and Koji Kaya' Department of Chemistry, Faculty of Science and Technology, Keio University, 3- 14- 1 Hiyoshi, Kohoku-ku, Yokohama 223, Japan Received: August 26, 1992; In Final Form: October 12, 1992
Indium-sodium (In,Na,) bimetallic clusters were produced by two independent laser vaporization methods. Ionization potentials (IPS) of the In,Nam clusters were measured up to m = 2 using a tunable ultraviolet laser combined with a time-of-flight (TOF) mass spectrometer. At small sizes (n = 3-1 5 ) , the ionization potentials decrease by 0.1-0.8 eV with the addition of N a atom@), whereas the IPS of larger In, (15 I n I 27) clusters do not decrease with N a addition. Moreover, IPSof In7Nal and In13Nal clusters are higher than those of In7 and Inl3, and the IP increments can be explained by electronic shell closings of the l p (8e) and 2p shell (40 e), where In atoms in the clusters are monovalent and trivalent, respectively. The electronic shell structure was also examined by a magic number in mass distributions of In,Na,- cluster anions; the In12Na3- cluster can be observed as magic numbers, corresponding to the 2p shell closing. In contrast, no electronic shell structure is observed in pure In, clusters around n = 13. These results indicate that Na atom addition can induce s / p hybridization in the In, clusters. W e also present mass distributions of aluminum-sodium cluster anions, Al,Na,-, whose feature can be understood by the electronic shell model.
I. Introduction For alkali-metal, noble metal, and group IIIB metal clusters, the electronic shell model successfully explains discontinuities in the size-dependent properties, such as electronic structures and chemical reactivities.' The treatment of the jellium model assumes that the clusters can form a spherically symmetric potential well into which their valence electrons are to be filled.* The jellium model for the Woods-Saxon potential predicts the electronic shell closings when the total number of valence electrons is equal to 2,8,18,20,34,40,58,68,70,and so on. Especially, pure alkalimetal clusters have been extensively investigated both experimentally and the~retically.~-~ Both indium and aluminum belong to group IIIB in the periodic table, and they have two s-valence electrons and one p-valence electron. In their clusters, the electronic feature is characterized only by one p electron in a small cluster because an energy gap exists between the s and p bands. As the cluster size increases, the s and p bands hybridize and the composed atoms are characterized as trivalent atoms. In aluminum clusters, it has been revealed that the s/p hybridization occurs around n 5 by measuring their ionization potentials (IPs).~On the other hand, in indium clusters, it has been reported that the In atoms are monovalent up to n 15, by their IPS and photoelectron spectroscopy;IOthe s/p hybridization in the indium clusters is not completed at the small-cluster size. Recently, we have developed a method to produce Na-contained bimetallic clusters by two lasers and adapted the method to examine the electronic structure of the Al, clusters.'' Since the Na atom has only one valence electron, the electronic structure could be revealed by the IP variations of Al,Na,,: when one valence electron of the added N a atom completes the electronic shell closing of the cluster, the IP of the corresponding Al,Na, cluster increases compared to that of the Al,Na,,-, cluster. Otherwise, the added Na atom decreases the IP of the cluster because the IP of Na atoms is relatively low compared with that of the AI, cluster. In this paper, we examined the electronic structure of the In, cluster by adding Na atoms to the In,, clusters and measuring their IPS or magic numbers which appear in mass distributions of In,Na,- cluster anions. The features show that N a addition
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induces s/p hybridization of the In,, cluster around n 13. Moreover, mass distributions of aluminum-sodium cluster anions, Al,Na,-, are also presented in comparison with those of the In,Na,- cluster anions. Since it is known that A1 atoms are trivalent even at small n, this feature can be understood by the electronic shell structures of the mixture between trivalent A1 atoms and monovalent Na atoms.
