J. Phys. Chem. 1985,89, 3269-3273
3269
Photodissociation of Weakly Bound Ion-Molecule Clusters: The Kr*CO,+ Cluster Martin F. Jarrold, Andreas J. Illies, Winfried Wagner-Redeker, and Michael T. Bowers* Department of Chemistry, University of California, Santa Barbara, California 93106 (Received: February 13, 1985) The results of a laser-ion beam study of the photodissociation of the Kr.C02+cluster are presented. The photodissociation spectrum, product branching ratios, product angular distributions, and product relative kinetic energy distributions are reported. The photodissociation spectrum has a broad maximum of 2.4 X lo-'' cmzat around 500 nm. Two ionic products were observed: Kr+ (around 60%) and COz+. The product angular distributions could be adequately fit by the expression P(0) = 1 0P2(cos e) with a value of 1.55 for @ for the Kr+ product and 1.35 for the COZ+product, indicating "parallel type" transitions and lifetimes shorter than a rotational period. The product relative kinetic energy distributions peak at large values of relative kinetic energy indicating nonstatistical energy disposal. The kinetic energy distributions for the Kr+ product are bimodal at 650 nm. The two components probably arise from production of both spin-orbit states of Kr': 2P1/2and 'P3/2.
+
I. Introduction Ion-molecule clusters are interesting from several perspectives. In contrast to their neutral analogues, which are only bound in a van der Waals sense, ion-molecule clusters are often quite strongly bound. For this reason ion-molecule clusters play an important role as intermediates in chemical reactions.' From a more practical point of view, ion-molecule clusters are important constituents of the upper atmosphere and are present in highenergy environments such as lasers, plasmas, and discharges. Despite the practical and fundamental importance of ionmolecule clusters very little is known about the structure, bonding, and other physicochemical properties of these species. Studies of the spectroscopy of ions have lagged far behind studies of their neutral counterparts because of the problems in generating the large ion density that is required for spectroscopic studies. In the past few years, however, developments in both laser technology and experimental methods have made possible spectroscopicstudies of ions in favorable cases.2 In a recent series of papers we have reported the results of investigations of the photodissociation of a number of cluster ion^.^-'^ We have reported studies of simple symmetric dimer ions such as (NO)2+,3(N2)2+?(C02)z+$(S02)2+,'and (N20)2+8 and mixed clusters such as Kr.02+ and A P C O ~ + . ' The ~ mixed clusters are particularly interesting because they may be involved as intermediates in charge-transfer reactions and photoexcitation may probe the "collision complex" region of the potential surface important in the charge-transfer reaction. In this paper we report an extension of our studies to the Kr.C02: cluster. To our knowledge essentially nothing is known about tlus simple cluster ion. The Kr.C02+cluster is particularly interesting because the ground states of the Kr+ + C02and C02+ + Kr asymtotes are separated by only 0.230 eV." Thus, both sets of products could be formed by photodissociation. The energy separation between the Kr+ 2P3/z and Kr+ 2P1/2spin states is (1) See, for example, Maut-Ner, M. In "Gas Phase Ion Chemistry", Bowers, M. T., Ed.; Academic: New York, 1979. (2) See, for example, Dunbar, R. C. In "Gas Phase Ion Chemistry", Bowers, M. T., Ed.; Academic: New York, 1979. Carrington, A. Proc. R. Soc. London, Ser. A 1979, 367, 433. Woods, R. C. In "Molecular Ions: Geometric and Electronic Structura", Berkowitz, J., Groeneveld, K-O., Eds.; Plenum: New York, 1983. (3) Jarrold, M. F.; Illies, A. J.; Bowers, M. T. J . Chem. Phys. 1983, 79, 6086. (4) Jarrold, M. 222. (5) Jarrold, M. 214.
F.;Illies, A. J.; Bowers, M. T. J. Chem. Phys. 1984, 81, F.;Illies, A. J.; Bowers, M. T. J . Chem. Phys. 1984,81,
( 6 ) Illies, A. J.; Jarrold, M. F.; Wagner-Redeker, W.; Bowers, M. T. J . Phys. Chem. 1984,88, 5204. (7) Jarrold, M. F.; Illies, A. J.; Bowers, M. T. J . Chem. Phys. 1985, 82, 1832. (8) Misev, L.; Illies, A. J.; Jarrold, M.
