J. Phys. Chem. 1992, 96,4211-4216
4271
Structures and Bonding Properties of Ca-0 Clusters Inferred from Mass Spectral Abundance Patterns Paul J. Ziemann and A. W. Castleman, Jr.* Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received: November 6, 1991; In Final Form: February 18, 1992)
Calcium oxide clusters were produced by a gas aggregation technique and investigated using time-of-flight mass spectrometry. Depending on the source conditions, either singly or doubly charged clusters dominate the mass spectrum. Singly charged (CaO),Ca,+ (m = 0-5) clusters are observed for higher flows of inert carrier gas, whereas primarily doubly charged (CaO),Ca2+ (n = 3-100) clusters are observed at lower flows. The mass spectral abundance patterns indicate that the clusters have cubic structures resembling pieces of the fcc lattice of solid CaO. The most stable structures appear to be cuboids, cuboids with a complete terrace, and cuboids which have one 0 atom vacancy for each excess metal atom in the cluster. The latter structures may be stabilized by the donation of electrons from excess metal atoms into the vacancies, creating defects similar to solid-state color centers, or by the formation of metal-metal bonds.
Introduction One of the most powerful tools at the chemist’s disposal is the periodic table. For as one endeavors to unravel the mysteries of a particular chemical system, there is usually much to be learned by the systematic investigation of closely related systems, the properties of which can oftentimes be varied gradually by judiciously substituting one element for another. This is the approach we have taken in a study of metal oxide clusters. Recently, we reported the raults of investigations of singly’ and doubly2charged Mg-O clusters, in which mass spectrometry and theoretical techniques were employed to obtain information on the growth, fragmentation, and structural and electronic properties of these clusters. Here we report the results of an investigation of Ca-O clusters. Calcium is positioned directly below Mg in the group IIA metals. Therefore, like Mg, which has two 3s valence electrons, the chemical properties of Ca are determined primarily by the behavior of its two 4s valence electrons. Although Ca-O and Mg-O clusters can be expected to have similar properties, interesting differences are also possible since Ca has a larger radius and polarizability as well as a lower ionization potential and both the bulk metal and metal oxide are more strongly bound that their Mg ~ u n t e r p a r t s . ~ For . ~ example, whereas recent investigations indicate that pure Mg5 and Ca6 clusters have similar structural properties, in collision-induced fragmentation studies of small, stoichiometric, or nearly stoichiometric Mg-O and Ca-O clusters, the primary fragmentation pathways are observed to be quite different for the two systems.’ The Ca-0 cluster distributions obtained in the present study allow an extensive comparison with those obtained previously for Mg-O clusters. In addition, observations on new metal-rich cluster series yield structural information on clusters with compositions between the stoichiometric metal oxide and pure metal, which can be compared with the results of a recent theoretical investigation of the ”metalization” of ionic clusters.8
Experimental Section The apparatus used in these experiments is described in detail el~ewhere.~Basically, inside a liquid nitrogen-cooled source (1) Ziemann, P.J., Castleman, Jr., A. W. J . Chem. Phys. 1991, 94, 718. (2) Ziemann, P. J., Castleman, Jr., A. W. Phys. Rev. B 1991, 44, 6488.
(3) CRC Handbook of Chemistry and Physics, 63rd ed.; Weast, R. C., Astle, M. J., Eds.; CRC Press: Boca Raton, FL, 1982-1983. (4) Ha, T. In Molecular Elecfro-Optics; Part 1; O’Konski, C. T., Ed.; Marcel Dekker: New York, 1976; p 508. ( 5 ) Martin, T. P.,Bergmann, T., GBhlich, H., Lange, T. Chem. Phys. Lerr. 1991. 183.
119.
