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On the nature of NiAr+
Volume 152. number 6
CHEMICAL
PHYSICS LETTERS
25 November
1988
ON THE NATURE OF NiAr’ Dan LESSEN and P.J. BRUCAT Department of Chemistry,
Universrly of FIorda,
Galnesvtllej FL 326 Il. USA
Received 28 August I988
N&r+ ions generated in a supersonic expansion of Nr+ with a 1% Ar/He carrter gas arc mtccted into a tandem mass spectrometer and photodissociated with visible tunable laser radiation. A photofragmentation feature observed at 17984 cm-’ is believed to correspond to the dissociation threshold of NiAr+ to excited state (ZF,,L) Ni+ and ground state Ar. This assignment ytelds a prediction of 0.55 eV for the binding energy (Di) of NiAr +. The “NiAr+ ib”NiAr+ isotope shift of this spectral feature indicates the vibrational frequency in the ground state of NiAr+ to be 235 -t 50 cm-‘. Thus NiAr+ is a surprisingly “stiff” and strongly bound diatomic, discussed in light of a simple electrostatic (charge-induced-dipole) model of the internuclear forces in this class of molecules
1. Introduction How is it possible to determine the degree of internal excitation of an isolated molecular ion passively and routinely? Any method capable of providing such information would facilitate the spectroscopic and chemical study of gas phase ions of al sorts. However, it is not easy to conceive of an easy way of accomplishing such a measurement of reactive species in intrinsically low abundance. Consider a molecular ion which is observed to undergo unimolecular decomposition, In this case the level of internal excitation of the species must be above the lowest dissociation limit of said molecule, i.e. the molecule is very hot. Utilizing this concept one can fabricate an analog to the ion under investigation, an ion with a very weakly bound adduct such as a rare gas atom. The new molecular complex is relatively unstable; decomposition by loss of the rare gas atom occurs at a much lower internal temperature than that necessary for the decomposition of the “bare” ion. This threshold decomposition temperature will be defined by total binding energy of the rare gas atom to the ion and the vibrational (and possibly electronic) partition function of the complex. If the binding of the rare gas adduct is weak enough and does not perturb the intrinsic nature of the ion to which it is bound, the presence of the rare 0 009-2614/88/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division)
gas atom will indicate an upper limit to the temperature of the ion, making the atom a “hot” or “cold” thermometer. To utilize weakly bound rare gas adducts as “thermometers” for ions we must first understand the details of their binding to the ions of interest. Many ions with weakly bound adducts have been studied to shed light on the nature of ion solvation #I. In our laboratory transition metal cluster ions are of particular interest and generation of these ions in association with one [2] or more [3] rare gas atoms has recently been accomplished. As a benchmark for these studies, the study of rare gas atoms clustered around single transition metal atomic ions has been undertaken. The following is a description of the results obtained for one such ion, NiAr+.
2. Experimental A full description of the experimental apparatus and procedure used in this study will appear shortly [ 41. Briefly, transition metal atomic and cluster ions are generated in a laser-driven-plasma supersonicexpansion ion source. This source is similar to that used to produce positively and negatively charged w’ For an excellent review of these types of studies, see ref.
B.V.
[ 11. 473
Volume152.number 6
transition metal cluster ions [ 51 but has been mod-
ified to optimize the internal cooling of the ions subsequent to their formation. The effective internal cooling of the cluster ions from this source is evidenced by the observation that trace amounts of argon in the ion source gas supply routinely generate [ 3 ] metal cluster ions with weakly bound charge-induced-dipole rare gas adducts (M,,~Ar,t ). Limitation of the atomic ion number density upstream of the expansion orifice by reduction of the laser vaporization energy makes it possible to generate almost exclusively MArT ions (0 $ ni 15 ) and in particular MAr’, the featured ion of this study. After generation and cooling the ions are entrained in a pulsed helium molecular beam, traverse two differentialpumping orifices, and pass collisionlessly into a specialized time-of-flight mass spectrometer. There, the ion ensemble is accelerated through 1.45 kV and focused down a 2.45 m flight tube and into the entrance aperture of a 127” electrostatic sector energy analyzer. Primary mass spectra are obtained from the time-of-arrival distribution detected at the exit aperture of the energy analyzer by a dual microchannel plate electron multiplier. Secondary or photofragment mass spectra are derived from the electrostatic analyzer transmission at the same time of arrival as the parent ion, but at a potential corresponding to the ratio of fragment to parent masses times the parent transmission potential. Photoexcitation of the ions is accomplished by coaxial Nd:YAG-pumped dye laser irradiation of the ion beam ( 5-25 ) x 1O-6 s prior to passage into the energy analyzer.
3. Results Fig. 1 shows the photofragmentation of NiAr+ +Ni++Ar as a function of dissociation laser frequency on the interval from 17400 to 18100 cm-‘. Ar+ is not observed as a photoproduct of NiAr+ at these photon energies as would be cxpccted from the large disparity in the ionization potential of the two atoms(IP(Ar)=15.755eV,IP(Ni)=7.633eV) [6]. Therefore, NiAr+ photofragmentation is monitored as the Ni+ fragment ion current transmitted by the electrostatic sector. Under the normal operating conditions of the mass spectrometer, a trace amount of Ni+ from NiAr+ is observed from collision-induced 474
I$ November 1988
CHEMICAL PHYSICS LETTERS
-I r
NiAr’+ ~~
hv
+
Ni j-t
Ar
E (3 c 0
Laser
Frequency
(wavenumbers)
Fig. 1. Relative photofragmentation (ion current) of NiAr*+Ni* +Ar as a function of laserfrequency over the interval 17400-1800 cm- ‘. A jump in the one-photon photofragmentation is observed at 17984 cm ‘, indicating a threshold for producing excited *F7,2 Ni+ ions. This establishes the binding energy of NiAr+ as 0.55 eV.
