J . Phys. Chem. 1987, 91, 2463-2468
2463
Formation Mechanism of Large Clusters from Vaporized Solid Material Isao Yamada,* Hiroaki Usui, and Toshinori Takagi Ion Beam Engineering Experimental Laboratory, Kyoto University, Sakyo, Kyoto 606, Japan (Received: June 17, 1986)
Large vaporized-metal clusters are formed by ejecting the metal vapor from a heated crucible into a high vacuum region through a nozzle. Formation kinetics of the cluster beams are discussed in terms of classical nucleation theory. Computer simulations of cluster formation during the supersonic expansion process have also been carried out. The results show that the nucleation and growth rates of metal vapors are sufficiently high to produce the large clusters that are observed.
Introduction The ionized cluster beam (ICB) technique proposed by Takagi et al. in 1972 uses vaporized-solid-material clusters in the size range of 100-2000 atoms/cluster to form surface deposits of various kinds of metallic, semiconductor, and insulating materials.' The results have shown that high-quality films can be formed on a wide variety of substrate surfaces at low temperature, even with materials from which films could not previously be produced.* The advantages of this technique for film formation are due to the unique physical and chemical properties of the cluster state as well as to the effects of the kinetic energy and chemical activity that can be achieved with charged particles. Very recently, the applications of large clusters have started to extend into a variety of fields of material science and te~hnology.~This extension is mainly because techniques of forming large clusters have been developed and partly because their unique characteristics, unlike those of either small clusters or bulk material, have been re~ealed.~.~ Large clusters can be created by condensation of supersaturated vapor produced by adiabatic expansion through a small nozzle into vacuum. The vapor for the cluster formation can be produced simply by heating solid-state material to a sufficient temperature so that its vapor pressure is high enough to result in supersonic flow downstream of the nozzle in so-called pure vapor expansion.' Classical nucleation theory had led to the common belief that metals would have little tendency to condense due to their high surface tension us6 However, in 1983, the authors and their co-workers pointed out that the barrier heights and nucleation rates for metals are similar to those of gases in spite of their high surface t e n ~ i o n . ~This result comes from the fact that the nucleation barrier height is characterized mainly by u l k T instead of u alone, where k and T a r e Boltzmann's constant and temperature, respectively. This conclusion has been confirmed by Yang and Lu for some different metals and semiconductors.s These theoretical considerations could explain the experimental results that have been obtained on cluster formation with metal (1) Takagi, T.; Yamada, I.; Kunori, M.; Kobiyama, S. Proc. Int. Conf. Ion Sources, 2nd 1972; Osterreichische Studiengesellschaft fiir Atomenergie: Vienna, 1972; p 790. (2) For example, Takagi, T. Proceeding of the International Workshop on Ionized Cluster Beam Technique, Tokyo, 1986, Takagi, T., Ed.; Research Group of Ion Engineering, Kyoto University: Kyoto, 1986; p 1, and references therein. (3) For example, International Symposium on Metal Clusters; Ruprecht-Karls-Universittit: Heidelberg, 1986. (4) Yamada, I.; Stein, G. D.; Usui, H.; Takagi, T. Proceedings of the Sixth Symposium on Ion Sources and Ion-Assisted Technology, Tokyo, 1982, Takagi, T., Ed.; Research Group of Ion Engineering, Kyoto University: Kyoto, 1982; p 47. (5) Gspann, J. International Symposium on Metal Clusters; RuprechtKarls-Universittit: Heidelberg, 1986; p 58. (6) Stein, G. D. Proceeding of the International Ion Engineering Congress Kyoto, 1983, Vol. 2, Takagi, T., Ed.; Institute of Electrical Engineering of Japan: Tokyo, 1983; p 1165. -(7) Yamada, I., ref 6, p 1177. (8) Yang, S.-N.;Lu, T.-M. J. Appl. Phys. 1985, 58, 541.
0022-365418712091-2463$01.50/0
vapors. Evaluations of cluster size, for example, by time-of-flight methods, retarding field energy analysis, electrostatic energy analysis, and transmission electron microscopy have clearly shown that clusters with 102-103atoms can be formed from expansion of pure metal vapor^.^-'^ In terms of classical nucleation theory this paper describes the formation process for large clusters during the nozzle expansion of vaporized solid material. Numerical calculations of the nucleation rate and computer simulations of the vapor expansion show that efficient nucleation occurs in metal vapor and that the resulting clusters grow over considerably greater distances along the nozzle flow axis than do those of normally gaseous materials. The calculations reveal the differences in the nucleation tendencies for vaporized solid materials and normal gases.