(7) Saunders, W. A. Phys. Rev. B I! (8) Rajagopal, G., Bar]
chamber where the pressure is about 5 Torr, Ca metal at 850-950 O C is evaporated from a boron nitride crucible that is resistively heated by a tungsten wire and heat shielded around the circumference and on the bottom by a tantalum sheet. The metal vapor is entrained in 1000-2500 sccm of cold He and up to 5 sccm of N 2 0 , whereupon it cools and undergoes concomitant clustering reactions. The residence time in the source is about 50 ms and is relatively constant for all He flows since the pressure is nearly proportional to the flow rate. Gas exits in the source chamber and travels through an inverted U-tube into a flow tube a t about 0.5-Torr pressure, where an additional H e flow is introduced to maintain the total flow close to 4000 sccm. Most of the gas is pumped away by a Roots pump at the end of the flow tube, but a small fraction passes through an on-axis hole in a sampling cone and into the ionization region. The sampled flow tube effluent is then ionized by a focused excimer laser beam of 308-nm wavelength (XeCl excimer gas, 4.03-eV photons) and accelerated to about 2-kV energy in an electrostatic lens assembly before entering the detection region for time-of-flight mass analysis. Typical laser powers, averaged over a 1-cm2 iris through which the beam passes prior to entering the vacuum chamber, are about 5-20 mJ/pulse. Average power densities are about 0.15-0.6 MW/cm2, but because the beam is focused, the power densities in the ionization region are much higher. Fourier transform techniques were used to remove high-frequency noise from the mass spectra, but great care was taken to ensure that this procedure had no significant effect on the cluster peak heights.
Results and Discussion The effect of source conditions on the Ca-O cluster distribution is similar to that observed previously for Mg-0 clusters.’v2 In particular, only Ca+and Ca2+are observed when pure He is used as a carrier gas, and upon adding a few sccm of N 2 0 to the source, peaks corresponding to Ca-0 clusters appear in the mass spectrum. Likewise, only singly charged Ca-O clusters are observed when the He flow is greater than about 2000 sccm (high flow regime), whereas doubly charged clusters dominate the mass spectrum when the flow is reduced to about 1000 sccm (low flow regime). The shift in the distribution from singly to doubly charged clusters is apparently due to the production, at low flow, of clusters containing more excess metal. This added metal probably reduces the ionization potentials of the clusters, allowing them to be doubly ionized before they lose the weakly bound excess metal atoms through evaporative cooling. Within the two flow regimes the primary cluster series are the same for the Ca-O and Mg-O systems, the dominant stoichiometries being (MO),or (MO),M (M denotes a metal atom), although many of the Ca-O clusters also contain more than one excess metal atom. (9) Farley, R. W., Ziemann, P. J., Castleman, Jr., A. W. 2.Phys. D 1989, 14. 353.
0022-365419212096-4271$03.00/0 0 1992 American Chemical Society
4272 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 30
Ziemann and Castleman
TABLE I: Magic Numbers and Proposed Structures of (CaO),Cn*+ Clusters"
20
10
. "
..
. . 8
I
11
14
22
I 4 0
>
(CaO),Ca2+
structure
(CaO),Ca2+
5 7 9 11 13 16 19 22 25 27 29 31
3X2X2-1 4X2X2-1 5 X 2 X 2 -1 4X3X2-1 3X3X3 3X3X3+3X2 4 X 3 X 3 + 3 X l 5X3X3 5X3X3+3X2 5X3X3+5X2 5X4X3-1 7 X 3 X 3 4X4X4-1
45 47 49 52 55 57 59 62 61 72 74 79 82
6X4X3-1 5 X 5 X 3 5X4X4-1 l X 4 X 3 - 1 5X5X3+4X3
92 95 97 99
33 35 37 39 41 43
87
structure 6X4X4-1 5X5X4- 1 7 X 5 X 3 7X4X4-1 5X5X4+5X3 6X5X4-1 5X5X5 9X5X3 5X5X5+5X4 6X5X5-1 8X5X4-1 11X5X3 7 X 5 X 5 8X5X4+5X5 8X6X4-1 13X5X3 8X5X5-1
> 3
"The a X b X c designation corresponds to a cuboid structure with a, b, and c atoms along each edge, the -1 refers to an 0 atom vacancy, and the a X d designation refers to a complete terrace on an o X b or
w 2
face of a cuboid. When more than one structure can be used to explain the occurrence of a maximum, the most symmetric and/or compact one is listed.
