Ionization potentials of gold—sodium (AunNam) bimetallic clusters

Volume 2 11, number 6

CHEMICAL

PHYSICS LETTERS

Ionization potentials of gold-sodium Kuniyoshi

Hoshino,

Takashi

Naganuma,

Katsura

27 August 1993

(Au,Na, Watanabe,

) bimetallic clusters

Atsushi

Nakajima

and Koji Kaya

Department of Chemistry, Faculty of Science and Technology, Keio Universify, 3-14-l Hiyoshi, Kohoku-ku, Yokohama 223, Japan Received 20 May 1993; in final form I8 June 1993

Ionization potentials of gold-sodium (Au,Na,) bimetallic clusters were measured by a tunable ultraviolet laser combined with a time-of-flight mass spectrometer. The ionization potentials of Au,Na,,, clusters generally decrease with the number of Na atoms, but discontinuities in the ionization potentials of Au,Na, clusters were observed when the total number of valence electrons in the clusters tills an electronic shell. This result indicates that valence electrons of both Au atoms and Na atoms are delocalized in the clusters, forming the electronic shell.

1. Introduction

Alkali metal clusters have been well studied experimentally and theoretically for a decade. Magic numbers of the alkali metal clusters have been observed in mass distribution [ I], ionization potential (IP) [ 2-41, and electron affinity (EA) [ 51. The magic numbers can be successfully explained by a jellium shell model assuming a free-electron-like behavior of the valence electrons in a spherical potential. In addition to alkali metal clusters, clusters of divalent metal [ 61, trivalent metals [ 71, and coinage metals [ 8,9 ] also show the electronic shell effect in IPs and EAs. We recently have reported that the electronic shell effect was also observed in the bimetallic clusters composed of aluminium or indium (trivalent metal) and sodium (monovalent metal) by measuring their IPs [ l&l 1 ] : in the aluminiumsodium (Al,Na,) and the indium-sodium (In,Na,) clusters, the electronic shell is formed by all of their valence electrons. Yamada and Castleman have also reported that magic numbers were observed in mass distributions of bimetallic (monovalent metal-monovalent metal and trivalent metal-monovalent metal) clusters [ 121. However, in cobalt-sodium bimetallic clusters, the electronic shell effect cannot be observed [ 131, which can be attributed to half-filled d orbitals of cobalt atoms having [Ar]3d74s2 electronic configuration. Since d electrons are localized around the atoms, they cannot contribute to the for0009-2614/93/S

06.00 0 1993 Elsevier Science Publishers

mation of an electronic shell. In contrast, clusters of coinage metal (copper, silver, and gold) having fullfilled d orbital5 show the electronic shell effect in mass distributions, IPs, and EAs [8,9] in the same manner as observed in alkali metal clusters. In this work, we produced gold-sodium (Au,,Na,) bimetallic clusters by a laser vaporization method and determined the IPs of the bimetallic clusters by photoionization method. Though either Au, or Na. clusters show the electronic shell structure in themselves, each metal atom has a different character: an Au atom has closed d orbitals and a high ionization potential of 9.23 eV [ 141, whereas an Na atom has no d electron and a low ionization potential of 5.14 eV [ 141. We applied a two-laser vaporization method [ 15 ] to produce the Au,Na, clusters, and the electronic structure of the bimetallic clusters was discussed by the measurement of the IPs.

2. Experimental Details of experimental setup have been provided elsewhere [ lo]. Briefly, the Au,Na, bimetallic clusters were generated by a laser vaporization using two pulsed Nd3+ :YAG lasers ( 532 nm) . Each laser light was focused on the two rotating and translating rods (Au and Na), and the produced metal vapors were cooled with a pulsed He carrier gas ( 10 atm stagnation pressure). An experimental advantage of this

B.V. All rights reserved.

