J. Phys. Chem. 1994, 98, 11321-11330
11321
Lanthanide Oxide Cluster Ions Generated by Vacuum Laser Ablation: Metal Valence Effects on Cluster Compositions John K. Gibson Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6375 Received: June 1, 1994; In Final Form: August 25, 1994@
Pulsed XeCl excimer laser ablation (A = 308 nm; irradiance x 10 MW cm-2) of hydrated lanthanide oxalates into vacuum has been found to generate significant yields of lanthanide oxide cluster ions, the compositions of which were analyzed by reflectron time-of-flight mass spectrometry. Cluster ions, Ln,O,+, were identified for all of the lanthanides studied: Ln = Sm, Eu, Gd, Tb, and Yb. In each case, ions with metal contents (m) of up to at least 26 were detected, with the largest cluster identified being Tb63094+ (mass = 11 5 16 amu). A distinctive sequence of cluster compositions was found for each Ln, apparently reflecting the relative accessibilities of the +2, +3, and +4 oxidation states. On the basis of the observed cluster stoichiometry distributions, it was concluded that trivalent metal sites were dominant in each series of Ln,O,+. However, divalent Eu sites were ubiquitous, and tetravalent Tb sites were also assigned. In addition, more metal-rich cluster ions were identified for Sm and Gd. A double-chain polymeric lanthanide oxide schematic structure is postulated to provide a coherent picture of the observed clustering behaviors.
Introduction
TABLE 1: DivalenUTrivalent Promotion Energies1*and Redox Potentialsmfor Selected Lanthanides
Since the development of the technique some 30 years ago, pulsed laser ablation (and vaporization) of solids has found broad application in the deposition of thin films of a wide variety of materials, perhaps the most important of which are high-T, superconducting oxides. Many laser ablation studies have focused on the properties and performance of such deposited materials, especially films of YBa2Cu307-6 (YBCO) and other high-Tc oxides, including several in which various lanthanides have been substituted for y t t r i ~ m . ~A, ~substantial effort has also aimed at characterizing laser-solid interactions4 and ablation vapor plume^.^ At the laser irradiance levels generally necessary to achieve appreciable material transport (e.g., z108 W cm-2) YBCO ablation plumes are typically comprised primarily of atoms, small oxide molecules (e.g., MO), and their ions.6 Using substantially lower irradiance levels on YBCO (e.g, %lo5W cm-2), somewhat larger vapor species (Le., small clusters), such as Y404, Y2CU208, and BazCugO6, have been generated.’ Laser ablation of other oxide materials under comparable conditions has been found to similarly generate polyatomic vapor species, including (MgO), with n up to 8.8It has been postulated that the formation of large oxide clusters may explain the retention of oxygen stoichiometry in the preparation of YBCO films by vacuum laser ablation? and a mass spectrometric study of YBCO vacuum ablation using a mixture of 1064 532 nm light at irradiances of %lo8W cm-2 revealed a distinct series of large (to > 10 000 amu) yttrium oxide clusters.10 The primary cluster composition pattern found in this latter work suggested the sequential addition of Y203 units in forming increasingly large clusters. Vacuum Nd:YAG laser ablation of pure sesquioxides has been demonstrated to generate smaller Y,O,+ l1 and La,O,+ l2 clusters. The application of laser ablation for generating novel and large vapor cluster species has steadily increased since the discovery of c 6 0 and related fullerenes produced via this appr0a~h.l~ As generally applied, the technique uses an inert or reactive buffer gas to induce aggregation of small laser-
+
@
Abstract published in Advance ACS Abstracts, October 1, 1994.
0022-365419412098- 1132 1$04.5010
E( f”-2sLf”-3d‘s2)/eV
Sm Eu
1.92 3.11
Gd
Tb
-1.36” 0.04
Yb
2.88
E0{M3+IM2+}N -1.5 -0.35 b b -1.1
Gd[4P5d16s2]atomic ground-state configuration. Not determined ( Yb(I1) > Sm(I1) (>>Tb(II) and Gd(II), neither of which chemical oxidation states are known). Similarly, the tetravalent stability ordering for those lanthanides for which this valence state is reasonably accessible is Ce(1V) > Pr(1V) Tb(IV).20 Accessing and characterizing lanthanides in divalent or tetravalent oxidation states under extreme conditions (e.g., high-temperature, high-pressure, or highly oxidizing/ reducing conditions) can differentiate the chemistries of the members of this otherwise rather homogeneous series of elements. As a fundamental class of compounds, the solid lanthanide oxides have long received considerable attention with regard to the formation of distinguishing phases which contain the metal component in divalent, tetravalent or intermediate valence oxidation states. In Table 2 are indicated the known solid oxides of the lanthanides,21the compositions of which clearly reflect the stability trends summarized above. All of the lanthanides readily form a very stable trivalent sesquioxide. In accord with their relatively large promotion energies from the 4f"-26s2atomic ground-state electronic configuration to the 4fhP35d16sztrivalent bonding state configuration, Sm, Eu, and Yb also form solid monoxides. Similarly, the tetravalent 4fh-45dz6s2configurations for Ce, Pr, and Tb are of sufficiently low energies relative to the ground-state configurations that each of these lanthanides forms a dioxide. In addition to the near-stoichiometric LnzO3(s), LnO(s), and LnOz(s) compounds containing the metal species in essentially discrete oxidation states, several other solid oxide phases comprising lanthanides in mixed (or intermediate) oxidation states are known. Cerium, praseodymium and terbium each form a series of LnnI/IV01,5+x (0 x < 0.5) phases.21The Eu(IVII1) mixed valence compound, Eu304 (=Eu"Euz1"04), forms under moderate conditions,22while it has been suggested that Yb304 forms only under high-temperaturehigh-pressure
condition^.^^ Equilibrium (Knudsen effusion)z4 and pseudoequilibrium (Langmuir e v a p o r a t i ~ nstudies ) ~ ~ of the vaporization behaviors of solid lanthanide oxides have identified the atomic vapor, Ln(g), and the monoxide, LnO(g), (along with O/OZ(g)) as the primary gaseous products. The relative abundances of the metal-containing species, LnO(g) and Ln(g), correlate with the variations in the monoxide dissociation energiesz4 Thus Ln(g) is the overwhelmingly dominant vaporization product for Eu and Yb, which exhibit the smallest lanthanide monoxide dissociation energies, whereas LnO(g) is also important for all other Ln. Polyatomic gaseous lanthanide oxides, including LnzO(g), LnzOz(g), and LnOz(g), have also been identified as