E
Y
: z
aXc
!-
z 1
0
11
14
20
17
26
23
I
3 I, 62
n
74
are also present in high abundance up to about (Ca0)9+ and (Ca0)14Ca+,whereas (CaO),Ca?+ and (CaO),Ca:+ clusters are present in low abundance. The (CaO),Ca2+ and singly charged cluster peaks are resolved up to about n = 29 [(Ca0),4Ca+ is the largest resolved peak], beyond which point the latter appear as humps on the sides of the doubly charged cluster peaks. This overlap may increase the apparent intensities of (CaO),Ca2+ clusters, but the intensities of the singly charged clusters decrease so sharply beyond (CaO),,Ca+ [the peak to the right of (Ca022Caz+]that the contributions are probably small. The (CaO),Caz+ cluster distribution exhibits a pronounced pattern of magic numbers that can be explained in terms of the exceptional stabilities of cubic structures which resemble pieces of the fcc lattice of solid CaO. The three most stable structural types are compact cuboids and cuboids with a single 0 atom vacancy or a complete terrace. These can be written as (1) u X b X c, (2) a X b X c - 1, and (3) u X b X c u X d , where the a X b X c designation refers to a cuboid with (I,b and c atoms along each edge, the -1 refers to an 0 atom vacancy, and the a X d designation refers to a complete terrace on an u X 6 or a X c face of a cuboid. For some of the maxima there is more than one structure that can be used to explain the exceptional stability of the corresponding cluster, but the predominant isomers are probably the structures that are the most symmetric and compact. The magic numbers and proposed structures are listed in Table I. Recently, mass spectra of doubly charged (CaO),Ca2+ clusters were produced by using a source similar to ours and the abundance minima were explained in terms of the instabilities of the corresponding clusters relative to those with cuboid structure^.'^ However, there is no discussion of the maxima observed, and it is difficult to determine their locations from the mass spectrum presented. The magic numbers in mass spectra of (CaO),Ca2+ clusters are similar to those observed for (MgO),Mg2+ clusters, although there are some differences. The most important discrepancies are in the range n < 11, where maxima occur at n = 5 , 7 , and 9 for (CaO),Caz+ clusters, compared to a lone maximum a t (MgO)8Mg2+. One possible set of structures for these three (CaO),Ca2+ clusters begins with a 3 X 2 X 2 - 1 structure and continues by the addition of square (CaO)* units until a 5 X 2 x 2 - 1 structure is reached. In collision-induced fragmentation studies of small (CaO),+ and (CaO),Ca+ clusters ( n I 8) the
+
04 24
I
,
,
I
28
I
,
I
'
32
,
,
I
I
36
TI ME-OF- FLIGHT ( YS Figure 1. Time-of-flightmass spectra of (CaO),Ca2+clusters. In the upper two spectra the (CaO),Ca2+peaks were darkened to make it easier to differentiate them from the singly charged cluster peaks, but in the bottom spectrum this was unnecessary.
The distributions of both singly and doubly charged clusters exhibit pronounced patterns of abundance which result from laser-induced fragmentation of the clusters during the ionization process. The maxima, or so-called "magic numbers", appear in the mass spectra because clusters that are more stable than their neighbors are more resistant to fragmentation and thus acquire enhanced relative ab~ndances."'-~' (CaO),Ca*+ Clusters. A mass spectrum of Ca-0 clusters obtained in the low flow regime is shown in Figure 1. The major series of peaks corresponds to doubly charged (CaO),Ca2+ clusters with n = 3-100. Singly charged (CaO),' and (CaO),Ca+ clusters (IO) Mark, T. D., Castleman, Jr., A. W. Adu. At. Mol. Phys. 1985, 20, 65. (11) Castleman, Jr., A. W., Keesee, R. G. Annu. Reu. Phys. Chem. 1986, 37, 25. (12) Martin, T. P. Phys. Rep. 1983, 95, 167. (13) Ens, W., Beavis, R., Standing, K. G. Phys. Reu. Let?. 1983, 50, 27.
(14) Martin, T. P., Bergmann, T. J . Chem. Phys. 1989, 90, 6664.
The Journal of Physical Chemistry, Vol. 96, No. 11, I992 4273
Structures and Bonding Properties of Ca-0 Clusters
A
0 4 , 0
1
I
,
I
I
3
I
I
6
I
9
I
I
1
12
I
I
I
i
15
1010,
I
10 10,
1
(cao),ca;
2
(cao),ca; 8
84
6-
6
7
10
4-
4
2-
oi
13
I
I
I
1
b
3
0
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6
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I
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9
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8
2
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o0 - L , , , , . , , , , , , , , , l 0
3
6
9
12
15
Figure 2.