11. Experimental Section The experimental apparatus and methods have been published elsewhere,l2 so a brief description is given here. Two target rods of indium (In) (or aluminum (Al)) and sodium (Na) were vaporized by two pulsed Nd3+:YAG lasers (532 nm, Quantray DCR-2A and DCR-2). The In,Na, or Al,Na, bimetallicclusters were generated and thermalized in a channel ( I = 8 cm, d = 3 mm) by collisions with He carrier gas. Neutral clusters in the beam were intersected with the light of an ionization laser, a tunable ultraviolet laser or an ArF excimer laser, to ionize the clusters. The photoions were accelerated in a static electric field (repeller, +4.5 kV; extractor, +3.0 kV). On the other hand, cluster anions in the beam were directly accelerated with a fast pulsed electric field (-4 kV, Velonex Models 350 and V1736). Thus, the accelerated photoions or anions were mass-selected by a commercial time-of-flight (TOF)spectrometer (Jordan), and the ions were detected by a dual multichannel plate (Galileo). The ion signal was accumulated in a transient oscilloscope (LeCroy 9400A) coupled with a microcomputer, and the mass spectra of the bimetallic clusters were measured by averaging 700-800 outputs. In the IP measurement, photon energy was changed at 0.030.05-eV intervals in the range 6.0-4.2eV; in addition, mass spectra were measured by the ionization of an ArF laser to monitor the abundance and composition of the In,Na, clusters. The fluences of both the tunable UV laser and the ArF laser were kept at 200-350 pJ/cm2 in which the ion intensities of the ionization mass spectra by the tunable UV laser were normalized by both the laser fluence and the ion intensities of ArF ionization mass spectra. The IPS of the In,Na, clusters were determined from the final decline of the ion intensities as a function of the photon 0 1993 American Chemical Society
The Journal of Physical Chemistry, Vol. 97, No. 1 , 1993 87
IndiumSodium Bimetallic Clusters
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Mass Number (m/z) Figure I. T O F mass spectrum of In,,Na, (9 5 n 5 20,O 5 m 54) clusters ionized by an ArF excimer laser (193 nm, 6.42eV). Mass peaks of the clusters are labeled according to the notation n-m, denoting the number ofInatoms(n) andNaatoms(m). Themixingratiooftwometalelements could be controlled easily by the fluence of individual lasers. Since a mass number of five N a atoms (23 X 5 = 11 5 u) coincides with that of one In atom (1 15 u), a large amount of N a atoms prevents us from unique mass assignment. The laser fluencesofvaporizationof indiumand sodium rods are typically 8 and 1-2 mJ/pulse, respectively.
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energy. The uncertainty of the IPS could be estimated to be typically f0.06 eV. 111. Results and Discussion
A. Ionization Potentials of In,Na, (m= 0, 1, and 2) Clusters. Figure 1 shows a TOF mass spectrum of In,Na, clusters ionized by the ArFexcimer laser. These bimetallicclusters were produced under the laser vaporization conditions of -8 and -2 mJ/pulse for the In rod and the N a rod, respectively. Under this Na-poor condition, the bimetallic clusters are mainly composed of In atoms, containing N a atoms as a minority. Since an In atom (1 15 u) has a 5 times larger mass unit than a N a atom (23 u), a mass peak of InnNam+5inevitably coincides with that of III,,+~N~,. Therefore, it is required to assign mass peaks uniquely under the Na-poor condition. Figure 2 shows IPSof pure In,,clusters as a function of n (number of In atoms). Rayane et aLl3 have also measured the IPSof pure In,,clusters by electron impact ionization. Their obtained IPS are consistent with ours determined by photoionization, though there are some discrepancies at small n, which are caused by experimental conditions such as a difference in the production method (gas condensation/laser vaporization). Group IIIB elements are characterized by the s2p1configuration, and the s-orbital binding energy increases with nuclear
charge, resulting in the larger s/p separation for the heavier elements: Al, 3.6 eV; Ga, 4.7 eV; and In, 4.3 eV.I4 Therefore, the group IIIB metal clusters of the heavier elements should be characterized by the p electron (bands) for small n, where the composed atoms are monovalent. As the cluster size increases, the s and p bands hybridize, and the atoms in the clusters become trivalent. Therefore, the physical and chemical properties of the In,, and Al,, clusters are expected to be different especially a t small n, though both the In atom and the A1 atom belong to group IIIB elements in the periodic table. In aluminum clusters, a closing of the electronic shell corresponding to 40electrons (2p shell) is clearly observed between n = 13 and 14,in the IP measurementg as well as in the electron affinity (EA)15J6measurement; a discontinuous drop is observed in the IPS and EAs between them. This result originates from the fact that the s/p hybridization is completed before n = 13, and the aluminum atom is trivalent in the cluster. Schriver et al. have found the onset of s/p hybridization around n = 5 from the IP mea~urement.