Review of Relevant Data from Previous Experiments As background for the calculations reported here we will summarize briefly some pertinent experimental results on the free jet expansion of metal vapors. Figure 1 shows translational velocities determined from time-of-flight (TOF) measurements on free jets of silver vapor at various source t e m p e r a t ~ r e s . ~The ~,~~ dashed lines in the figure show translational velocities attained in isentropic expansions at Mach number M given as u = ( y k T o / m [ ( y- 1 ) / 2 + M-*])"' where the ratio of specific heats y is 5 1 3 for monatomic vapor. In every case the ejected vapor attains supersonic translational velocities. Their corresponding Mach numbers increase with increasing source temperature. Moreover, as the nozzle size (Reynolds number) increases the velocity also increases indicating the enhanced conversion of thermal energy to streaming kinetic energy to be expected for gas expansion. Figure 2 shows similarly the terminal translational temperatures in the free jets of silver vapor as determined from TOF measurements. In every case it is much lower than the source temperature and decreases as the nozzle diameter and source temperature increase, showing the expected increase in terminal Mach number with increasing source Reynolds n ~ m b e r . ' ~ ,These '~ results clearly indicate the kind of (9) Yamada, I.; Takagi, T. Proceedings of the Tenth International Symposium on Molecular Beams, Cannes, 1981; DRET & CEA: France, 1981, D ' VII-Bl. ( I O jTheeten, J. B.; Madar, R.; Mircea-Roussel, A,; Rocher, A,; Laurence, G. J. Cryst. Growth 1977, 37, 317. (11) Yamada. I.; Takagi, T. Thin Solid Films 1981. 80, 105. (12) Yamada, I.; TakGka, H.; Inokawa, H.; Usui, H.; Cheng, S. C.; Takagi, T. Thin Solid Films 1982, 92, 137. (13) Usui, H.; Takaoka, H.; Yamada, I.; Takagi, T. Prcceedings of the Fifth Symposium on Ion Sources and Ion-Assisted Technology, Tokyo, 1981, Takagi, T., Ed.; Research Group of Ion Engineering, Kyoto University: Kyoto, 1981; p 175. (14) Yamada, I.; Takagi, T.; Younger, P. R.; Blake, J. SPIE Advanced Applications of Ion Implantation, Vol. 530; International Society of Optical Engineers: Washington, 1985; p 75. (15) Usui, H.; Ueda, A,; Yamada, I.; Takagi, T. Proceeding of the Ninfh Symposium on Ion Sources and Ion-Assisted Technology, Tokyo, 1985; Takagi, T., Ed.; Research Group of Ion Engineering, Kyoto University: Kyoto, 1985, p 39.
0 1987 American Chemical Society
Yamada et al.
2464 The Journal of PhysicaI Chemistry, VoI. 91, No. IO, 1987
VAPOR PRESSURE (Torr ) 5 10 20
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1700 1800 1900 2000 CRUCIBLE TEMPERATURE (K)
Figure 1. Translational velocity of Ag clusters measured by the timeof-flight method. The broken lines indicate translational velocities attained in isentropic expansions at the Mach numbers indicated.
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Figure 4. TEM images of Ag clusters collected on carbon films at liquid nitrogen temperature (A) and at room temperature (B) for different deposition times.
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CRUCIBLE TEMPERATURE ( K 1 Figure 2. Translational temperature of Ag clusters measured by the time-of-flight method.
supersonic expansion that should lead to high degrees of supersaturation in view of the fact that the source vapor is saturated to begin with. Similar velocity analysis on silver atoms evaporating from an open crucible, i.e. Langmuir evaporation, indicated temperatures nearly equal to the crucible (source) temperature and thus the absence of any gas dynamic expansion. The important question is whether clusters are actually produced in these expansions. A direct answer comes from determinations (16) Anderson, J. B.; Fenn, J. B. Phys. Fluids 1%5,8,780. (1 7 ) Anderson, J. B. Molecular Beams and Low Density Gasdynamics, Wegener, P. P., Ed.: Marcel Dekker: New York, 1974; Chapter 1 .