(CaO)z unit has been observed to be very table.^ For n 1 11 there are also a few differences between the magic numbers in (CaO),Ca2+ and (MgO),Mg2+ mass spectra, but usually these can be explained in terms of slightly different preferences for cubic structures. For example, the maxima at (Ca0)41Ca2+and (Ca0)43Ca2+are apparently due to the exceptional stabilities of 7 X 4 X 3 - 1 and 5 X 5 X 3 4 X 3 structures, respectively, whereas a maximum occurs at (Mg0)42MgZ+which is probably due to the 5 X 5 X 3 5 X 2 structure. A more interesting feature in the mass spectra is the difference in the intensities of magic and nonmagic (CaO),Ca2+ clusters, which is much more pronounced than was observed for (MgO),Mg2+ clusters. It may be that Ca-O bonds have more ionic character than Mg-O bond^^^,'^ and that the greater charges on the Ca and 0 ions require relatively complete cuboid structures for high stability, whereas clusters with partially completed terraces easily undergo fragmentation. (CaO),Ca,+ Clusters. The mass spectra obtained under high flow conditions are composed of singly charged clusters of the series (CaO),,Ca,+, with m = 0-5. Plots of the relative peak intensities as a function of cluster size are shown in Figure 2. The
+
+
(15) Binks, J. H., Duffy, J. A. J . Solid State Chem. 1990, 87, 195. (16) Phillips, J. C. Rev. Mod. Phys. 1970, 42, 317.
actual mass spectra are not presented because the high density of cluster peaks makes it too difficult to follow trends in intensities when all the series are observed together. The relative abundance of a particular cluster series depends on the laser power used for ionization, as is shown in Figure 3. The cluster distribution shifts toward clusters with less excess metal as the laser power is increased, indicating that the metal-rich clusters fragment by losing Ca atoms or Ca+ ions. The metal-rich clusters apparently are not produced by the lass of 0 atoms from stoichiometric clusters. Over the range of powers used in these experiments (5-20 mJ/pulse), the (CaO),Ca+ series was always more abundant than the (CaO),,+ clusters. This is apparently due to the higher stabilities of (CaO),Ca+ clusters, rather than particularly favorable photoionization cross sections, since in sputtering experiments7 they are also more abundant than the stoichiometric clusters (the only other series observed) even though no photoionization takes place. These mass spectra differ from those of Mg-O clusters, in which (MgO),+ clusters are generally more abundant than (MgO),Mg+ clusters, and singly charged clusters never contain more than one excess metal atom. The presence of more excess metal on Ca-0 than Mg-O clusters probably results from the slightly stronger metal-oxygen and metal-metal bonding in the Ca-0 system. (The atomization energies are 11.01 and 10.39 eV for solid CaO and MgO, re-
Ziemann and Castleman
4214 The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 TABLE II: Manic Numbers and Prooosed Structures of (CaO).Ca,+ Clusters magic numbers and structures cluster
(CaO).+ . .,.
4 2 X 2 X 2
3
(CaO),Ca+
2 2 2 3 3 4 4 5 5
(CaO),Ca2+ (CaO),Ca3+ (CaO),Ca,+ (CaO),Ca,+
x 2 x 2-1 X 2 X 2 - 2
6
9
3 X 2 X 2 5 3 x 2 x 2-1 4 3 X 2 X 2 - 2
3 X 3 X 2
9 X2X2-3
4 X 3 X 2 - 3
X 2 X 2 - 4
6 5 X 2 X 2 - 4
12" 4 X 3 X 2 11 4 x 3 x 2-1 10 4 X 3 X 2 - 2
8 3 x 3
x 2-1
7 3 X 3 X 2 - 2 12 5X3X2-3
1 5" 5 X 3 X 2 13"J 3X3X3 13 5 X 3 X 2 - 2
X2X2-5
"The signal intensity was too small to observe this magic number, but it was observed for C a - O clusters generated by sputtering.' bThe signal intensity in mass spectra of pure singly charged clusters was too small to observe this magic number, but it was observed when doubly charged clusters were also present. ,-
I
12
i
ii
(a) LOW POWER h
.-cIn. 8C
3
-
< 6v
> I-
(I, 4Z
E -
2 210
W
16
(b) HIGH POWER
I-
z
13
d
Figure 4. Plot of the peak intensities of (CaO),Ca32+ clusters, obtained from a time-of-flight mass spectrum.