minor high-temperature equilibrium vaporization products for several lanthanides?6 The species formed by the higher energy laser ablation process will presumably differ from those from equilibrium vaporization. Lanthanide metal clusters, Lnn, have been generated up to n = 30-50 for several lanthanides by the gas-aggregation technique using either a thermalz7928or laser ablationz9 source of lanthanide atoms. Oxygen contamination in the aggregation atmosphere was found to result in some highly metal-rich oxygen-containing clusters (e.g., Tbz10).29Using a similar synthetic approach, substantially more oxygen-rich lanthanide oxide clusters (e.g., Euz0027) have been generated for Eu, Yb, and Tm by intentionally introducing considerably greater concentrations of Oz(g) into the nucleation a t m o ~ p h e r e .In ~~ the latter work, the average metal valence (established by the net oxygen contents of the clusters) was correlated with the relative oxidizability of the three respective lanthanides. Thus, the average oxygen content, n, of the observed Ln,O,+ clusters was greatest for Tm and least for Eu. We have undertaken an investigation to further characterize the clustering behavior and systematics of several lanthanideoxygen systems using the alternate approach of laser ablation of oxygen-containinglanthanide solids into vacuum. In addition to supplementing the results of earlier investigations and extending such studies to other lanthanides, it was intended that this approach might provide novel and more detailed insights into lanthanide bonding and valence systematics. In particular, the vacuum ablation approach may allow for unique growth processes involving small metal oxide species, and may provide access to condensation mechanisms not operative in Lnn/OZ aggregation environments. The initial thrust of this effort previously focused on laser ablation of several pure lanthanide sesquioxides, LnzO&), into vacuum;31 most of the results obtained there were similar to those found under thermal equilibrium conditions (i.e., predominantly Ln+ and LnO+). The notable exception was Yb203(s), for which some minor concentrations of small polyatomic lanthanide oxide vapor species were identified. To induce a greater degree of oxide aggregation in the vacuum ablation plume, we have extended these studies to solid ablation target materials which comprise a relatively volatile component. For the work reported here, the lanthanide sesquioxalates, Lnz(CZO~)~*XHZO, were selected owing to their unusually high degree of hydration (x = 10 for Ln = La-Ho; x = 6 for Ln = E ~ - L u ~ ~It) was . anticipated that the release of hydration waters into the ablation plume might induce the formation of small lanthanide oxide species and their subsequent aggregation into larger clusters. Such a highly hydrated solid should additionally provide a plume atmosphere which is rich in oxygen-containing species and might thus be especially conducive to oxide cluster growth. Using electron impact ionization to analyze the neutral component of oxalate laser ablation plumes, we have c o n f i i e d that CO(g) and COz(g) are also major products, suggesting a mechanism similar to that established for thermal decomposition following d e h y d r a t i ~ nLnz(C204)3(s) :~~ LnzO3(s) 3CO(g) 3Coz(g). Although the ultimately ablated lanthanidecontaining solid may be essentially the sesquioxide, the volatile products of both the dehydration and the decomposition of the oxalate precursor should provide a dense ablation plume in which cluster growth is promoted.
+
-
+
Experimental Section The laser ablation mass spectrometer system is essentially comparable to those used elsewhere for similar studies on other materials (e.g.. refs 7 and 14), and the particular experimental
J. Phys. Chem., Vol. 98, No. 44, 1994 11323
Metal Valence Effects on Cluster Compositions procedures used here have been described p r e v i ~ u s l y . The ~~ apparatus consists of an excimer laser whose output is focused normal onto the surface of a solid target in the source region of a reflectron time-of-flight mass spectrometer (RTOF-MS). The laser (Lumonics EX744) was operated at 308 nm (XeC1) with a typical output of -300 mJ pulse-' ( h h m % 15 ns) at repetition rates of up to 100 Hz; most experiments were carried out at 10 Hz. The fluence on the target was moderated by an adjustable iris in front of a 25 cm focal length quartz lens which focused the laser output onto the target, located -2 cm from the axial center of the RTOF-MS flight tube. The targets could be rotated to new ablation spots but were not moved during irradiation and were generally maintained stationary for several thousands of laser shots on the same spot. Approximate irradiances on the targets were established by measuring the net transmission through the iris and other optical components and determining the approximate area of the final ablation craters (an area of -1 m2was used throughout). The use of the net ablation spot size neglected changes in beam size (and spatial energy profile) upon altering the transmission aperture size. The optimal irradiance levels for most of the results reported here were in the approximate range of 3-17 MW cm-2 (50-250 mJ cm-2). Turbomolecular vacuum pumping maintained the base pressure in the mass spectrometer below Pa; however, during ablation, the average background pressure could increase by up to an order of magnitude. The RTOF-MS (Comstock, Inc.) consists of a 1.2 m long flight tube (each direction) operating at 2 kV and a high-transmission gridless reflector. All of the spectra referred to here were obtained in the reflecting mode of operation. Although the ion source region of this instrument is equipped with an electron gun for electron impact ionization, all of the results reported here were for positive (+1) ions formed during the ablation process (negative ions and neutral species were not analyzed). The ablation plume expanded across the entrance to the spectrometer, normal to the flight tube longitudinal axis. Ion detection was with a dual-microchannel-plate detector, the output of which was recorded on a 500 MHz digital storage oscilloscope (Tektronix TDS 544A). Mass calibration was accomplished with electron-impact ionized perfluorophenanthrene (c&24, M W = 624 amu) which was leaked into the ion source. The hydrated oxalates were all supplied as powders (JohnsonMatthey) of 99.99% purity with an unspecified degree of hydration. The targets were prepared by uniaxially compressing the pure oxalate powders under lo9 Pa into 5 mm diameter, -2 mm thick pellets. The high degree of hydration of the gadolinium oxalate powder in particular was evidenced by the ejection of liquid water from the die-and-piston assembly upon compression. Previous results for lanthanide oxide ablation under similar conditions31had established the most effective general approach for the high-resolution detection of relatively large, low-energy ions formed in the ablation plume. Using this approach, ions were extracted from the expanding ablation plume into the flight tube (by a pulsed +200 V repeller plate) after some time delay, Id, following the laser pulse. For the oxalate studies, a time delay of -20 ps was found to provide optimal sampling and detection of the large lanthanide oxide ion clusters formed.