F.;Bowers, M. T. Chem. Phys., in
prcss. (9) Jarrold, M. F.; Misev, L.; Bowers, M. T. J. Chem. Phys. 1984, 82, 4369. (10) Illies, A. J.; Jarrold, M. F.;Wagner-Redeker, W.; Bowers, M. T. J . Am. Chem. Soc., in press.
0022-3654/85/2089-3269$01.50/0
relatively large, 0.666 eV." The energetics of the Kr.C02+ system are thus quite complex and are summarized in Figure 1. The dissociation energy of the Kr-C02+cluster given in the figure (0.74 eV) is a recent measurement12 which is discussed in section IV. The range of photon energies employed in the experiments reported here is shown in Figure 1. It is evident that at 650 nm the available energy is close to the threshold for production of Kr+ in the higher spin-orbit state, while this threshold is substantially exceeded at 458 nm. The charge-transfer reaction Kr+ + C 0 2
-
COz+ + Kr
(1)
may be related to the present studies because the reaction could proceed through a long-lived Kr+.C02 intermediate. The charge-transfer reaction has been the subject of several and occurs at close to the collision rate for both spin-orbit states of Kr+.15 The paper is organized as follows: in section I1 the experimental methods are briefly reviewed; the results are presented in section 111and this is followed by Discussion (IV) and Conclusions (V) sections. 11. Experimental Section The experimental apparatus and methods have been described in detail e l s e ~ h e r e ~and - ~ will only be briefly reviewed here. A schematic diagram of the experimental apparatus is shown in Figure 2. The experiment consists of a reverse geometry mass spectrometer (VG Instruments, ZAB-2F), an argon ion laser (Coherent, Innova 20), and dye laser (Coherent, Model 590). Cluster ions were generated in a cooled (approximately -80 "C), high-pressure ion source, accelerated to 8 kV and mass selected by the magnet. The ion beam was then crossed with the focussed laser beam a t the intermediate ion beam focus in the second field-free region. The photoproducts were energy analyzed by an electrostatic analyzer using an energy-resolving power of 2000-2700 fwhm and detected with an electron multiplier. Data were accumulated by using pulse-counting techniques in a multichannel analyzer. The laser beam was modulated with an electronic shutter and any background product component removed by up-down counting. The data were analyzed off-line with an IBM 9000 computer. The ion source was operated with the extraction voltage set to zero and at a total pressure of between 0.1 and 0.12 torr (approximately 30% Kr and 70% CO,).Gases were obtained from commercial sources: COz from Matheson (bone dry) and Kr from (11) Rosenstock, H. M.; Draxl, K.; Steiner, B. W.; Herron, J. T. J . Phys. Chem. ReJ Data 1917, 6, Suppl. No. 1. (12) Derai, R.; van Koppen, P.A. M.; Bowers, M. T. to be submitted for publication. (13) Landenslager, J. B.; Huntress, W. T.; Bowers, M. T. J . Chem. Phys. 1974,61,4600. (14) Hartland, P. W.; Ryan, K. R. Int. J. Mass Spectrom. Ion Phys. 1975, 18, 215. (15) Adams, N. G.; Smith, D.; Alge, E. J . Phys. B 1980, 13, 3235.
0 1985 American Chemical Society
3270 The Journal of Physical Chemistry, Vol. 89, No. 15, 1985 +2-
2
PHOTON ENERGY, eV 3.0 2.8 2.6 2.4 2.2 2.0
458 nm
t i -
>-
l-
(L
~
5 Kr+(*P,,,)
Kr+(*P3,,)
z 0-
+ C0,C'Z)
+ C0,C'Z)
hv
*' 3 22
0
%?i
8I - I2- 2 -
-
MIRRORS
0
--
8\, Q ,
I
I I
I
500 600 WAVELENGTH, nm
700
Plot of the total photodissociation cross section against wavelength for Kr.C02+. The dashed line drawn through the data points is only a guide. Figure 3.