2016-
TIME-OF-FLIGHT (PSI Figure 3. Time-of-flight mass spectra of (CaO),Ca,+ clusters obtained at (a) 5 mJ/pulse and (b) 10 mJ/pulse laser power. The numbers above the peaks correspond to the number of excess Ca atoms in the cluster (m). Plots for each value of m are shown in Figure 2.
spectively, and 1.85 and 1.52 eV for solid Ca and Mg, respectively.)) The (CaO),Ca,+ cluster distributions exhibit abundance patterns which, especially for m = 0-3, bear a definite relationship to one another. For (CaO),+ clusters, magic numbers appear at n = 4, 6, and 9, and although the intensities are too low to accurately analyze the distribution of clusters larger than these, magic numbers have been observed at n = 4, 6,9,12, and 15 for (CaO),+ clusters generated by sputtering.' This is the same pattern that we observed for (MgO),+ clusters, except that (MgO)*+was also a local maximum. The magic numbers in our (CaO),Ca+ cluster distribution a r u r at n = 3,5, 8, and 11, which is identical to the (CaO),+ cluster pattern, shifted to smaller sizes by one unit. The intensities are too low to determine magic numbers beyond n = 11, but a maximum occurs at n = 13 in the (CaO),Ca+ cluster distribution obtained when doubly charged clusters are present (low flow regime) and also in the sputtering
Figure 5. Proposed structures of (Ca0),2+, (CaO),,Ca+,(CaO)&a2+, and (CaO)&a,+ clusters.
experiments mentioned earlier. These magic numbers are the same as those observed for (MgO),Mg+ clusters, exept for the absence of a maximum at (MgO)3Mg+. The magic numbers in the (CaO),Ca2+ and (CaO),Ca,+ mass spectra are further shifted from the n = 4, 6,9,12, and 15 pattern by two and three units, respectively. Although no magic number appears at (CaO)6Ca3+, there is one at (Ca0)6Ca32+in the mass spectrum of doubly charged (CaO),Ca32+ clusters shown in Figure 4. There is no maximum at (CaO)Ca3+, but this could be expected since the pattern of magic numbers for the larger (CaO),Ca3+ clusters suggests that the three excess Ca a t o m preferentially bind to three different 0 a t o m which are part of a 3 X 2 rectangle or hexagon, whereas there is only one 0 atom in the (CaO)Ca3+ cluster. The progressive shift in the magic number patterns of (CaO),Ca,+ clusters is highly suggestive of a relationship among the structures that are responsible for the exceptional stabilities of these clusters. Proposed structures based on these patterns are listed in Table 11. The magic numbers observed for (CaO),' clusters can be explained in terms of cubic structures, and the preferred structures for metal-rich clusters with m = 1-3 are
Structures and Bonding Properties of Ca-0 Clusters apparently the same cuboids with one 0 atom vacancy for each excess metal atom in the cluster. An example of the proposed change in structure in going from (Ca0)12+to (CaO)&a3+ is shown in Figure 5. (For simplicity these are drawn as perfect cubic structures, although somewhat distorted forms would be more realistic.) The successive removal of 0 atoms from one face of the 4 X 3 X 2 cuboid leads to stoichiometries which all appear as magic numbers in the mass spectra, eventually resulting in a structure composed of a 3 X 3 X 2 metal oxide cuboid and a layer of metal atoms occupying all the 0 atom sites on a 3 X 2 face. Two plausible structures for (CaO)&a2+ are shown, although recent calculations on metal-rich alkali halide clusterss (discussed in more detail later) suggest that the upper one, with the neighboring vacancies, will have more metal-metal bonding and should therefore be more stable. The proposed structures corresponding to the magic numbers of the other (CaO),Ca,+ clusters can also be generated by successively removing 0 atoms from one face of the parent clusters. For m = 1-3 the parents are the most stable (CaO),+ cluster structures, and for m = 4 and 5 they are 4 X 2 X 2 and 5 X 2 X 2 cuboids. The latter two cuboids are the likely structures for the nonmagic (Ca0)8+and (CaO)lo+clusters and are the smallest parent structures that will allow four and five vacancies on a single face. Two plausible structures for (CaO)4Ca4+clusters can be generated from a 4 X 2 X 2 cuboid. One is obtained by removing four 0 atoms from one of the 4 X 2 faces to produce a rectangular (CaO), structure covered by a layer of four Ca atoms. The other is obtained by removing two 0 atoms from each of the 2 X 2 faces on the ends of the cuboid, leaving a 2 X 2 X 2 cube with one excess Ca atom bound to each of the 0 atoms. The magic number at (Ca0)&a4+ can be ascribed to a structure similar to the second of these, generated from a 5 X 2 X 2 cuboid by removing two 0 atoms from each of the 2 X 2 end faces and leading to a 3 X 2 X 2 structure with two excess metal atoms on each end. The magic number at (CaO)&as+ also appears to correspond to a layered structure, which can be generated by removing five 0 atoms from a 5 X 2 face of a 5 X 2 X 2 cuboid, leaving a (CaO)S rectangle and a layer of five Ca atoms. The apparent stability of these layered structures is very interesting, and it is worth noting that there exists a Ca2N compound in which layers of Ca atoms are located between CaN layers, stabilized by the donation of excess electrons