Results The laser ablation spectrum obtained for Yb2(C204)3*xH20, shown in Figure 1 (top), exemplifies the general features typical of all of the spectra discussed here. The time scale was defined such that the repeller voltage pulse, which extracted ions from the ablation plume into the flight tube, occurred at t =- 0, this
EXPANDEDREGION
11-24)
t
ION EXTRACTION
PULSE (MI
Yb,O:
I
6. 0
50
100
150
200
250
3W
350
400
450
310
320 330
t (rs)
r
c? I;
J 230
240
250
260
270
280
290
300
t (W)
Figure 1. Top: laser ablation mass spectrum for Ybz(C204)3*xH~O (irradiance = 4 MW cm-2 (60 mJ cm-2); delay = 21 ps). The very intense Yb+ peak was due to ablated ions which entered the flight tube
prior to the ion extraction pulse. All of the subsequent, sharp peaks were due to ions pulse-extracted after a 21 ps delay following the laser pulse. The d n peak identifications refer to the formulae, Yb,O,+. Bottom: Expanded portion of top spectrum. following the laser pulse with some variable time delay (td = 24 ps for the Figure 1 spectrum). All of the comparatively broad peaks up to and including the very intense Yb+ peak at -45 pus (the second Yb+ peak is discussed below) were attributed to relatively high-energy ions in the leading edge of the ablation plume, which had spontaneously leaked into the flight tube prior to the ion extraction pulse. Since only a small fraction of the plume ions should have directly impinged upon the flight tube entrance grid without the application of an extraction voltage pulse, such extensive ion leakage suggested the formation of a substantially larger amount of primary plume ions than indicated by the spectra. Along with some lighter ablation products (e& H+, Of, etc.), this initial leakage component of the spectrum was dominated by the very intense Ybf (or other Lnf) peak. The weaker, broad peak near 100 ps was also attributed to such leakage ions but was not assigned (the mass assignments for these leakage peaks were rather uncertain). The more relevant portion of the spectrum shown in Figure 1 (top) is that due to ions extracted into the flight tube by the ion repeller pulse (at t 0), 24 ps after the laser pulse. The first of these much narrower and more highly resolved features was the second Yb+ peak, which appears as a sharp spike on the high-mass (long-time) edge of the dominant Yb+ leakage peak. That the leakage component of the spectra was generally more intense than the extracted component suggests that the
11324 J. Phys. Chem., Vol. 98, No. 44, 1994
plumes were essentially comprised of a dense leading edge of ions (in which space-charging effects were dominant), and a more dilute tail region from which smaller ion bunches could be cleanly extracted with a modest (4-200 V) voltage pulse. All of the sharp peaks at longer flight times were assigned to ytterbium oxide ions, Yb,O,+ (designated by their m h assignments in Figure 1 and elsewhere), which had been extracted into the flight tube by the repeller pulse. Peaks due to these cluster ions were not evident in the primary, leakage component of the spectrum and were absent from spectra obtained using very short (e.g., 210 ps) or long (e.g., 240 ps) delay times, indicating an approximate temporal window (i.e., -10-40 ps) during which the cluster-containing portion of the plume traversed the region in front of the spectrometer entrance. These observations suggested that the oxide cluster ions were formed in a (cooler) portion of the plume, behind the primary, highenergy ion component (which was dominated by Yb+). That the results imply postablation cluster growth by aggregation in the ablation plume-as opposed to direct emission of large clusters from the solid target-is consistent with the representation of yttrium oxide cluster growth offered by Becker and Pallix.lo For all of the Ln2(C204)3*xHzO studied, it was the delayextracted ion spectra which were of primary interest and which are considered here. Ablation of each of the Ln2(C203)3-~H20 resulted in spectra exhibiting a series of cluster ions assigned as pure binary oxides. The presence of substantial concentrations of hydrogen-containing species (especially H2O and its fragments) in the ablation plume suggested the possibility of H-incorporation into the "oxide" cluster ions. The resolution between neighboring ion peaks (MIhM) was limited to -200 using the laser ablation ion source (MIAM was '1000 for electron impact and laser ionization of gaseous neutrals). However, the accuracy of the peak maxima mass assignments was better than the peak-to-peak resolution, and it was possible to assign the maxima to hydrogen-free oxide ions for masses up to -1000 amu (e.g., to L&Os+). For Tb, for example, a peak assigned to Tb4OgHzO+, at 750 amu, could be confidently differentiated from Tb407+, which would have appeared at 748 amu. Although, chemical valence considerations in the context of the observed cluster growth systematics, as discussed below, suggested the addition of (hydrogen-free) sesquioxide (-LnzO3-) units in building up to the larger clusters, the possibility of some hydrogen incorporation into clusters could not be excluded. Thus whereas pure binary oxide clusters were considered dominant, some smaller contribution from hydrogen-containing speciesespecially at higher masses where both resolution and sensitivity were diminished-was possible. The presence of significant amounts of carbon in both the target and the ablation plume (e.g., as CO(g) and C02(g)) suggested the further possibility of C-incorporation in some clusters. In particular, a C4 cluster component could not be resolved from an 0 3 component (both 48 amu). However, the cluster size distribution systematics again substantiated the interpretation that the dominant cluster series were comprised of binary metal oxides. Specifically, the observed mass differences between neighboring cluster ion peaks generally corresponded to LnO, LnOz, or their composite, LnzO3; although the latter moiety could not itself be practically differentiated from LnzC4, the former two were definitively attributable to oxide units. For each oxalate, optimal oxide cluster ion formation/ detection was restricted to a fairly narrow range of laser irradiances (%3-17 M W cm-2) and extraction delay times (%20-25 ps); the cluster ion intensities generally diminished rapidly outside of these parameter limits. Presumably a minimal
Gibson
TABLE 3: Observed Ln,,,O,+ with m min avgLnvalence Sm Eu 1io 1 Y YO 111 211 212 213 313 314 414 415 416 516 511
518 610 613 611 618
619 a
3 1.5 2.5 3.5 2.3 3 2.3 2.8 3.3 2.6 3 3.4 0.2 1.2 2.5 2.8 3.2
Y"
5
Y Y Y
6 Gd
Tb
Yb
Y Ya
Y Y
Y" Y
Y
Y Yb
Y
Y Y Y
Y
Y
a
Y
Y
Y
Yb
Y
Y
Y Y
Y Y Y
Y Y
Y
Y Y
Y
Most intense peak. Also observed: Tb&HzO+; TbzO3*2HzO+;
Tb406*H20+.