571

Volume 21 I, number 6

CHEMICAL

27 August 1993

PHYSICS LETTERS

method is that we can control the ratio of two components in the bimetallic clusters by adjusting their laser fluences. After the growth of the cluster in a channel (2 mm diameter and 8 cm length), the clusters were expanded to form a cluster beam and the bimetallic clusters were photoionized by the light of an ultraviolet laser. An ArF excimer laser (6.42 eV) or the second harmonic of a dye laser pumped by a XeCl cxcimer laser was used as the ionization lasers. The photoions were accelerated in a static electric field and were mass-selected by a time-of-flight (TOF) mass spectrometer equipped with a reflectron. The ions were detected by a dual multichannel plate and the signal was digitized and averaged in a digital oscilloscope (LeCroy 9400) coupled with a microcomputer (NEC PC-980 1). The photoionization mass spectra of bimetallic clusters were measured by averaging 700-800 outputs, at 0.03-0.05 eV intervals in the range of 4.2-5.9 eV. The fluence of ionization lasers was kept at 200-350 pJ/cm2 in order to avoid a multiphoton process. After normalizing the ion intensities by the laser fluence, the IPs of the Au,Na, clusters were determined from final decline of the ion intensities as a function of the photon energy.

0

0

2 Number

4

6 of

6

0

10

Na atoms

2

2 Number

4

4

6

8

6 6 of Na atoms

Fig. 1.Ionization potentials of Au,Na, clusters as m (number ofNaatoms, O
10

10

a function of (b) AurNa,,,, that the IP is circle.

3. Results and discussion 3.1. Ionization potentials of Au,Na, function of Na atoms (m)

clusters as a

Figs. 1 and 2 show IPs of Au,Na, clusters (n= 6l3)asafunctionofNaatoms(m,O
4a 0

2

4

6

8

10

40

0

2

4

6

8

10

4.O+j-edd - Number’01

NH atoms

-

Number

of Na atoms

Fig. 2. Ionization potentials of Au.Na, clusters as a function of m (number of Na atoms, O
creases largely with coverage of alkali atoms on the metal surface [ 161. As shown in figs. 2b-2d, Au, iNa,, Aui2Nam ( m = 6 and 8 ) and Au,~N~~ (m = 5 and 7) have relatively

Volume 2 1 I, number 6

CHEMICAL

high IPs in spite of the Na atom additions. Hereafter we use the abbreviated notation n-m, which corresponds to Au,Na,. A discontinuous drop of IP occurs when one more Na atom is added to the 11-7, 12-6, 12-8, 13-5, and 13-7 clusters. The phenomenon can be explained by the electronic shell model. It has been known that the spherical jellium model holds well for alkali clusters [ 1-5 ] and coinage metal clusters [8,9]. The model predicts the clusters are electronically stabilized at n= 8, 18, 20, 34, 40, 58, ... . Since both the Au atom and the Na atom are monovalent, all of 11-7, 12-6, and 13-S clusters have 18 valence electrons in the clusters, and both 12-8 and 13-7 clusters have 20 valence electrons in the clusters, which correspond to 1d and 2s shell closing, respectively. Therefore, the clusters with closed electronic shells are electronically stabilized and show high IPs. When one Na atom is added to these closedshell clusters ( 11-7, 12-6, 12-8, 13-5, and 13-7), the IPs decrease discontinuously as shown in figs. 2b2d. This is because the one excess valence electron opens a higher shell, resulting in the IP drop. Other discontinuities in the IPs are observed at 11-3, 122, and 13-l clusters. These clusters have 14 valence electrons which satisfy the 1d subshell closing [ I, 171. The discontinuities depend only on the total number of atoms in the Au,Na, clusters, n+ m. These results indicate that in the Au,Na,, the 6s electrons of Au atoms and the 3s electrons of Na atoms are delocalized in the Au,Na, clusters, forming electronic shell although there is a large energy gap (4.09 eV) between 6s of the Au atom and 3s of the Na atom. In addition to the electronic shell structures, an obvious even-odd alternation can be observed in IPs. As shown in figs. la-ld, for example, Au6Nam and AusNam clusters have high IPs with an even number of Na atoms (m), and have relatively low IPs with odd m. In contrast, Au,Na, and Au,Na, clusters have high IP with odd m, and have relatively low IP with even m. This means that the alternation is determined by the total number of valence electrons (n+m) in Au,Na, binary clusters. In order to see in more detail the even-odd alternation of IP values, we will discuss in section 3.2. the IP variation against the number of gold atoms n.