~ In indium clusters, however, no evidence for the electronic shell closing is obtained between n = 13 and 14 in the IP measurement as shown in Figure 2. This indicates that the s/p hybridization is not completed yet up to n 13 and an electronic shell is formed only by p electrons. Indeed, the discontinuity in IPS can be observed at n = 8 and 18,and the IP patterns can be explained by l p (8e) and Id (18e) shells, where In atoms are monovalent. Moreover, photoelectron spectroscopy for In,- cluster anions also provides evidence for the monovalent character of In atoms around n = 10.Io It should be noted that the IP of In, is extremely lower than those of neighbors, which seems to be attributed to the stability of the In7+ cation.I7 Figure 3 shows IPSof In,,NaI and InnNa2clusters as a function of n (number of In atoms). At small sizes (n = 3-15), the ionization potentials decrease by 0.148eV with the addition of N a atom(s), whereas the IPS of larger In,,(1 5 I n I 27) clusters do not decrease with N a addition. Since the IP of a N a atom (5.14eV)18is almost equal to those of the larger In,,clusters, the decrease in IP becomes small. In the series of In,,NaI clusters, there is a discontinuous drop between n = 13 and 14. Besides, there is another drop between n = 7 and 8,though the difference between them is not so large compared with that at n = 13 and 14. In both cases, the IPSof In7 and Inl3clusters slightly increase with N a addition, while those of Ins and In14 clusters largely decrease by more than 0.5eV with it. These IP patterns indicate that an electronic shell is formed, because an electronic shell closing is characterized by a high IP and the next shell opening corresponds to a discontinuous drop of IP with size n. In our previous work, the IPS of AlI3Naland A123NaIare higher than those of AIl3and Al23. The IP change in AI13Naland A1z3NaI can be explained by the electronic shell model: By the addition of one Na atom to the pure aluminum clusters, the total number
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The Journal of Physical Chemistry, Vol. 97, No. 1 , 1993
of valence electrons strictly satisfies the shell closing of 2p and 3s (70 e), respectively, and this electronic stabilization increases the IP. When only p electrons of indium atoms contribute to form an electronic shell, In7NaI and In13Nalhave 8 and 14 electrons, respectively. The former corresponds to the closing of the l p electronic shell. The latter could correspond to a subshell closing associated with a spheroidal distortion,' although it does not correspond to any electronic shell in the treatment of the WoodsSaxon potential. On the other hand, if both the s and the p electrons contribute to form the electronic shells, In7Na1 and In13Nal have 22 and 40 valence electrons, respectively, where only the latter corresponds to closing of the 2p electronic shell. There are two possibilities for the stability of the In13Nalcluster, but it can be attributed to the 2p shell (40 e) in terms of magic numbers in the mass distributions of In,Na,- cluster anions, as mentionedinsection B.l: Although InllNaz-clusteranionsshould have a magic number in the nonspherical shell of 14 e, it could not be observed as a magic number. Furthermore, there is no discontinuous drop in IPS between Inlo and In's. On the other hand, the stability attributed to the 2p shell closing is consistent with the appearance of the magic number of the In12Na3-cluster anion. In the series of In,Nal clusters, therefore, the valency changes from monovalent to trivalent between n = 7 and 13. As discussed previously, only the p electron contributes to the electronic shell in the pure indium cluster a t least up to n 13, because the s/p hybridization is not completed. The 2p shell closing of the In13Nalcluster indicates that N a addition induces s/p hybridization of the pure Inl3cluster and the In atoms become trivalent. As described in section B.l, the promotion of s/p hybridization with Na addition can also be observed in In,Na,cluster anions around n = 13. In the series of In,Naz, there is also a discontinuous drop in IPS between n = 6 and 7. This feature is also attributed to the electronic shell, but there are two possibilities: 8 electrons ( l p shell) where the In atom is monovalent and 