irradiance was necessary for adequate material removal. It can be speculated that above "17 MW cm-2 the daposited energy was too great to allow for aggregation of small species into clusters. Although it appeared that the extent of cluster formation (i.e., detection) was generally diminished for the initial laser shots on a pristine target site, after -1000 shots the degree of clustering did not apparently further correlate with the number of accumulated shots. The necessity for activation of a new ablation site suggests that the formation of a crater (or indentation) may have been required for the effective formation of clusters. The mechanism of cluster formation (e.g., interaction with the walls of a crater or condensation in an expansion nozzle created by a crater) is not particularly relevant to the following interpretations of the observed cluster ion distributions. The morphologies of the five types of oxalate pellets were somewhat varied following the ablation experiments. The entire Gdz(C204)3*xH20 and Yb2(C204)3-xH20 pellets both expanded dramatically and largely fragmented in the mass spectrometer, presumably reflecting extensive dehydration in vacuum; despite the loss of integrity, a deep ( 2 2 mm) ablation crater could be identified in the latter pellet. In contrast, the EUZ(CZO~)~*XHZO, Sm~(C204)3*xH20, and Tb2(C204)3*xH20targets remained intact and hard, with the deepest craters being from -0.2 mm (Sm and Tb) to -2 mm (Eu) deep. Of course, the final crater depth was dependent upon the number of laser shots incurred by a particular target site, which was somewhat variable. The ions identified in the delay-sampled ablation plumes are enumerated in Tables 3 and 4 (throughout, d n values denote specific Ln,O,+); the valence assignments are discussed below. It should be reemphasized that no postablation ionization was invoked, and only the positive ion (Le., I+) products in the ablation plumes were analyzed-neutrals and negative ions were not detected. Although representative spectra were selected for these tabulations, the features were essentially similar for all of the cluster ion spectra for a given lanthanide oxalate. However, the absolute and relative cluster ion intensities could vary significantly, depending upon the ablation and detection parameters. The distinction between small (m 5 6 ) and large ( m 1. 7) Ln,O,+ clusters is somewhat arbitrary but reflects the approximate onset of the dominant composition and abundance patterns established in the systematic series of larger ion clusters. Although only clusters up to Ln230,+ are enumerated in Table 4, substantially larger clusters were also identified, up to
Metal Valence Effects on Cluster Compositions
J. Phys. Chem., Vol. 98, No. 44, 1994 11325
TABLE 4: Observed Ln,,,O,+ for 7 5 m i 23 lanthanide valence distribution observed intensity m/n avg 2 3 4 Sm Eu Gd Tb 23 0.71 (Ln-rich) 712 718 719 7/10 813 814 817 819 8/10 811 1 8/12 912 915 911 1 9/12 9/13 1017 loll0 10112 10113 10114 10115 1118 11/14 11/15 11/16 12/10 12/13 12/16 12/17 12/18 13/11 13/17 13/18 13/19 14/12 14/16 14/19 14/20 14/21 15/20 15121 15/22 16/19 16/22 16/23 16/24 17/24 17/25 18/22 18/25 18/26 18/27 19/27 19/28 20125 20128 20129 21/30 21/31 22/28 2213 1 22/32 2313 1 23/34
2.43 2.71 3.00 0.88 1.00 1.88 2.38 2.63 2.88 3.13 0.55 1.22 2.56 2.78 3.00 1.50 2.10 2.50 2.70 2.90 3.10 1.55 2.64 2.82 3.00 1.92 2.25 2.75 2.92 3.08 1.77 2.69 2.85 3.00 1.79 2.36 2.79 2.93 3.07 2.73 2.87 3.00 2.44 2.81 2.94 3.06 2.88 3.00 2.50 2.83 2.94 3.06 2.89 3.00 2.55 2.85 2.95 2.90 3.00 2.59 3.00 2.95 2.74 3.00
Intensity
4 2 0
3 5 7
0 0 0
(Ln-rich) (Ln-rich) 5 3 1
0
(Ln-rich) 3 5 7 7
4 2 0
5 7 9
0
5 7 9 9
67
0 0 0 1
94 21
7 9 11
0
9 11 11 9 11 13
0 0 0
0
11 13 13 11 13 15
100 6 8
100
13 15 15 15 17
0
15 17 17 17 19
82
81
17 19 19 21
60 40
0 0 0
50
19 21 21 23
35
58
63 23
5 7
41
45
67 15
3 5
38
28
59
16 67 71
82
41
8
0 0
60 33
1 0
40
0 0
53
0 0
37 52 4
1 0
0
52 36
61 6
37 63
45 34
0 0 1 0 0
0 0 0 0
21
75
37 43 3 33 66
45 27
23
88
37 35 27 30 19
50
(Ln-rich) 3 1 2 0
6 11
49
13
0 0 1
(Ln-rich) 3 1 2 0
74
57 97
(Ln-rich) 3 1 0 2
88
12
0 0 0
0
20
0 0 0
20
17
L 00
37 41
= 100 for most intense Ln,O,+
20
160
185
210
235
260 t
56 27 100 40
1
(Ln-rich) 3 1 0 2 0
42
47 60
0 0 0
(Ln-rich) (Ln-rich) 3 1 0 4 2
11 20
135
(Ln-rich) 4 2 0
7 18
(Ln-rich) (Ln-rich) 3 1
65
66 40 100 53
(Ln-rich) 4 2 0
100
7
(Ln-rich) (Ln-rich) 5 3 1
23 37 8 9
(Ln-rich) (Ln-rich)
Yb
11
83
(m 2 7) peak for each
Ln .
SmssOs2+, EU26036+, Gd37055+, Yb6109lf, and Tb6309.1+,the last of these at 1 1 5 16 amu. The composition patterns established by the observed cluster sequencing up to LQ~O,,+ was
285
310
335
360 385
(IN
Figure 2. Laser ablation mass spectra for Gd2(C20&-xHzO (irradiance x 9 MW cm-* (140 mT cm-2)): (a) delay = 21 ps; (b) delay = 24 p s .
essentially extended with the systematics exhibited by the continuation of the series to larger cluster compositions. Representative laser ablation mass spectra are shown in Figures 1-5, for each of the five lanthanide oxalates studied. The extensive clustering evident for each lanthanide was characterized by a relatively smooth variation in intensities for the large oxide ion clusters, with no clear indication of any local abundance maxima or “magic number” compositions. The two spectra shown for Gd~(C204)3*xH20 in Figure 2 (same intensity scale for both) were obtained under essentially identical ablation conditions, using only slightly different sampling delay times (Le., 21 vs 24 ps); the obvious discrepancies between these spectra illustrate the potential for significant variations in relative and absolute peak intensities. The results for europium were distinctive in that optimal cluster formation required slightly greater irradiances (-15 MW cm-2 for E U ~ ( C ~ O ~ ) ~ * X vsH5Z11 O, MW cm-* for the other Ln2(C204)3~H20);in addition, a significantly lower relative yield of large ( m > 6) Eu,O,+ clusters was found (see Figure 3). The especially sharp peaks for Tb clusters (Figure 4) reflect that this was the only monoisotopic lanthanide studied (naturally occumng abundance, terbium-159 = The typical mass spectral resolution is illustrated in the expanded portion of the spectrum in Figure 3 (bottom), where several Eu,O,+ are clearly differentiated from EumOn+l+.As noted above, the precision of the peak maxima mass assignments was found to be substantially greater than the peak-to-peak resolution. The mass resolution was sufficient to quantitatively confirm the proper lanthanide isotopic distributions only for the Ln+ peaks; however, an appropriate qualitative peak-broadening was evident with increasing size for the polyisotopic lanthanide oxide clusters.