27 August 1993

PHYSICS LETTERS

3.2. IPs of the Au,Na, clusters as a function of Au atoms (n) In section 3.1, IP variation against the number of Na atoms m was discussed. The existence of the electronic shell model for the stabilization of the binary clusters was clearly established. In addition to this, even-odd alternation of the IP values which manifests itself by the removal of orbital degeneracy due to the structural distortion from spherical symmetry, was also discussed. Because an Na atom works as an electron donor, m-dependent IP variation is always affected by IP decrement due to Na doping. Instead, an Au atom has high IP and n-dependent IP of Au,Na, clusters keeping m constant may give more precise information on even-odd alternation of IPs. Fig. 3 depicts n-dependent IP variation of Au,Na, clusters taking m constant (m= 3, 4, 6 and 7). Figs. 3a and 3b clearly exhibit even-odd alternation in the region of the cluster size (n + m) less than 16. Taylor et al. have reported the photoelectron spectra of gold cluster anions and have observed the even-odd alternation of electron affinity (EA) [9]. The large amount of alternation of EA (1 eV) of gold cluster anions was ascribed to the wide energy splitting of the orbitals and the large atom-atom interactions in the clusters. In Au,Na, clusters, the observed amount of alternation is as large as pure gold clusters whether

.._ -7.0 > U6.5

6

7

6

...-.__.-._

9

10 11 12 13

--_

7

8

9

10 11 12 13

9

10 11 12 13

0

$6.0 e

6

(b)-...--. m=4

t

t

t

5

6 7 8 Number

of Au atoms

Fig. 3. Ionization potentials of Au,Na, clusters as a function of n (number of Au atoms, 6
573

Volume 2 11, number 6

CHEMICAL

PHYSICS LETTERS

or not one chooses n (number of gold atoms) or m (number of sodium atoms) as a variable to investigate the even-odd alternation. This indicates that the atom-atom interactions of Au,Na, clusters is as strong as that of gold clusters even though binding energy of sodium dimer (0.74 eV) [ 181 is several times smaller than that of gold dimer (2.29 eV) [ 191. By the cooperative interaction of gold and sodium, sodium and gold atoms mix up together to form stable binary clusters. In other words, a sodium atom in the gold cluster plays the role of both an electron donor to gold atoms and a binding partner to form a stable bond with the gold atom. The feature of Au,Na, clusters discussed above may be described as localized molecular orbitals instead ofjellium model. However, once the cluster size (n+m) goes to 16 or more up to 20, even-odd alternation of IP smears out and clear evidence of the electron shell structures in the cluster stabilization appears. Figs. 3c and 3d show that IP values increase smoothly from the cluster size 16 to 18, then drop discontinuously at 19 and increase again at 20. The occurrence of magic numbers at 18 and 20 which correspond to the shell closing of Id and 2s shehs, irrespective of the choice of variable ,n or m, firmly establishes the fact that valence electrons from Na and Au atoms contribute almost equally to the closing of the electronic shell. The electronic character of the Au,Na, clusters described above may also influence the surface reactivity of the clusters. The study on the adsorption of NO and O2 on the Au,Na, clusters is in progress in our laboratory.

Acknowledgement This work is supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture. One of the authors (KH) expresses his gratitude to the Japan Society for the Promotion of Science for Japanese Junior Scientists.