20 electrons (2s shell) where the In atom is trivalent, although we cannot identify which shell corresponds to the electronic shell of InbNaz at present. B. Mass Distributionsof Io,Na,- and Al,Na,- Cluster Anions. In the mass spectra, it has been found that neutral or charged metal clusters with a certain number of atoms are more abundant than others. These "magic numbers" have been characterized by electronic and/or geometric factors.3*'9 In In, clusters, their IPS are relatively high so that an exact abundance of neutral In,, cluster cannot be determined by the use of a commercial UV laser. Then we examined the stability of In,Na,- cluster anions in the beam. In this section, we discuss the electronic shell structures of In,,Na,- and Al,Na,- cluster anions from the point of view of magic numbers. B.1. In,Na,- Cluster Anions. The intensity distributions in In,,-, In,Nal-, InnNaz-, In,Na3-, and In,Na4- cluster anions are separately shown in Figure 4. The distributions were obtained from the mass spectrum adjusted for medium-sized clusters around n = 15 at appropriate voltages for a series of deflection plates. When one pays attention to the intensity distribution of In,- in Figure 4a, a step in the intensity distribution can be found between n = 13 and 14. A similar distribution has already been reported by Gausa et al:I0 they have observed a slightly enhanced intensity for n = 13. The slightly enhanced intensity of 111'3- coincides with the electronic shell closing for 40 electrons if s/p hybridization is completed in the cluster anions. They have also measured the photoelectron spectra of Inl3-, in which no clear evidence of electronic shells is shown, and have concluded that the intensity enhancement might be attributed to a geometric shell and In atoms are monovalent.I0 Indeed, 13 atoms cluster can take a close-packing structure, such as an icosahedral structure. In the series of In,Nal-and InnNa2-(Figure 4b and 4c), there are no clear magic numbers, although the intensity of In12Nal-
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Figure 4. Mass distributionsof In,Na,- clusters: (a) Inm-,(b) In,Nal-, (c) In,Nal-, (d) In,Na3-, and (e) InnNa4-. There are several magic numbers of Inls-, InllNaJ-, and InllNar-,which are attributed to an electronic structure.
seems to be slightly enhanced. In the series of InnNa3-,however, there is a conspicuous intensity step between n = 12 and 13, indicating that the In12Na3-cluster anion is stable geometrically or electronically. When one considers the geometric structure of a 15-atom cluster, there is one stable structure of close packing: a part is body-centered cubic (bcc). Moreover, its stability can also be explained by the electronic shell model; the total number of electrons in InlzNa3- is 40 e, which strictly satisfies the 2p shell closing when In atoms are trivalent. Although the geometric and electronic stabilizations are exclusive, it is reasonable that the electronic factor can be attributed to the stability of the In12Na3cluster anion because the IP pattern of neutral In,Nal clearly shows the closing of the 2p shell at the In13Nal cluster. The result indicates that Na atoms induce s/p hybridization in the indium cluster anion, as well as the neutral In13Nal cluster described in the preceding section. In the series of InnNa4-, furthermore, there is a local minimum at n = 12, which is attributed to the instability of the InlzNa4cluster anion. When one N a atom is added to the InlzNa3- cluster anion which is stable because of the closing of the 2p electronic shell, a valence electron from the N a atom should open the next electronic shell ( I f ) , forming the In12Na4- cluster anion. The Inl2Na4- cluster anion is so electronically unstable that the abundance is relatively small compared to neighbors. As mentioned above, neutral In13Naland the InlzNa3-cluster anion are stabilized by the electronic shell effect in which In atoms should be trivalent but In13- is stabilized geometrically and the In atoms are monovalent. Since one valence electron is weakly bound in the N a atom, the difference between In13- and In13Nalis only the existence of the Na+ ion core. Therefore, the result ofs/p hybridization manifests that theNa+ ioncore perturbs the electronic structure of In. electronically (or geometrically).
IndiumSodium Bimetallic Clusters
The Journal of Physical Chemistry, Vol. 97, No. 1, 1993 89
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clusters: (a) AI,,-, (b) AI,Nal-, (c) AI,Na2-, (d) AI,Nal-, and (e) A1,Na4-. There are many magic numbers of A113-, A h - , All zNal-, A h N a l - , Al~gNaz-,A112Na3-,AlZzNaj-, All INa4-, and A1z1Na4-, which are attributed to an electronic structure.