Discussion The formation of large lanthanide oxide clusters from the vacuum laser ablation of hydrated lanthanide oxalates implies a special stability of the observed compositions. Each of the lanthanides exhibited a unique clustering sequence, suggesting a dependence of cluster compositions and structures on the accesibilities or stabilities of lanthanide valence states. In addition, the undetermined and variable extent of hydration of the different oxalates could have affected the clustering behaviors. The essential appearances of the lanthanide oxide ion cluster mass spectra obtained here were similar to the yttrium oxide cluster spectra (from YBCO laser ablation) reported by Becker and Pallix.lo They reported that the extensive clustering evident using ~ 3 0 0MW cm-2 of 1064 532 nm light
+
Gibson
11326 J. Phys. Chem., Vol. 98, No. 44, 1994
E
70
95
120
145
170
195
220
245
270
295 320
1 (es)
140
165
190
215
240
265
290
315
340
365
390
t (CIS)
Figure 5. Laser ablation mass spectrum for Sm2(C204)yxH20(irradiance R 11 MW cm-2 (160 mJ cmF2);delay = 21 ps).
The average formal lanthanide valence in an oxide cluster ion, LnmOn+,is given by (2n l)/m, where the +1 net ion charge is assigned to the metal component of the cluster. The values for the average lanthanide valence for a given cluster ion composition have been included in Tables 3 and 4. To achieve a more detailed assessment of the nature of the observed cluster ions, it is instructive to distribute these net valences among the component lanthanide sites. The discrete valence 170 175 180 185 190 195 2 W 205 210 215 220 distribution assignments given in Table 4 are based upon the t (0) following assumptions: Ln(II1) sites are dominant; Ln(1V) sites Figure 3. Top: laser ablation mass spectrum for Eu~(C~O&.XH~O are possible only for Tb among the lanthanides studied here; (irradiance 15 M W cm-2 (230 mJ cm-2); delay = 21 ps). Bottom: and Ln(I1) sites are possible for all of the lanthanides, but more Expanded portion the top spectrum. Shows resolution of Eu,O,+ from likely for Sm, Yb, and especially Eu. In addition, the stoichiEumOn+lf. ometries of some highly metal-rich (denoted as “Ln-rich’ in Table 4) cluster ions for Sm, Gd, and Yb (e.g., GdgOs+) suggested the possibility of low-valent lanthanide-rich moieties (e.g., metallic regions within oxide clusters). In the following discussion, some of the notable details of the clustering systematics indicated by the results outlined in Tables 3 and 4 and Figures 1-5 are considered for each individual lanthanide. Ytterbium. The observed ytterbium oxide ion cluster composition sequence (Tables 3 and 4, Figure 1) can be resolved into two distinct series components, or subseries. The primary component is described by Lnm01/2(3m-1)+ with odd values of m (Le., d n = 314, (5/7), 7/10,9/13, 11/16, 13/19, ...). The second, slightly less intense component is represented by Ln,0112(3~-2)+, with m even (Le., 4/5,6/8,8/11, 10/14, 12/17, 14/ 20, ...). For the odd clusters, the average valence is 3.00, and Tb,O: each Yb site can be assigned as trivalent. For the even clusters, a single divalent ytterbium site must be invoked to satisfy the charge balance (see the valence distributions given in Table 4). 70 95 120 145 170 195 220 245 270 295 320 In addition to satisfying the observed net valences, any t (CIS) postulated cluster structures should be consistent with the Figure 4. Laser ablation mass spectrum for Tb2(C~O&.xH20(irradidiscrete addition of Yb2O3 units in the construction of subseance = 11 MW cm-2 (160 mJ cm-2); delay = 21 ps). quent clusters in each of the compositional subseries. The general absence of Ln203+ from the ablation spectra (except disappeared above ~ 4 0 MW 0 cm-2. That their useful irradiance for Ln = Tb; see Table 3) presumably reflects the resistance of range was approximately an order of magnitude greater than most Ln to oxidation above the +3 valence state.20 However, that identified here as optimal for clustering may reflect the it is likely that neutral Ln2O3 are important cluster components; use of a more volatile target material and/or shorter wavelength in this regard, neutral Y203 has been identified as an important light in the present work. With vacuum laser ablation of pure product of YBCO ablation.’ Cluster formation apparently sesquioxides, Mele et al. generated series of MmOln(3m-1)+ proceeded without significantly favoring any “magic number” clusters for M = Y (to m = 15)” and M = La (to m = 23).12 compositions. Certain relatively small oxide vapor molecules, The cluster stoichiometry sequences found in that work were including P4010and Sb406,34form small cage structures. Given the observed regular addition of Yb203 units without any also similar to some of those observed here.