SHUTTER
LASER
I
PRODUCTS
0
--6°--€s-0
400
Kr.CO:
k A
I
3
0
-
-
-
B 0.5 -
W
\ b,-
o.o!, , + , , , , , , 0.0 0.2 0.4 0.6 0 . 8
-
, I ,
1.0 1.2 1 . 4 ' 1.6 1.8 2.0
PRODUCT RELATIVE KINETIC ENERGY, eV LABORATORY ENERGY, eV
Figure 5. Peak shapes for Kr+ from photodissociation of Kr.C02+at 514
nm with the laser polarization at Oo and 90° with respect to the ion beam direction. The points are the experimental data and the line is a computer simulation with b = 1.55 (see text). this falloff in the fraction of Kr+ formed occurs in the same wavelength region as the decline in the total photodissociation cross section. B. KP Kinetic Enerm Distributions and Angular Distributions. The products of photodissociation are not generally distributed isotropically because the probability of photon absorption is related to the projection of the electric dipole transition moment on to the electric vector of the laser beam. Thus, photon absorption occurs with a larger probability for particular orientations of the molecule in space than for others. With plane polarized light the photofragments have an angular distribution of the general form:l9 P(6) = (4a)-'[1
+ /~P,(COSe)]
(3) In this expression P(6) is the probability per unit solid angle that the products recoil at an angle 6 with respect to the electric vector, P,(cos 6) is a second degree Legendre polynomial in cos 6 and 0 is the asymmetry parameter which can have values between +2 and -1. Photofragment angular distributions can provide information on the type of transition occurring on photon absorption and also on the lifetime of the excited ion. In the present work, information about the product angular distribution is derived from comparison between the measured peak shapes (with Oo and 90' between the laser beam polarization and the ion beam direction) and a computer simulation which uses the measured product relative kinetic energy distribution and eq 3 for the product angular distribution. A value for /3 is deduced by adjusting 0 to give the best match between the computer simulation and the measured peakse3 The simplest model we can use for the product angular distribution is one in which p does not vary with the product relative kinetic energy and thus has a single value. With this model it was possible to achieve a good match between the simulation and the peaks measured for the Kr+ product with 0' and 90' between the laser beam polarization and the ion beam direction. An example is shown in Figure 5 for a wavelength of 514 nm. The points are the experimental data and the lines are the computer simulation. The value of /3 used for the computer simulation shown in Figure 5 was 1.55. The same value for /3, f0.05, fit the measured Oo and 90° peak shapes over the whole wavelength range studied (458-650 nm). Thus the angular distribution of the product Kr+ does not change significantly with the products relative kinetic energy or with the wavelength of the photodissociating light. Product relative kinetic energy distributions are derived from the peak shape measured with the laser beam polarization at the "magic angle" of 54.7O with respect to the ion beam direction. With this configuration the measured peak shape is independent (19)
Zare, R. N. Mol. Photochem. 1972, 4, 1. Busch, G . E.;Wilson, K.
R.J . Chem. Phys. 1972, 56, 3638.
Kr.CO:
1
+ hv X
Kr++C02
-D
(b)
= 458 nm
2 2
'3/2
, , , , , , , , , , , , ,-,?i, 1.0 1.2 1.4 1.6 i . 8 PRODUCT RELATIVE KINETIC ENERGY, eV
0.0 0.2 0.4 0.6 0.8
'I
2.0
Figure 6. Relative kinetic energy distributionsfor the Kr+ product from the photodissociation of Kr.C02+ at (a) 650 nm and (b) 458 nm. The small oscillations were not reproducible.
of the product angular distribution and only contains information on the product relative kinetic e n e r g i e ~ .The ~ product relative kinetic energy distribution can be derived from the "magic angle" peak by taking the derivative of the peak and changing the energy axis to the center-of-mass frame. Finally, the product relative kinetic energy distributions were corrected for instrumental discrimination. The procedure employed has been described in detail in ref 3. Product relative kinetic energy distributions for the Kr+ product measured with wavelengths of 458 and 650 nm are shown in Figure 6. Both distributions peak at fairly large values of relative kinetic energy, which indicates nonstatistical energy disposal. The 650-nm kinetic energy distribution is clearly bimodal. There is a sharp maximum at around 0.3-eV relative kinetic energy and a broader maximum at 0.5 eV. As the wavelength is reduced the bimodality becomes less obvious and is not evident in the product relative kinetic energy distributions for wavelengths of 5 14 nm and shorter. The small oscillations evident in the 458-nm distribution shown in Figure 6 are statistical noise and were not reproducible. A plot of the average product relative kinetic energy against the photon energy is shown in Figure I. As the photon energy is reduced the average product relative kinetic energy initially falls but then for photon energies in the range 1.9-2.1 eV (650-590 nm) the average apparently remains constant. C. COz+ Kinetic Energy Distributions and Angular Distributions. The C02+product Oo and 90° peak shapes were analyzed in the same way as described for the Kr+ product. Values for /3 of 1.35 f 0.10 adequately fit the measured peak shapes over the whole wavelength range studied (458-650 nm). Thus the C02+ product angular distribution does not change significantly with either laser wavelength or product relative kinetic energy. Product relative kinetic energy distributions measured for the COz+product at 650 and 458 nm are shown in Figure 8. The
Jarrold et al.