of cluster size as obtained by the retarding field method which measures the energy of a charged cluster. In combination with a knowledge of velocity, this energy becomes a measure of mass and thus the number of atoms in the cluster. Figure 3 shows cluster size distributions obtained from such retarding field measurements on a beam from a jet of silver vapor. The clusters were generated by using a graphite crucible with a cylindrical nozzle of 1 mm diameter at crucible temperatures of 1800 and 1900 K. The saturated vapor pressures at these temperatures are estimated to be 7.7 and 19 Torr, respectively.'* The distributions are broad, the number of atoms per cluster ranging from several hundred to two t h o u ~ a n d . ' ~ .Both ' ~ the beam intensity and the mean cluster size increase with increasing source temperature. We have also made measurements of cluster size with an electrostatic energy analyzer that showed energies in the range from 80 to 170 eV, in good agreement with the retarding field measurements.' Transmission electron microscope (TEM)observations have also been made on silver clusters. The experiment was performed by collecting the clusters on carbon films cooled by liquid nitrogen.
'
(18) Smithells, J. C. Metals Reference Book;Butterworths: London, 1967; Vol. 1. 4th ed, p 262.
The Journal of Physical Chemistry, Vol. 91, No. 10, 1987
Large Clusters from Vaporized Solids The deposition time was controlled so as to avoid coalescence of clusters on the surface. Figure 4 displays typical TEM images of clusters that have been collected on substrates at liquid nitrogen temperature and a t room temperature. These clusters were produced by using a graphite crucible with a nozzle diameter of 1 mm and a source temperature of 1680 K, the corresponding saturated vapor pressure being 2.2 Torr. The silver clusters collected on the cooled substrate range from 2 to 5 nm and suggest a range from a few hundred to a thousand atoms per cluster. In the case of the room temperature substrate, the clusters appear to have smeared out and lost their original shape. This observation suggests that they broke up upon impact so that their constituent atoms migrated over the surface, a finding that is consistent with our picture of the film formation process by the ICB technique.’ It is noteworthy that we have also obtained some information on the state of aggregation of atoms in a cluster by means of electron diffraction observations. For this purpose, a metal vapor cluster source was installed in the diffraction chamber of an electron microscope and the cluster beam was crossed by high energy electron beam. The diffraction pattern for an antimony cluster beam showed a halo pattern indicating a lack of long range ordering! Similar results have been obtained recently for tellurium and bismuth cluster beams. This absence of apparent crystallinity is characteristic of an amorphous or liquid state and probably plays a significant role in the film-forming process that seems to be unique to the ICB method.
Theory and Computations Classical nucleation theory shows that the Gibbs free energy change AG* for the formation of a cluster of radius r is given by AG* = (AG)max= 16?rcr3/3[(kT/vC) In SI2
(2)
where u, is the molecular volume in a cluster, and S is the saturation ratio P / P , , P , being the equilibrium vapor pressure. With the assumption of a thermally stable steady state, the nucleation rate J for cluster formation can be expressed by
J = K exp(-AG*/kT)
(3)
where K is a factor which varies much more slowly with P and T than does the exponential term. Frenkel19 gave K as K = (P/k7‘)2v,(2~/~m)‘~2
(4)
Since the formation free energy of a cluster AG* depends on surface tension to the third power in eq 2 and appears in the exponent of the nucleation rate expression, the value of J is quite sensitive to variations in CT. This observation has led to the oversimplified conclusion that materials of high surface tension are more reluctant to form clusters than materials with low values of U , The situation is, however, more subtle. Figure 5 shows the results for calculations of the dimensionless nucleation energy AG/kT that must be overcome to add an atom to a cluster of radius r for several materials. The calculation was made according to eq 2 under conditions of P = 10 Torr at temperatures which give S = 100. For the surface tension CT, the flat plane value u, was used in the calculation. In spite of their high surface tension, with the exception of Hg, it is found that the nucleation energy barriers of metals are not high as compared to gases. Figure 6 , a and b, shows respectively equinucleation rate curves for A1 and Ar on pressure-temperature (P-T)diagrams. The P-T relations under dry isentropic expansion are also represented by dotted lines in those figures. The Mach numbers denoted by M a r e also indicated along these lines, assuming that the expansion starts from saturated vapor a t 10 Torr. The equinucleation rate curves for A1 are located much closer to the equilibrium vapor pressure curve compared to those for Ar. The curves show that a sufficient nucleation rate is obtained at an earlier stage in the expansion for A1 vapor than for Ar. When the expansion starts from a saturated vapor of Po = 10 Torr, a J o f loz5clusters/(cm3 s) can be obtained at a Mach number M = 0.8 for Al, whereas (19) Frenkel, J. Kinetic Theory of Liquids; Dover: New York, 1955.