+
Metal Valence Effects on Cluster Compositions
,0-1e Ln
y-0-
I
o
I
-
J. Phys. Chem., Vol. 98, No. 44, 1994 11327
Ln
I ' o
l
0 m-6 2
L
L
-1 m-3 2
(b) LnmOim7,,m EVEN (415,618, 8/11, 10114, 12/17,...) r
1
/
e Ln
0
0
Lnll
L
Figure 6. Postulated structural configurations of the two dominant series of Ln,O,+: (a) m odd; (b) m even. Except for the single Ln"
site in structure (b), all other metal sites are LnIn. significant stability maxima, it seems unlikely that the larger clusters identified here should be represented by expanding cages. Similarly, local abundance maxima would have been expected if the oxide clusters were described by a threedimensional lattice construction analogous to that postulated for (MgO),+ clusters.35 Rather, the relatively smoothly varying abundance patterns obtained here would suggest a polymeric chain structure, represented by the sequential addition of repeating -[Yb203]- units; the distinction between odd-m and even-m polymeric cluster subseries could be at the termini of such chains, where the uniform valence difference would be accommodated. A polymeric sesquioxide structure consistent with these considerations is exhibited in the solid state by valentinite, a crystalline form of Sb203,36in which two -0Sb-0-Sb-0-Sbchains are connected into a double chain by Sb-0-Sb cross-linkages. Corresponding polymer-like structures which can be postulated for the two Yb,O,+ cluster subseries are shown schematically (for a generic Ln) in Figure 6, with no attempted suggestion of detailed bond parameters (all Sb-0-Sb bonds are significantly bent in valentinite, for example). Although fully linear chains are indicated for simplicity, some branching is possible, if not probable. Both of the speculative structures in Figure 6 invoke the fundamental -0-M-0-M-0unit upon which valentinite is based, and each is presumed to terminate at one end with a 0-M+(III)-0 site where the f l ion charge is localized. The suggested difference between the two structures is at the other chain end, where one (Figure 6a) terminates with two Ln(II1) sites and the other (Figure 6b) with a unique Ln(I1) site. As required, all of the Yb (Ln) sites are trivalent in the postulated odd-m cluster configuration and only the one (terminal) site is divalent in the even cluster configuration. The schematic structures proposed in Figure 6 are consistent with the observations and have a basis in solid-state chemistry. Lacking additional evidence for these structural configurations, their primary value in the present context is in representing the observed clustering systematics and in interpreting the observed lanthanide valence effects. This rudimentary speculative cluster configuration model does not attempt to address factors such as the degree of covalent vs ionic metal-oxygen bonding (the assigned discrete Ln valences clearly represent a simplification in this regard). Gadolinium. In the case of Gd, as with Yb, the dominant series of clusters is represented by LnmO1/2(3,-l)+with m odd (Le., 3/4, 5/7, 7/10, 9/13, 11/16, ...), in which all Gd sites are
L
'm-2 2
Figure 7. Modified postulated structures for some secondary Ln,O,+ series. Unlabeled Ln sites are all nominally trivalent; Ln,* denote (metallic) lanthanide-rich sites.
considered to be trivalent. A possible structural configuration for this primary series is again that shown in Figure 6a. The corresponding even subseries evident for Yb (e.g., Figure 6b) was essentially absent for Gd, possibly reflecting the low stability of the required Gd(I1) site. For Gd there were additionally some important Gd-rich cluster ions, the stoichiometries of which implied the assignment of a substantially reduced valence to several Gd sites. Rather than assigning a formal reduced discrete valence to isolated Gd sites, it may be more reasonable to postulate the incorporation of lanthanide-rich (metallic) regions within oxide cluster structures derived from those shown in Figure 6. For example, the observed clustering sequence, Gdm01/2(3m-lo)+ with m even (Le., 8/7, 10/10, 12/13, 14/16, ...) can be rationalized by structure 6a, with the terminal 0-Ln+(III)-0 site replaced by a more metallic 0-GQ+-O site; this modified structure is shown in Figure 7a. Similarly the sequence, GdmOl/2(3,-17)+with m odd (Le., 7/2, 9/5, lU8, 13/11, ...) could be attributed to clusters with structure 6b, with the 0-Ln(I1)-0 site replaced by a 0-G&-0 site, as shown in Figure 7b. The particular stability of the G& moiety is further suggested by the observation that G&+ was the only pure metal cluster ion identified in these studies. This interpretation of the observed metal-rich gadolinium oxide stoichiometries implies an instability of Gd(II) sites, but invokes the existence of even lower (formal) valence polyatomic metallic Gd sites. Europium. Like Yb, Eu exhibits the LnmOl/2(3,-2)+( m even) cluster series, which contains one Ln(I1) site (Figure 6b). In contrast to Yb, Eu does not form the Lnm01/2(3,-1)+(m odd) series, in which all Ln sites are trivalent, but does exhibit the four additional clustering sequences: (S2) Eum01/2(3,-3)+(5/6, 7/9, 9/12, 11/15, ...); (S3) E~~O1/2(3~-4)+ (4/4, 6/7, 8/10, 10/13, ...); (S4) EU,01/2(3~-5)+(7/8, 9/11, 11/14, 13/17, ...); and (S5) EUm01/2(3m-6)+(8/8, 10/12). Consistent with the particular stability of divalent europium, the clusters in these four
Gibson
11328 J. Phys. Chem., Vol. 98, No. 44, 1994 europium-specific series are assigned two (for series S2), three (for S3), four (for S4), or five (for S5) divalent europium sites. In the context of the structural configurations suggested in Figure 6, these reduced valence europium clusters can be represented by structure 6a with one (for series S2) or two (for S4) crosslinking oxygens removed, or by structure 6b with one (for S3) or two (for S5) such oxygens absent. That even the larger europium clusters (e.g., Eu200n+)did not incorporate additional (> 5) divalent sites suggests that the favored locations for the missing one or two Eu-0-Eu cross-linkages are at one or both of the terminal chain positions. The resulting postulated structure for E~~O1/2(3~.5)+ (S4), for example, is represented in Figure 7c. Although the divalent state of Eu is slightly more stable than that of Yb, these two elements are typically quite similar in their oxidatiodreduction behaviors.2o For the observed ytterbium cluster series, either 0 or 1 Yb(II) sites were assigned, whereas for the europium clusters 1-5 Eu(II) sites were assigned. This apparent manifestation of the greater tendency towards divalence for europium would suggest a particular sensitivity of the oxide cluster compositions to the redox properties of the metal component. It was also noted that large oxide cluster ion formation was not as predominant for europium as for the other lanthanides, including ytterbium; this cluster yield effect may have reflected variable experimental parameters, such as the extent of hydration of the target materials, or perhaps differing oxalate decomposition pathway^.^^,^^ Terbium. The dominant cluster series for Tb was that represented by Ln,01/2(3~-1)+(3/4,5/7,7/10,9/13, ...), in which all Tb sites are trivalent (Figure 6a). In addition, there was a small contribution from a Tbm01/2(3m-2)+series (4/5, 6/8, 8/11, 10/14, ...), in which one divalent Tb site has been assigned (Figure 6b). The additional secondary cluster sequence, TbmO3&2+(213, 4/6, 6/9, 8/12, 1005,...), was unique to terbium. To account for the relative excess of oxygen therein, each of these Tbm03d2+requires assignment of a single Tb(1V) site. These oxygen-excess clusters can be represented by the configuration shown in Figure 6b, with a terminal double-bonded oxygen added, thereby replacing the Ln(I1) site with a O=Tb(IV) site; this modified postulated structure is shown in Figure 7d. Such a structural interpretation implying the unique specificity of the (terminal) tetravalent terbium site is consistent with the observation that only one such site was ever present, even for clusters as large as TblsOn+. The apparently unique higher-valent terbium oxide cluster series further demonstrates the sensitivity of oxide cluster formation to the relative stabilities of the lanthanide valence states. Also unique to terbium were mass spectral peaks assigned to hydrated oxide ion clusters: Tb203*H20+,Tbz03*2H20+,and Tb406*H20+. The distinctive hydration of these particular clusters, each of which is considered to include a Tb(1V) site, suggests that the tetravalent site may be particularly favorable for water coordination. Samarium. As with all of the other Ln, except Eu, the principal cluster series was that represented by LnmOi/2(3m-i)+ (7/10, 9/13, 11/16, 13/19, ...), in which all Sm sites are considered trivalent. In distinct contrast to Yb and Eu, the LnmO1/2(3,-2)+series, with a single Ln(I1) site per cluster, was not evident for Sm. The distinguishing behaviors of Sm, Eu, and Yb with regard to the formation of clusters with divalent sites is a clear manifestation of the order of divalent lanthanide stabilities noted above: Eu(I1) > Yb(I1) > Sm(II). Indeed, divalent samarium was not significantly evident in the observed cluster compositions.