3272 The Journal of Physical Chemistry, Vol. 89, No. 15, 1985
TABLE I: Dissociation Energies of Some Selected Ion-Molecule Clusters cluster Kr2+
GO,),++ Kr.C02
Ar.C02+ Kr-02+ N2.02’
NyNO’
..
118
212 214 216 PHOTON ENERGY, eV
2j8
210
Figure 7. Plot of the average center-of-mass product kinetic energies against photon energy for the photodissociation of Kr.C02+ (C02+(0) and Kr+ (0)).
+ Kr
(a)
Do0, eV
ref
1.176 0.70 0.74 0.26 0.33 0.24 0.22
20 23 12 24
9 25 26
charge transfer by Kr+ resulting in formation of COz+. The COz+ ions then undergo association reactions yielding (CO&+ and Kr.COZ+. These clusters can subsequently undergo a series of “ligand switching” reactions to ultimately yield Krz+which is very strongly bound.20 The relative intensities of the main ions in the mass spectrum were approximately COz+ (18), (COz)z+ (l), Kr.COZ+(2), and Krz+ (10). The fact that the observed intensities of (C02)*+and Kr-COz+in the mass spectrum are similar suggests that the dissociation energies of KrC02+and (COz)2+are similar. Subsequently the equilibrium
+
(COz)z+ Kr
Kr.C02+
+ COz
(4)
was studied by Derai et a1.12 using a drift tube ion source. From the temperature dependence of the equilibrium constant a value for AH of -0.065 eV was derived for reaction 4. Converting this value for AH to 0 K21-22 and combining it with the 0 K dissociation energy of (COz)z+z3yields a value of 0.74 eV for the 0 K dissociation energy of the Kr.C02+ cluster. This value for the dissociation energy of Kr.COz+ is compared with the dissociation energies of other ion-molecule clusters in Table Table I illustrates that symmetric clusters or dimers are generally strongly bound (>0.6 eV) and that asymmetric clusters are generally weakly bound (-0.2-0.3 eV). Kr.COz+, however, does not fit these generalizations. It is as strongly bound as the symmetric clusters. Unlike the other mixed clusters in Table I, the components of the Kr.COZ+cluster (Kr and C02) have ionization potentials that are quite similar (see Figure 1). This similarity results in extensive delocalization of the positive charge and hence stronger chemical bonding.24 Put another way, the reason the Kr.C02+ cluster is so strongly bound is “resonance stabilization” due to the two species Kr+-C02 and Kr.C02+. Photoproduct angular distributions provide information on the nature of the electronic transition and the lifetime of the excited molecule. This information is contained in the asymmetry parameter. Positive values of the asymmetry parameter indicate a “parallel type” transition (in this case relative to the Kr-CO2+ bonding axis) and negative values, a “perpendicular type” transition. Values of the asymmetry parameter outside the range 0.5 > p > -0.25 indicate a lifetime shorter than a rotational period. For photodissociation of K r C 0 2 + the values of the asymmetry parameter determined from the experimental data were 1.55 f 0.05 for the Kr+ product and 1.35 f 0.10 for the C 0 2 +product. These values indicate that both products were produced by I.9*20923-26
0.0 0.2 -0.4 0.6 0.8 1.0 i . 2
1.4 f.6 1.8 PRODUCT RELATIVE KINETIC ENERGY, eV
1
E
l
0.04 ,
+ hv-
Kr.CO:
l
CO:
+ Kr
(b)
\
, ,
,
, ,
,
,
,
v\,
-
--,
i.0 1.2 1.4 1.6 1.8 PRODUCT RELATIVE KINETIC ENERGY, eV
0.0 0.2
0.4 0.6 0.8
,
, 2.0
Figure 8. Relative kinetic energy distributions for the C02+product from the photodissociation of Kr.C02+ at (a) 650 nm and (b) 458 nm.