from the Ca atoms [i.e., (Ca2+),(e-)(N3-)] .I7 We should emphasize that in the above discussion we are not proposing that the metal-rich clusters are actually produced by the loss of 0 atoms from clusters containing less excess metal. As we mentioned earlier, the effect of laser power on the distributions (Figure 3) indicates that this is not the case. We are merely treating the stoichiometric clusters as “structural parents”, in order to clarify the structural relationships. The exceptional stability of vacancy structures may be due to the donation of electrons from excess Ca atoms into the vacancies, leading to the formation of defects similar to the F and Fc centers found in bulk, solid CaO.Is This effect has been shown to stabilize vacancy structures in neutral alkali halide clusters containing one extra metal a t ~ m , ’ ~ as - ~well ’ as clusters with compositions up to, but not including, the pure metal.8 In the latter investigation it was observed in calculations that as halide atoms are successively removed from the cluster, excess electrons effectively replace them in the cluster lattice, such that the cubic structure of the stoichiometric alkali halide parent is maintained until all the halide atoms have been removed. Only then does the structure change significantly, as it transforms to the geometry of the pure metal cluster. Furthermore, for clusters containing multiple vacancies, (17) Cotton, F.A. Wilkinson, G. Advanced Inorganic Chemistry, 5th ed.; Wiley: New York, 1988; p 154. (18) Henderson, B., Wertz, J. E. Defects in Alkaline Earth Oxides; Wiley: New York, 1977. (19) Honea, E. C., Homer, M. L., Labastie, P., Whetten, R. L. Phys. Rev. Lett. 1989, 63,394. (20) Landman, U.,Scharf, D., Jortner, J. Phys. Reu. Lett. 1985.54, 1860. (21) Rajagopal, G., Barnett, R. N . , Nitzan, A., Landman, U.Phys. Rev. Lett. 1990, 64, 2933.
The Journal of Physical Chemistry, Vol. 96, No. 11, 1992 4215 the most stable structures are those in which the vacancies (or excess metal atoms) are all on the same face, as close together as possible. The calculations suggest that these metal-rich clusters may have a significant amount of “metallic” character and that it may be correct to think of them as being composed of a mixture of “metallic” and ionic components. Although our experiments were performed on a different material from those used in the calculations, our results agree very well with the predictions of exceptional stability for cuboid structures in metal-rich clusters and the formation of clusters containing segregated metal layers. In addition to enhancing the stability of vacancy structures in alkali halide clusters, the presence of an excess electron may lead to unstablez0 or complete cuboid structures (e.g., 3 X 3 X 3), depending on the nature of the ionic interactions. When stable, such cuboids have much lower vertical and adiabatic ionization potentials than vacancy clusters, since the excess electron is bound in a diffuse surface state rather than localized in an anion vacancy. The 3 X 3 X 3 structure is evidently quite stable for the magic (CaO)13Ca+,which could have one excess electron since Ca is divalent. However, the low relative abundance of (CaO) &a2+ clusters indicates that the single-vacancy structure formed by removing one 0 atom from a 3 X 3 X 3 cube cannot readily accommodate the excess electrons (possibly three, depending on the binding energy of CaZ+-e-) provided by two Ca atoms. It may be that the cluster has three excess electrons and that two can occupy a spin-paired state in the vacancy while the other occupies a diffuse surface state, which in this case leads to an unstable structure. A magic number does appear at (CaO) 13Caz+,which probably corresponds to the double-vacancy structure formed by removing two 0 atoms from a 5 X 3 X 2 cuboid. Such a structure would allow two spin-paired electrons to occupy one vacancy while the unpaired electron occupies the other, analogous to F and F+ centers in the bulk material. It has been shown that in the minimum-energy state of a neutral Na14F12 cluster with a 3 X 3 X 3 - 1 structure, a single surface vacancy is occupied by two spin-paired electrons.8 It would be worthwhile to investigate the stabilities and structures of metal-rich Ca-O clusters using the computational techniques which have been employed so successfully for the alkali halides. Since the strength Ca-Ca bonding may not be as strong as in alkali metal clusters, due to the closed subshell configuration of the Ca atoms, it would be interesting to compare the properties of metal-rich Ca-0 and alkali halide clusters. In a recent theoretical study of Mg, clusters (n = 1-7),zz it was observed that the bond energies of singly charged clusters are significantly larger than those of the neutrals, since the electron is removed from an antibonding orbital. A similar effect would be expected for Can clusters and could lead to an enhancement of the bonding between Ca atoms on a Ca-O cluster surface, where the 0 atoms to which the Ca atoms bind may also draw electrons out of antibonding orbitals. A thorough investigation of these clusters should provide further insight into the properties of mixed ionic-metallic systems.