As with Gd, several Sm cluster compositions were identified which suggested a metal-rich site in what might be envisioned as heterogeneous oxide/metal clusters. The samarium clusters in the series, Smm0l/2(3,-1o1+ (8/7, 10/10, 12/13, 14/16, ...), correspond to those in the isologous Gd series, similarly suggesting the assignment of a terminal -O-Sq+-Osite in each (see Figure 7a). The unique 8/4 and 10/7 samarium clusters can be explained by a structure involving the replacement of the terminal -0-Ln+-0site in Figure 6a with a -O-Sw+-Osite. Lanthanide Valence Configuration Effects. The discrete valence distribution approach invoked in the above discussion has served to clearly differentiate the oxide clustering behaviors of the lanthanides, and the resulting interpretation correlates well with the known relative stabilities of the I[,ILI,and IV lanthanide valence states. Laser microprobe mass analysis (LAMMA; Nd: YAG laser) of LazOs(s) and HozO3(s) previously identified the primary trivalent lanthanide cluster ion distributions seen here.38 However, those two lanthanides do not (readily) occur in additional valence states and the distinctive (nontrivalent) cluster compositions were not reported there. Both the LAMMA results38 and those reported here for excimer laser ablation contrast with results for photoionization of transition metal oxide clusters.39 It was found that multiphoton ionization simulated high-temperature conditions, resulting in fragmentation to oxide clusters dominated by lower valence metal sites, reminiscent of the species formed by equilibrium high-temperature vaporization. The discrete metal site valence model invoked above is considered most relevant to lanthanide oxide cluster ions, which are likely insulators with a localized ion charge. However, as an altemative to discrete metal valence assignments the approach applied by Brechignac et aL30 to Eu, Tb, and Yb oxide clusters considers the average valence of the aggregate lanthanide component of the clusters as a function of cluster size (where size = number of lanthanide atoms). The average aggregate lanthanide valence for each particular cluster composition observed in the present study is given in Tables 3 and 4. Note that in contrast to Brechignac et al., we have considered the f l ion charge in establishing the net metal valence (i.e., valence = [2n l]/m, rather than 2nlm). The inclusion of the net ion charge in cluster valence considerations is particularly appropriate here, since no postionization was invoked and the observed clusters were thus formed with an intrinsic +1 charge. The average aggregate lanthanide valence for clusters with a given number of lanthanide atoms was obtained as an intensityweighted average of the calculated valences for all of the observed clusters with that particular number (m)of metal atoms (IILnmOn+]= mass spectral peak height for ion LnmOn+):
+
avg Ln, valence =
C{I[L~,O,+I ([2n + 1I / ~ ) I / ~ { I [ L ~ , O , ~ I n
n
The results are plotted for Eu and Yb in Figure 8 and for Tb and Gd in Figure 9. The greater tendency for Eu to form lower valence clusters than Yb is clearly evident. The two anomalously low-valent Yb points (at m = 12 and m = 14) reflect the observed metal-rich clusters, Yb12010+ and Ybl4012+; as with several similarly distinctive Gd and Sm cluster compositions, these apparently peculiar Yb clusters are speculated to be of a heterogeneous constitution, characterized by a “metallic” ytterbium-rich region and a saturated oxide region. The loss of detailed information resulting from considering only average valence effects is further illustrated by the results shown in
J. Phys. Chem., Vol. 98, No. 44, 1994 11329
Metal Valence Effects on Cluster Compositions 3.0- 0 e, 0
e,
9
-
0
2.8
e . . . . . . . . . . . e . . 0 . 0 oooo ooooo 0
0
0
oo 0
_I C
e
? -X
0 : Eu
2.4
t 1
Number of Metal Atoms in Oxide Cluster
Figure 8. Average metal valence, (2n+l)/m, vs number of metal atoms, m in Ln,O,+, for Ln = Yb, Eu. t
-
e,
C
9
-C I
e,
01
e
P
0 : Tb
A : Gd
1
6
11
16
21
26
31
Number of Metal Atoms in Oxide Cluster
Figure 9. Average metal valence vs number of metal atoms for Ln = Tb, Gd.
Figure 9. Although the tendency of Tb to form a tetravalent state is evident, the average Gd valence is even lower than that of Eu for several cluster sizes. We have attributed this apparent reduced valence effect to clusters with metallic regions, as opposed to oxide clusters with several isolated, formally lowvalent Gd sites (e.g., Gd(I1) or Gd(1)); the particularly low average Gd,O,+ valence for even values of m reflects the dominance of the species schematically represented in Figure 7a. Although average valences for clusters formed under oxygen-saturated conditions may provide insights into variable metal-valence effects,30 it is clear that such mean effects can be of less significance under other cluster-formation conditions, such as those invoked here. In summary, the more detailed interpretation of oxide cluster compositions, which assigns discrete metal valences within the clusters, has provided greater insights into the subtleties of valence and bonding variations among the lanthanides. A fundamental goal in studying the formation and properties of clusters is to elucidate the transition from atomic/molecular to bulk (condensed state) behavior. As anticipated, the average lanthanide valences in the small (e.g., m 5 6 ) oxide clusters were substantially lower than in the most stable condensed oxides (i.e., LnzO3). Rather, the small oxide molecular ions were predictably more comparable to the equilibrium lanthanide oxide vaporization products. Essentially, as Ln2O3 units were sequentially attached to the smaller species to generate the series of larger clusters, the bulk-state valence (i.e., Ln(II1)) was asymptotically approached. Similar clustering series have been reported for a trivalent lanthanide-like element, and for lanthanum.12 The yttrium results in Particular'O suggested that cluster growth occurred in the ablation plume, without direct ejection from the solid of large clusters, and a comparable
growth mechanism was likely operative here. Given that the fundamental unit of all of the observed cluster series was Ln2O3, it is vexing that the Ln203+ ion was evident only in the case of Tb. However, it is possible that the neutral Ln2O3 species were nonetheless important (but unobservable) plume constituents. Postionization, for analysis of the neutral components of the ablation plumes, would be useful for addressing this and other issues. As illustrated in Figure 9, the behavior of terbium was particularly unique in that the bulk-state valence was essentially achieved for even the smallest oxide vapor species (Le., for all ions beyond Tb+). Other notable features of the variable approach in the lanthanide oxide cluster ion series to bulk-state compositions and valences have already been considered above (e.g., the particular importance of metallic, Gd-rich oxide cluster ions).