distributions peak at large values of relative kinetic energy indicating nonstatistical energy disposal. There is no evidence of any bimodality in these distributions. The small oscillations in the distributions are statistical noise and were not reproducible. Average product relative kinetic energies derived from the distributions are plotted in Figure 7 against the photon energy. The average relative kinetic energies for the COz+product are smaller than for the Kr+ product despite the fact that COz+ is the lower energy product; i.e., there is more energy available to the C 0 2 + products than to the Kr+ products (see Figure 1). IV. Discussion Before proceeding to discuss the photodissociation of Kr.C02+ we will first consider the ion chemistry which generated the KrCO2+ clusters. The primary processes occurring in the ion source are electron impact forming Kr+ and COz+ and rapid
(20) Abouaf, R.; Huber, B. A.; Cosby, P. C.; Saxon, R. P.; Moseley, J. T. J . Chem. Phys. 1978,68, 2406. (21) The conversion is given by -moo = -maTJ[Cp(KrC02+) C,(CO,) - Cp((C02)2+)- C,(Kr)] dT. The integral can be estimated by using statistical thermodynamics but this requires values for the vibrational frequencies of Kr.C02+ and (C02)2+.The important modes are the low-frequency ones associated with the cluster bonding. These modes are of sufficiently low frequency that their contribution, at the appropriate temperature, could be approximated as kT. Actually, we estimated a reasonable set of vibrational frequencies for KrC02+ and used the frequencies for (C02)2+, estimated from the measured AS (given in ref 22), for evaluating the integral term. (22) Illies, A. J.; Jarrold, M. F.; Bass, L. M.; Bowers, M. T. J . Am. Chem. SOC.1983, 105, 5775; also see ref 9. (23) Derived from AH determined from equilibrium measurements, see ref 22. (24) Pratt, S . T.; Dehmer, P. M. J . Chem. Phys. 1983, 78, 6336. (25) Janik, G. S.; Conway, D. C. J . Phys. Chem. 1967, 71, 823. (26) Turner, D. L.; Conway, D. C. J . Chem. Phys. 1976, 65, 3944.
+
+
The Kr.C02+ Cluster "parallel type" transitions and the lifetime of the excited clusters is less than a rotational period which is consistent with dissociation on directly repulsive surfaces. From the plot of the average kinetic energy against the photon energy, shown in Figure 7,it is evident that as the photon energy increases the average product kinetic energies increase only slightly. For the C 0 2 +product the average kinetic energiis increase from 0.41 eV at 650 nm to 0.48 eV a t 458 nm. On going from 650 to 458 nm only 8% of the increase in the photon energy appears as product relative kinetic energy. The balance, 92%, must be channeled into internal excitation of the C02+product. The energy partitioning in this product channel is rather unusual. A simple impulsive model2' predicts the fraction of the available energy, EAV,channeled into product relative kinetic energy, ET, is ET/EAV= PBC/PF (5) where wBc is the reduced mass of the atoms at the end of the breaking bond and I . L ~is the reduced mass of the fragments. For the CO, + Kr system this model predicts that a minimum of 36% of the extra photon energy should appear as product kinetic energy, which is much more than the 8% observed. It seems likely that over the wavelength range studied most of the extra energy must be deposited directly into the C 0 2 + vibrational modes by the electronic transition. The vibrational modes in C 0 2 and C 0 2 + are widely enough spaced17 that structure might have been observed in the product kinetic energy distributions. Other than the apparent bimodality in the 650-nm Kr+ data (Figure 6a) definative structure was not unambiguously observed. The 590-nm Kr+ distribution does appear to show some reproducible structure, especially on the low-energy side of the peak in the distribution where the instrument resolution is highest2* The spacings appear best correlated with the bending mode v2 of COz but could also be due to combination modes. Since this structure was not clearly observed at other wavelengths it cannot be assigned. For the Kr+ product the change in the average kinetic energies is larger than observed for the C02+product. On going from 650 to 458 nm around 21% of the increase in the photon energy appears as product relative kinetic energy. The balance must be channeled into internal excitation of the products; either as electronic excitation of Kr+ (Le., Kr+ 2P112rather than Kr+ 2P3/2) or as vibrational-rotational excitation of C02. As note above, the 650-nm Kr+ relative kinetic energy distribution shown in Figure 6a is bimodal. The two maxima in the distribution probably arise from production of Kr+ in the 2P1/2 and 'P3/2 electronic states. Arrows labeled zP112and 2P3/2in Figure 6 show the location of the kinetic energy thresholds for production of the spin-orbit states from ground-state Kr.C02+. Products originating from ground-state Kr.C02+ with relative kinetic energies greater than the arrow labeled 2P112must be produced in the lower-energy 'P312 spin-orbit state. Thus the lower-energy maxima in the kinetic energy distribution probably arises from Kr+ 2Pl/2and the higher-energy maxima arises from Kr+ 2P312. An approximate deconvolution of the two peaks in the 650-nm distribution suggests the Kr+ product is produced approximately 77% in the 'P3/2 state and 23% in the 2P1/2state. The sharp drop in intensity at the 2P1/2threshold in the 650-nm Kr+ distribution suggests that at this wavelength the amount of Kr+ produced in the 2Pl/2state is restricted by energetic con(27) Busch, G. E.; Wilson, K. R. J . Chem. Phys. 1972, 56, 3626. (28) Jarrold, M. F.;Illies, A. J.; Bowers, M. T.Chem. Phys. 1982,65, 19.