574

27 August 1993

References [ I] W.D. Knight, K. Clemenger, W.A. de Heer, W.A. Saunders, M.Y. Chou and M.L. Cohen, Phys. Rev. Letters 52 (1984) 2141; W.A. de Heer, W.D. Knight, M.Y. Chou and M.L. Cohen, Solid State Phys. 40 ( 1987) 93. [Z] E.C. Honea, M.L. Homer, J.L. Persson and R.L. Whetten, Chem. Phys. Letters 17 1 ( 1990) 147. [ 3 ] M.M. Kappes, M. Schar, U. Rothlisberger, C. Yeretzian and E. Schumacher, Chem. Phys. Letters 143 ( 1980) 25 1. (4 ] Ph. Dugourd, D. Rayane, P. Labastie, B. Vezin, J. Chevaleyre and M. Broyer, Chem. Phys. Letters 197 (1992) 433. [ 5] J.G. Eaton, L.H. Kidder, H.W. Sarkas, K.M. McHugh and K.H. Bowen, in: Physics and chemistry of finite systems: from clusters to crystals, Vol. 1,eds. P. Jena, S.N. Khanna and B.K. Rao (Ktuwer, Dordrecht, 1992) p. 493. [4] M. Rupple and K. Rademann, Chem. Phys. Letters 197 (1992) 280. [ 7 ] K.E. Schriver, J.L. Persson, EC. Honea and R.L. Whetten, Phys. Rev. Letters 64 ( 1990) 2539. [S] I. Katakuse, T. Ichihara, Y. Fujita, T. Matuo, T. Sakurai and H. Matuda, Intern. J. Mass Spectrom. Ion Processes 67 (1985) 229; 74 (1986) 33; M.B. Knickelbein, Chem. Phys. Letters 192 ( 1992) 129; G. Alameddin, J. Hunter, D. Cameron and M.M. Kappes, Chem. Phys. Letters 192 ( 1992) 122. [9 ] K.J. Taylor, C.L. Pettiette-Hall, 0. Cheshnovsky and R.E. Smaley, J. Chem. Phys. 96 (1992) 33 19. [lo] A. Nakajima, K. Hoshino, T. Naganuma, Y. Sone and K. Kaya, J. Chem. Phys. 95 ( 199 1) 7061. [ 111 A. Nakajima, K. Hoshino, T. Sugioka, T. Naganuma, T. Taguwa, Y. Yamada, K. Watanabe and K. Kaya, J. Phys. Chem. 97 (1993) 86. [ 121 Y. Yamada and A.W. Castleman Jr., J. Chem. Phys. 97 (1992) 4543. [ 131 K. Hoshino, T. Naganuma, Y. Yamada, K. Watanabe, A. Nakajimaand K. Kaya, J. Chem. Phys. 97 ( 1992) 3803. [ 141 C.E. Moore, Analysis of optical spectra, NSRDS-NBS Circular No. 34 (US GPO, Washington, 1971); R.C. Weast, ed., Handbook of chemistry and physics, Vol. 61 (CRC Press, Boca Raton, 1980) p. E-69. [ 151 S. Nonose, Y. Sone, K. Onodera, S. Sudo and K. Kaya, J. Phys. Chem. 94 ( 1990) 2744. [ 161 H.P. Bonzel, Surface Sci. Rept. 8 (1987) 43; H.P. Bonzel and C. Ertl, eds., Physics and chemistry of alkali adsorption (Elsevier, Amsterdam, 1989). [ 171 W.D. Knight, W.A. de Heer and W.A. Saunders, Z. Physik D 3 (1986) 109. [ 181 Y. Wang, M. Kajitani, S. Kasahara, M. Baba, K. Ishikawa and H. Kato, J. Chem. Phys. 95 (1991) 6229. [ 19 ] G.A. Bishea and M.D. Morse, J. Chem. Phys. 95 ( 1991) 5646.