Conclusion The extensive formation of lanthanide oxide clusters by vacuum laser ablation of hydrated lanthanide oxalates has been demonstrated for Sm, Eu, Gd, Tb, and Yb. The particularly extensive cluster aggregation induced under these conditions is attributed to the generation of substantial local vapor densities in the ablation plume, primarily due to dehydration and decomposition of the oxalates. Previous studies of pure lanthanide oxide ablation under similar conditions to those employed here3' revealed a much smaller degree of cluster formation. The observed clusters were represented by series of Ln,O,+ clusters, the compositions of which suggested growth by the sequential addition of LnzO3 units. No particularly favorable ("magic number") compositions were evident among the larger clusters ( m > 6). The observed cluster sequences are consistent with the assignment of four types of lanthanide sites: (1) metallic (i.e., reduced valence); (2) divalent; (3) trivalent; (4) tetravalent. The assigned distributions of valence sites were consistent with the well-established relative stabilities of lanthanide valence states. Thus, all cluster series were dominated by trivalent sites, divalent sites were most ubiquitous for Eu but were also important for Yb clusters, and only Tb was found in the tetravalent state. In addition, Gd and Sm (and Yb, to a lesser extent) formed clusters characterized by excess metal content; this excess was considered to comprise "metallic" regions within oxide clusters. The observed clustering systematics are consistent with a postulated model structural configuration based upon the polymeric double-chain structure exhibited in a crystalline form of Sb2O3. Modifications to this basic structure provide a coherent picture of the observed deviations from the primary cluster abundance sequences. The speculative schematic structures offered in Figures 6 and 7 are particularly congruous with the observed cluster composition systematics when the f l charges are assigned as indicated. These model structures are offered to schematically represent the observed ion distributions and definitive structural assignments require additional evidence. A recent investigation of Alms,+ cluster ions with compositions ( d n ) analogous to the Ln,O,+ clusters identified here has suggested cage structures?0 The actual detailed structures of the lanthanide oxide cluster ions may at least involve some polymeric branching. On the basis of simple metal ion valence considerations, it is predicted that the cluster series should shift in composition for the corresponding neutral and negative ion oxide clusters. Establishing the clustering systematics for these alternative charge states could serve to further examine the validity of the postulated model.
11330 J. Phys. Chem., Vol. 98, No. 44, 1994
Gibson
Acknowledgment. This research was sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, US.Department of Energy under Contract DE-AC05840R21400 with Martin Marietta Energy Systems, Inc. References and Notes (1) Cheung, J. T.; Horwitz, J. S . MRS Bull. 1992,17(2), 30 (2) Neifeld, R. A.; Gunapala, S.; Liang, S. A.; Shaheen, S. A.; Croft, M.; Price, J.; Simons, D.; Hill, W. T. III Appl. Phys. Lett. 1988,53, 703. (3) Dousselin, G.; Pellan, Y.; Thivet, C.; Guilloux-Viry, M.; Padiou, 3.; Robinet, S.; Pemn, A. J. Alloys Comp. 1993,195, 195. (4) Leuchtner, R. E.; Horwitz, J. S.; Chrisey, D. B., In Laser Ablation in Materials Processing: Fundamentals and Applications, Materials Research Society Symposium Proceedings,Vol. 285, Braren, B., Dubowski, J. J., Norton, D. P., Eds.; MRS: Pittsburgh, 1993, pp 87-92. (5) Dye, R. C.; Brainard, R.; Foltyn, S. R.; Muenchausen, R. E. In Laser Ablation in Materials Processing: Fundamentals and Applications, Materials Research Society Symposium Proceedings; Braren, D. B., Dubowski, J. J., Norton, D. P., Eds.; MRS: Pittsburgh, 1993; Vol. 285, pp 15-26. (6) Schenck, P. K.; Bonnell, D. W.; Hastie, J. W. J. Vac. Sci. Technol. A 1989,7, 1745. (7) Moalem, M.; Olander, D. R.; Balooch, M.; Fluss, M. J. J. Vac. Sci. Technol. A 1992,10, 3292. (8) Olander, D. R.; Yagnik, S. K.; Tsai, C. H. J. Appl. Phys. 1988,64, 2680. (9) Dijkkamp, D.; Venkatesan, T.; Wu, X. D.; Shaheen, S . A,; Jisrawi, N.; Min-Lee, Y. H.; Mclean, W. L.; Croft, M. Appl. Phys. Lett. 1987,51, 619. (10) Becker, C. H.; Pallix, J. B. J. Appl. Phys. 1988,64, 5152. (11) Mele. A.: Consalvo. D.: Stranees. D.: Giardini-Guidoni. A.: Teehil. R. Appl. SUI$ Sci. 1989,43,398. (121 Mele, A.; Consalvo, D.; Stranges, D.; Giardini-Guidoni,A.; Teghil, R. Int. J. Mass Spec. Ion Processes f990,95, 359. (13) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985,318, 162. (14) Ross, M. M.; O’Keefe, A.; Baronakski, A. P. In Physics and Chemistry of Small Clusters; Proceedings of the International Symposium on the Physics and Chemistry of Small Clusters, Richmond, VA, 1986, Jena, P., Rao, B. K., Khanna, S . N., Eds.; Plenum: New York, 1987, pp 323-328. (15) Creasy, W. R.; Brenna, J. T. Chem. Phys. 1988,126, 453. (16) Hettich, R. L. J. Am. Chem. SOC.1989,111, 8582.
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