The Journal of Physical Chemistry, Vol. 89, No. 15, 1985 3273 straints. The fall in the total photodissociation cross section at wavelengths greater than 600 nm (see Figure 3) and the reduction in the fraction of Kr+ produced at these wavelengths (see Figure 4) are both probably the result of the operation of this energetic constraint on the production of the higher-energy Kr+ 2P1/2state. At wavelengths shorter than 650 nm it is likely that a larger proportion of the Kr+ product is produced in the 2P, state. From the 458-nm kinetic energy distribution shown in kigure 6b it is feasible that all the Kr+ product is produced in the 2P1/2state since essentially all the product intensity lies at kinetic energies less than the 2P1/2threshold (although this does not prove the product is 2pl/2).
V. Conclusions The Kr.C02+ cluster photodissociates in the visible region of the spectrum. The absorption has a broad maximum of 2.4 X cm2 at around 500 nm. Three sets of products are formed: Kr+(2P3/2)+ CO,, Kr+(2P1/2)+ C 0 2 , and C 0 2 + + Kr. The product angular distributions indicate that all the products are produced by "parallel type" transitions and the lifetime of the excited species involved is less than a rotational period; a result consistent with dissociation on directly repulsive surfaces. The product kinetic energy distributions peak at karge values of relative kinetic energy indicating that energy disposal is nonstatistical for all the product channels. For the COz+ product the average relative kinetic energies increase only slightly as the photon energies are raised. Most of the extra energy is probably deposited directly into the vibrational modes of COz+ by the electronic transition. For the Kr+ product the kinetic energy distributions are bimodal at small photon energies due 'to the operation of energetic constraints on the production of Kr+ 2P1/2.The same energetic constraints are probably responsible for the fall in the total photodissociation cross section above 600 nm and the reduction in the fraction of Kr+ product formed at these wavelengths. The Kr.C02+ cluster differs from the Ar.C02+ and Kr.02+ clusters we have studied p r e v i o ~ s l y . ~For . ~ ~Ar.C02+ and Kr.02+ the ionization potentials of the components differ by approximately the photon energy so the electronic transition induced by photon absorption is an intramolecular charge-transfer transition from on A+.B state to a B+.A state. Furthermore, for Kr.02+ and Ar.C02+ the A+.B and B+.A states are well separated in energy. For Kr.C02+, on the other hand, the components have ionization potentials which are quite similar so the A+.B and B+.A states have close to the same energy and there will be maqy states in the range accessible with the photon energies employed. The most straightforward interpretation of our results for the photodissociation of Kr.C02+ is that there are three directly repulsive states involved. Each surface responsible for one of the three ionic products (Kr' 'P3/2, Kr+ 2P1,2, and CO,'). However, because of the high density of electronic states (since A+.B and B+.A have close to the same energy) it is possible that there are several intersections between potential energy surfaces and that one or more products arises from surface hopping at an intersection. Acknowledgment. We gratefully acknowledge the support of the Air Force Office of Scientific Research, under Grant AFOSR-82-0035, and in part the National Science Foundation, under Grant CHE80-20464. W. W.R. also acknowledges the Deutsche Forschnungsgemeinschaft for a fellowship. We also gratefully acknowledge Tony OKeefe for being just one hell of a guy* Registry No. Kr', 16915-28-9; C 0 2 , 124-38-9; C02', 12181-61-2.