2465
P = IOTorr 60
40
F 2o Y
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a
0
- 20
- 40 Figure 5. AG/kT as a function of cluster radius for several materials. The pressure and the saturation ratio are fixed to P = 10 Torr and P/P, = 100, respectively.
much higher M or S are required for Ar to obtain the same nucleation rate. These results suggest that A1 vapor condenses more easily than Ar. Similar results are obtained for other metals and semiconductors. The formation process for metal clusters during a supersonic nozzle expansion was simulated by combining classical nucleation theory with one-dimensional flow equations. A cluster was assumed to grow from an embryo of critical radius at the maximum value of AG. The growth rate, determined by the difference between the rates of impingement and reevaporation of atoms, is given by dr/dt = (f/p,)(P/(Z?rRT)l/Z - P , / ( ~ T R T , ) ’ / ~ )
(5)
where ( is the sticking coefficient, pc the density of the cluster, T, the temperature of the cluster, and R is the gas constant. P, is the saturated vapor pressure at the surface of the cluster and is given by Thomson’s equation*O as P, = P, exp(2u/pcRT,r)
(6)
The one-dimensional flow equations for the conservation of mass, momentum, and energy are given as follows: (7)
du 1 -/I d P u--+--=O p dx dx du dT dcr u - + (1 - p ) ~- = hf dx dx dx
(9)
where p is the vapor density, A the cross sectional area of the flow, u the velocity, p the mass ratio of condensed to vapor phase, cp the specific heat of vapor, and hfgthe latent heat of condensation. These equations, together with the equation of state for the mo(20) Thomson, W. Proc. R. Soc. Edinburgh 1870, 7, 6 3 .
Yamada et al.
2466 The Journal of Physical Chemistry, Vol. 91, No. 10, 1987
,
1.0 I
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1
1
0 2 DISTANCE ( m m )
3
I
4
6
Figure 7. Variations of the normalized flow temperature T/To and pressure PIP, for AI vapor expansion along the flow axis.
A1 I "
1200 1400 1600 1800 2000 2200
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Figure 8. Mass ratio of clustered material to vapor of AI cluster beam along the flow axis.
v
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I
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Figure 6. Equinucleation rate lines for AI (a, top) and Ar (b, bottom) on pressuretemperature diagrams. Broken lines show the P-T relations of isentropic expansions from saturated vapor of 10 Torr. P, denotes
the equilibrium vapor pressure. nomer vapor, were solved simultaneously by Adams' method.2' Equations 7 to 9 can be transformed as
(21) FACOMFORTRANSSL II User's Guide; Fujitsu: Tokyo, 1981; p 459.
-2
DISTANCE FROM THROAT ( mm)
L L
where W = M/( 1 - p ) and X = hfg/cpT.The major problem in the calculation is that eq 10 has a singular point at hf = 1 - p. For a dry isentropic expansion, i.e. p = 0, M is fixed to unity at the nozzle throat where dA Jdx = 0, and the equation can be solved analytically. Usually, it is assumed that the nucleation takes place in the supersonic part of the flow, Le. downstream of the nozzle throat. In solving eq 10, it can be assumed that the vapor forms noncondensing isentropic flow up to a point whose Mach number is slightly larger than unity. At that point the numerical integration of eq 10 can be started, thereby avoiding the singular point. However, if the nucleation occurs upstream of the nozzle throat, the solution cannot be obtained so easily, because the singular point is now included in the range of integration. In the following calculations, we started the integration of eq 10 4 mm upstream of the throat. The initial condition, i.e. the Mach number at this point, was sought by repeating the integration until it resulted in continuous changes of flow properties at the singular point. This process is required because, in a subsonic flow, the flow properties at one point are influenced by those of the downstream region. Powas set to a saturated vapor pressure at a given crucible temperature To. The cluster temperature T, in eq 5 was assumed to be equal to T for simplicity. Flat plane surfaee tensions were used in the calculations. In a usual ICB source, the vapor is ejected through a short nozzle and is rendered to a free expansion, whose flow field cannot be described properly by one-dimensional equations like eq 7-9. In the following calculations, the crosssectional area of the flow was arbitrarily assumed as A = (rD2/4)[1 + ( 2 x tan ( 8 / 2 ) / 0 ) * ] (1 1) where x is the distance from nozzle throat, D is the throat di-
The Journal of Physical Chemistry, Vol. 91, No. 10, 1987 2467
Large Clusters from Vaporized Solids
1
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CLUSTER SIZE ( ATOMS/CLUSTER) Figure 9. Cluster size distribution of the A1 cluster beam for different
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source pressures Po The arrows indicate the sizes of clusters which have been nucleated at the nozzle throat. -2
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Figure 11. Nucleation rate J and saturation ratio S along the flow of AI
(a, top) and Ar (b, bottom) cluster beams.