Conclusions The mass spectra of Ca-0 clusters obtained in these experiments are similar in many ways to those observed previously for Mg-O clusters, although many of the Ca-O clusters contain more excess metal (beyond the stoichiometric composition) than did the Mg-O clusters. This is probably due to the slightly stronger metal-oxygen and metal-metal bonding in the Ca-O system. The magic numbers in mass spectra of (CaO),Caz+ clusters indicate that the clusters have cubic structures similar to pieces of the fcc lattice of solid CaO, with the most stable structures being cuboids and cuboids with an 0 atom vacancy or a complete terrace. A few of the magic numbers differ from those observed for (MgO),MgZ+ clusters, but these can usually be explained as resulting from preferences for slightly different cubic structures. The mass spectra of singly charged clusters are composed of clusters of the series (CaO),Ca,+, with m = 0-5. The abundance (22) Reuse, F.,Khanna, S.N., de Coulon, V., Buttet, J. Phys. Reu. B 1990, 41, 11743.
J. Phys. Chem. 1992, 96, 4276-4278
4276
patterns indicate that (CaO),+ clusters prefer cuboid structures and that the most stable structures for the metal-rich clusters are the same cuboids with one 0 atom vacancy for each exmetal atom in the cluster. It appears that the vacancies tend to aggregate on one face of the cluster, eventually leading to a segregated metal layer on a metal oxide cuboid. It may be that the vacancies are occupied by electrons donated from the excess metal atoms and that either these electrons or direct metal-metal bonding stabilizes the vacancy structures.
Acknowledgment. We gratefully acknowledge DuPont Chemicals for an unrestricted grant through the Department of Chemistry, and P.J.Z. thanks them for support through a Pennsylvania State University Particle Science and Technology Center DuPont Fellowship. We also thank Dr. Yasuhiro Yamada and Dr. Andreas Hartmann for helpful discussions during the course of this work and Dr. U. Landman (GIT) for some very helpful suggestions especially regarding the alternative mechanism shown in Figure 5 .
N
Double-Resonance Multiphoton Ionization Spectroscopy of the 6 Rydberg State of Ammonia Teruhiko Nishiya Institute for Molecular Science, Myodaiji, Okazaki 444, Japan (Received: November 12, 1991; In Final Form: January 16, 1992)
Double-resonance multiphoton ionization (MPI) spectra of the B-Rydberg-state of ammonia have been recorded from the symmetric stretching (YJ vibrational level of the ground electronic (X) state (X( 1,O,O,O)level). Symmetry and Franck-Condon (FC) apiderations suggest that the Y,’ vibronic (B(1,0,0,0)) state should be observed from the symmetric (s) inversion component of the X(l,O,O,O) state, but it is shown that only the antisymmetric (a) inversion component actually provides the double-resonance spectra in the wavelength range investigated. Rotational analysis of the bands indicates that an out-of-plane bending (vi) overtone level of the B state (B(0,3,0,0) level) contributes to the spectra. It is suggested that the B(1,0,0,0) state does not have sufficient stability to show double-resonance MPI spectra presumably because of rapid predissociation.