A theoretical calculation on the electronic states of the In,Na, clusters is in progress in our group. B.2. Al,,Na,- Cluster Anions. Although thes/p hybridization in the indium clusters can be induced by the addition of N a atoms around n = 13, it is known that the s/p hybridization in aluminum clusters is completed around n = 7. Therefore, the features in the intensity distributions of the Al,Na,- cluster anions can be clearly understood by the electronic shell structures as the mixture between trivalent A1 atoms and monovalent N a atoms. Figure 5 shows the intensity distributions of the AI,Na,- ( m = 0-4)cluster anions. As reported previously,12 the intensity distribution of pure A1,- clusters has two apparent magic numbers of n = 13 and 23. Both of them can be explained by the electronic shell structure; the total number of valence electrons of Al13- and Al23- is 40 and 70, respectively, and the numbers strictly satisfy the shell closings of the 2p and 3s shells, where A1 atoms are trivalent. Figure 5bshows theintensitydistributionoftheAI,Nal-clusters from n = 6 to 32. The intensities of Al13Nal-and A12fial- are low compared with those of neighboring clusters. As deduced from the electronic shell model, AlljNal- and A1z3Nal- are unstable because they have one more excess electron than the number satisfying the electron shell closing. In contrast, the intensities of Al12Nal-and A122Nal-are relatively high. This is because they are almost satisfied with the electronic shell though they lack two electrons to the close 2p and 3s shells strictly. Figure 5c shows the intensity distribution of the A1,Na2-clusters from n = 5 to 32, and there is one prominent peak at n = 19. Although it cannot beexplained by the electronic shell, thestability of the 60-electron cluster can be predicted to accompany the subshell closings of distorted geometry.20.21 In the IPS of the
copper cluster, a significant IP decrease is observed between n = 60 and 6 1.22 It should be noted that even-odd alternation of intensities can be clearly observed in the mass distribution of the Al,Na2- cluster anions compared to those of the A1,- and Al,Nal- clusters. The alternation can be explained by a common rule: when the total number of valence electrons is even, then the cluster is relatively stable because of the stabilization by a pairing of electrons. Similar alternation has been found in the mass distributions of noble metal cluster ions; in the mass distributions of cluster cations/ anions of copper, silver, and gold, the ion intensities of odd-n clusters are greater than those of even n.23 Since noble metal atoms are monovalent, the singly charged odd-n clusters have even numbers of total valence electrons. But no explanation has been made yet on the reason why N a addition enhances the evenodd alternation in abundance. Figure 5d shows the intensity distribution of AI,Na3- clusters from n = 7 to 32. In the distribution, All2Naj- and A122Na3appear as a peak and an intensity step in the mass distribution, respectively, as well as large even-odd alternation. These two magic numbers can be explained by the electronic shell model. Both of them strictly satisfy electronic shell closings of the 2p and 3s shells, respectively, so that they are stabilized electronically. In a series of Al,Na4-, as shown in Figure 5e, the intensities of the AllINa4-and AI2,Na4- cluster anions are relatively high compared with neighbors. Since the total number of valence electrons exceed those of strict electronic shell closings by 1 in the A112Na4- and A122Na4- cluster anions, both of them are electronically unstable and, instead, All INa4- and A1z1Na4become more abundant. The intensity distribution of Al,Na4is almost the same as that of Al,Nal-, though the number of aluminum atoms decreases by one from Al,Nal- to the corresponding A1,Na4-. This indicates that one A1 atom is electronically equivalent to three N a atoms.
IV. Conclusion In this work, we exemplified an electronic shell effect in In-Na and AI-Na bimetallic clusters. N a addition enables us to examine the electronic shell closing of In, and Al, clusters in IP discontinuity and magic numbers in mass distributions because we can add electrons one by one into the corresponding cluster. Especially in the In,Na, clusters around n = 13, Na addition induces s/p hybridization; one valence electron of the N a atom triggers the s/p hybridization.
Acknowledgment. We are grateful to Prof. S. Iwata (Keio University) for valuable discussions. This work is supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture. A. N . expresses his gratitude for partial financial support to this study from Kanagawa Academy of Science and Technology. References and Notes (1)
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Saunders, W. A.; Clemenger, K.; de Heer, W. A.; Knight, W.D. Phys. Rev. 1985, 832, 1366. (7) Bergmann, T.; Martin, T. P. J. Chem. Phys. 1989, 90, 2484. (8) Hoena, E. C.; Homer, M. L.; Persson, J. L.; Whetten, R. L. Chem. Phys. Left. 1990, 171, 147. (9) (a) Schriver, K. E.; Persson, J. L.; Honea. E. C.; Whetten, R. L. Phys. Reu. L e f f .1990,64, 2539. (b) Persson, J. L.; Whetten, R. L.; Cheng, H.-P.; Berry, R. S.Chem. Phys. Left. 1991, 186, 215.
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