I
-4
I
-2
I
0 2 4 DISTANCE R O M THROAT ( m m )
6
Figure 10. Variation of mass ratio of clustered material to vapor of A1 cluster beam along the flow for different surface tensions (a, top) and sticking coefficients (b, bottom).
ameter, and 0 is the flow divergence angle. D and 0 are fixed at 2 mm and 30°,respectively, because our aluminum film formation experiment shows that the angular distribution of the film thickness deposited by the 2-mm nozzle has a full width at half-maximum at about 30’. Figure 7 shows the variation with axial distance of normalized flow temperature Tf To and flow pressure PIPo for Al. As Po increases the promotion of cluster formation is shown by the increase in the dimensionless flow temperature relative to the
isentrope value. This increase is caused by the latent heat of condensation. Figure 8 shows the variation of mass fraction of clustered material along the flow axis. As expected, this mass fraction increases with increasing source pressure. The results in Figures 7 and 8 show that the condensation of A1 vapor starts upstream of the nozzle throat. Figure 9 shows the cluster size distributions for A1 at a distance of 6 mm from the throat for different Po.The cluster size increases with increasing Po. The large clusters in the size distribution are formed early in the expansion, the small ones later. The arrows in the figure indicate the size of clusters whose formation started at the nozzle throat. Larger clusters began their formation upstream in the subsonic region, smaller ones in the downstream supersonic flow regime. The simulation shows that a substantial portion of the large clusters began their formation upstream of the throat. In the calculation of cluster formation by classical nucleation theory, there are some ambiguities with respect to the cluster properties. For example, the surface tension cannot be specified with certainty because the “surface” of a cluster is not well defined. The concept of a sticking coefficient may be a gross simplification for such a surface. The effects of variation in the values of these factors on cluster formation calculations were characterized in
2468
J. Phys. Chem. 1987, 91, 2468-2474
terms of mass fraction of cluster material. Figure 10 shows the change of the cluster/vapor ratio during the expansion for the different values of the surface tension (a) and the sticking coefficient (b). Condensation takes place at an earlier stage of expansion as the surface tension decreases from the flat plane value. The influence of the surface tension is especially significant at the initial state of the cluster formation. Changes in sticking coefficient, on the other hand, have little influence on the nucleation rate but are more significant in the growth process. In order to clarify features of cluster formation in metal vapors, the nucleation rate for A1 clusters was compared to that for Ar. Figure 11 shows the changes in nucleation rate J and saturation ratio S during the expansion for A1 (a) and for Ar (b). In the calculations, it was assumed that Po = 20 Torr in both cases, To being adjusted to the saturation value at that pressure. For Al, a sufficiently high nucleation rate was obtained even by using the flat plane surface tension. For Ar, however, u had to be reduced to about 0.6 times the flat plane value to obtain comparable cluster formation. In the case of Al, the nucleation rate starts to increase upstream of the throat and reaches its maximum around the throat region. As the expansion proceeds, the nucleation rate decreases at first and then gradually increases again. In the case of Ar, on the other hand, there is no significant nucleation until the saturation ratio becomes very high. The maximum nucleation rate is attained in the region downstream of the throat. As the expansion proceeds, the nucleation rate decreases rapidly and vanishes completely. This comparison shows that the formation region for clusters is considerably more extensive for A1 than for Ar. These differences in nucleation and growth mainly reflect the fact that A1 starts to nucleate at a lower supersaturation ratio.