1. Introduction
Ammonia_has a pyramidal equilibrium geometry in its ground electronic (X) state. All of the observed excited electronic states of ammonia are Rydberg in character and have planar equilibrium geometries. A geometry change from pyramidal to planar causes each of the electronic transitions to be dominated by a long progression in the excited-state out-of-plane bending (vi) vibration as a consequence of the Franck-Condon (FC) principle. Therefore, it is short of spectroscopic information about the excited-state stretching vibrations in ammonia, although the stretching vibronic levels may play an important role in the predissociation of the excited states.l Among the first three singkt excited states, the predissociation of the vibronic levels of the A state has most extensively pee; studied by many researchers. Ashfold et al. have observed C-A dispersed emission spectra and have assigned the very diffuse bands to the combination levels of the symmetric strztching (q’) and out-of-plane bending vi) modes of the A state (A(l,n,O,O) levels).* And concerning the ’ state, Miller et al. reinvestigated the 2 + 1 resonance-enhanced multiphoton ionization (REMPI) photoelectron spectra in the vicinity of the b state origin of ammonia cooled in a supersonic jet.3 They reassigned the anomalous photoelectron spectra to the pro ression of the vl’ nu; combination levels of the state (&(l,n,O,O) levels). Related to the work presented here, Seelemann et al. first reported REMPI spectra of vibrationally excited state-selected ammoniae4 They prepared the combination level of the asymmetric stretching (v3) and asymmetric bending ( u 4 ) modes (X(0,0,1,1) level) by near-infrared (near-IR) laser radiation, and performed the 2 + 1 REMPI from that state by ultraviolet (UV)
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(1) Ashfold, M. N. R.; Bennett, C. L.; Stickland, R. J. Comments At. Mol. Phys. 1987, 19, 181. (2) (a) Ashfold, M. N. R.; Bennett, C. L.; Dixon, R. N.; Fielden, P.; Rieley, H.; Stickland, R. J. J . Mol. Speczrosc. 1986, 117, 216. (b) Ashfold, M. N. R.; Bennett, C. L.; Dixon, R. N. Faraday Discuss. Chem. SOC.1986, 82, 163. (c) Dixon, R. N. Chem. Phys. Lett. 1988, 147, 377. (3) Miller, P. J.; Colson, S.D.; Chupka, W. A. Chem. Phys. Lett. 1988, 145, 183. (4) Seelemann, T.; Andresen, P.; Rothe, E. W. Chem. Phys. Lett. 1988, 146, 89.
laser radiation. Allen et al. p e d the method to reveal FC disfavored vibronic levels of the B By rotational analysis of the spectra, they identified the B(O,n,l,O) states. Reported here are_double-resonance multiphoton ionization (MPI) spectra of the B state of ammonia from the X( 1,0,0,0) state. With consideration of the FC principle, chances of det_ectingthe B( 1,O,O,O)state should be much improved using _the X( 1,O,O,O) state as the intermediate state. Preparation of the X(l,O,O,O) state is performed by two excitation schemes. One is an excitation by I R laser radiation. The method is based on the experiment ,Of Seelemann et al.4 and is used in the hope-of observing the B(1,0,0,0) state with the IR excitation of the X( 1,0,0,0) state. And the other is an excitation by stimulated Raman process. Esherick et al. introduced ionization detected stimulated Raman spectroscopy.6 The-method is used with the stimulated Raman pumping of the X(l,O,O,O) state. Comparison of the doubleresonance spectra obtained by both excitation schemes may be able to resolve accidental degeneracies and to ascertain the character of the intermediate state. It is revealed that only the antisymmetric (a) inversion component of the X( 1,0,0,0) state can provide double-resonance MPI spectra in the UV wavelength range studied in !his work. Rotational analysis of the bands indicates that the B(0,3,0,0) state contributes to the spectra. The nonappearance of the B(l,O,O,O) state in this investigation is discussed. 2. Experimental Section
Since the experimental procedure is very similar to that of Seelemann et only the relevant features are described here. Pure ammonia gas is flowed through a glass MPI cell equipped with nickel parallel plates with a bias voltage of 80 V. The cell pressure is measured with a capacitance manometer (MKS, Baratron Type 220B) and is typically 90 mTorr. The ion signals from the cell are preamplified (Keithley, Model 427) and are processed with boxcar averagers (Stanford Research Systems, Model SR25O). ~
(5) Allen, J. M.; Ashfold, M. N . R.; Stickland, R. J.; Western, C. M. Mol. Phys. 1991, 74, 49. (6) Esherick, P.; Owyoung, A. Chem. Phys. Lert. 1983, 103, 235.
0022-3654/92/2096-4276%03.00/0 0 1992 American Chemical Society