Discussion At first glance, the foregoing calculations seem to explain and account for the experimental results. But it must be remembered that the classical nucleation theory used for the computations in effect identifies the nucleation rate for a particular vapor state with the equilibrium concentration of clusters of critical size for that state. Thus, it assumes local thermodynamic equilibrium and the question arises as to whether the rates of the gas dynamic constituent kinetic processes are sufficiently rapid relative to the rate of expansion to achieve the equilibrium that the theory presumes. In addition, the calculation involves obscure physical parameters such as surface tension, density, and sticking coefficient for clusters. Notwithstanding the fact that the present nucleation theory does not give quantitatively reliable results for cluster formation in supersonic expansions, it can be utilized to compare
on a qualitative basis the tendency for cluster formation of different gases. Our calculations imply that metal vapor is more likely to form clusters compared to gases under comparable expansion conditions. A comparison of the experimental results with the calculation reveals that large clusters can be formed at source pressures somewhat lower than those expected from the simulations. This difference could partly be attributed to the forementioned problems in the theory and partly reflect the rough assumption of the calculation conditions such as flow geometry. However, there remains one question from a molecular dynamic point of view. This question is whether or not at the gas densities involved in the cited experiments the three-body collision frequency is great enough to produce dimers at a rate sufficiently high to provide the equilibrium concentration of critical nuclei that the theory demands. This reservation stems from the reasonable assumption that formation of a dimer requires a three-body encounter and is the first step in the nucleation condensation sequence. Thus, the apparent agreement between the predictions of the classical equilibrium theory and the experimental results with metal vapors set forth in this paper have in some sense served to reemphasize the importance of understanding from a kinetic point of view how the requisite equilibrium state can be reached and maintained, in particular, how the initial aggregation steps that seem to require three-body encounters takes place. Such an understanding has not yet emerged. One point to be mentioned, in this connection, is that some metal vapors may contain a considerable fraction of polyatomic species even at thermal equilibrium, while common gases do not. It is reported, for example, in a mass spectrometric observation of ionic species in Ag vapor that more than 70%of the spectrum consists of Ag2+,Ag,', and Ag,+.22 The development of cluster formation outlined here is only one of several approaches that can be taken. More theoretical approaches should be given consideration, such as that of Knauer, which involves the nozzle wall as a third body.23 Further experimental studies will be necessary to test the validity of these theories. Acknowledgment. The authors express their thanks to Dr. J. Gspann of Karlsruhe University for valuable discussions on properties and formation of metal clusters. (22) Searcy, A. W.; Freeman, R. D.; Michel, M. C. J . Am. Chem. SOC. 1954; 76, 4050. (23) Knauer, W. Proceedings of the International Workshop on Ionized Cluster Beam Technique, Tokyo, 1986, Takagi, T., Ed.; Research Group of Ion Engineering, Kyoto University: Kyoto, 1986; p 41.
Production of Clusters by Laser Vaporization and Their Trapping in Rare Gas Matrices L. A. Heimbrook, M. Rasanen, and V. E. Bondybey*+ AT& T Bell Laboratories, Murray Hill, New Jersey 07974 (Received: June 17, 1986)
Pulsed laser vaporization is combined with matrix isolation to provide a new versatile technique for studies of metal atoms, clusters, and other reactive intermediates. The potentials of the new technique are discussed and it is shown that concentrations of metal dimers and higher clusters can be considerably enhanced compared with conventional matrix experiments. The technique is applied successfully to several metals with widely differing properties, and some new spectroscopic results are also reported. Spectra of several molecules, including Caz, Pb2, and PbO, are generated and, in particular, the molybdenum dimer spectroscopy and photophysics in a matrix are discussed.
Introduction There has been recently a considerable progress in our understanding of the spectroscopy, electronic structure, and bonding of metal dimers and small clusters. Instrumental in this progress
was the development of low-temperature techniques for studying these, intrinsically high temperature, species. A very productive low-temperature technique which was applied quite extensively in studies Of metal clusters is matrix
'Present address: Department of Chemistry, The Ohio State University, Columbus, OH 43210.
(1) Weltner, W.; VanZee, R. J. Annu. Rev. Phys. Chem. 1984, 35, 291.
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0 1987 American Chemical Society