dissociative processes. The study of such collisioninduced processes involving metal-argon complex ions will be reported elsewhere. The data presented here are in the form such that the smatl CID contribution to the dissociation has been nullified. To within the signal to noise of the present data, the photodissociation action spectrum of NiAr+ in the region of 18000 cm ’ appears as a featureless edge, presumably indicating the onset of a photodissociation threshold, i.e. the point at which the laser photon has just enough energy to produce (excited state) products with zero kinetic energy, Therefore we ascribe the edge energy of 2.23 eV ( 17984 cm-’ ) as the sum of the binding energy of NiAr+ and some promotion energy in the isolated Ni+ ion. Internal electronic excitation of the Ar atom is not energetically possible. The absence of any vibrational or electronic hot band features associated with the threshold at 17984 cm- ’ implies extensive cooling of the NiAr+ emanating from the supersonic-expansion ion source. Laser fluence dependence of the dissociation yield at 18020 cm- ’ (above the dissociation edge) shows a linear fragmentation response over a range of 0.5 to > 8.0 mJ/pulse cmW2. This indicates that the photodissociation at this energy involves a simple one-
Volume 152, number 6
CHEMICAL PHYSICS LETTERS
photon absorption event, leaving no ambiguity as to the value of the excitation energy imparted to the NiAr+ by the laser. By premise, the absorption spectrum of NiAr+ must be similar to the absorption of the bare Ni+ ion. Consultation of the energy levels [4] of Ni’ shows that no electric dipole allowed transitions would be expected within the manifold of states arising from the 3d9 configuration since all these states have the same overall parity. The lowest electronic state corresponding to 3d84p configuration (and electric dipole connected to the ground state) is 6.39 eV above the 2DS,2 ground state of Ni+. The photodissociation of NiAr+ appears to occur through at least weakly allowed one-photon transition at 2.3 eV, where no isolated Ni+ transitions are expected. The presence of the polarized argon atom produces an electric field in the vicinity of the Ni+ and may effectively “mix” states of opposite parity in the isolated ion. The spherical symmetry of the atomic ion is broken, making J and L, the total orbital angular momentum of the electrons with and without spin, no longer strictly good quantum numbers. One possible assignment of the photodissociation feature in fig. 1 is the threshold for production of 2F 7,Z Ni+ and ‘S Ar in a transition that derives its nature from the forbidden 3ds4s 2F,,2+3d9 2DS,z transition in isolated NiC. If this is indeed the case, then the NiAr+ ground state binding energy (0: ) is the difference between the Ni+ 2F7,2+2Dg,2 transition energy of 1.68 eV (13550.3 cm-‘) [4] and the observed threshold energy of 2.23 eV, or 0.55 eV. The isotope shift between the s8Ni40Arf and 60Ni40Arf photodissociation features is 0.8 f 0.15 cm-’ with the heavier isotope being shifted to higher energy. This shift corresponds to the difference in the vibrational energy of the ground state of these two species. Presumably, this vibrational energy is zero point, suggesting the ground state vibrational frequency of NiAr+ is 235 250 cm-‘.
4. Discussion Since there is little likelihood of any formal charge residing on the Ar atom in NiAr+, a good approximation to the nature of this molecule may be derived from a picture of an almost unperturbed Ni+ ion with
25 November I98 8
a polarized Ar atom bound to it by simple chargeinduced-dipole forces. Postulating the functional form of the repulsive interaction between the Ni+ and the Ar electron clouds, we may guess a singleparameter adjustable interaction potential of the form U(r)=ar-‘2-tq2ar-4, fq’Cr=
1.195X 10e3’ eV cm4,
where I is the Ni+-Ar internuclear separation, N is the polarizability [ 71 of argon, and a is an adjustable parameter. This approximate potential curve assumes a rotationless diatomic. Table 1 shows the application of this hypothetical electrostatic potential described above to the experimental data determined in this study. The columns indicate values of w,, the vibrational frequency of NiAr+ in its ground electronic state; r,, the ground state equilibrium Ni-Ar internuclear distance: O!, the energy required to dissociate NiAr+ in its zeropoint vibrational level to 2D5,2 Nit + ‘S Ar, and k, the vibrational force constant at r,. In the first row of this table, the experimental vibrational frequency is used to determine the force constant (in the harmonic oscillator approximation) and the second derivative of the potential surface at r,. This determines a and all other listed molecular parameters. In the second row of table 1, the experimentally determined dissociation energy, 0: =0.55 eV, is used to determine a and a second (and independent) set of derived molecular parameters. Comparison of the vibrational frequency determined from the assignment of the observed dissociation feature as production of ‘FTi2 Ni++ ‘S Ar leads to a predicted vibrational frequency of 289 cm- ’ in the context of our electrostatic model which is in remarkable agreement with the vibrational frequency of 235 + 50 cm-’ determined from an independent isotope shift measurement. This lends credence both to the threshold assignment and to the Table 1 Estimated molecular constants for NiAr+ W, (cm-‘)
@ (eV)
r, (10-8cm)
k
-235 289
0.416 0.550
2.074 1.935
71 117
( lo3 erg/cm’)
475
volume 152,number 6
CHEMICALPHYSICSLETTERS
validity of the model potential. Moreover, the vibrational frequency is very sensitive to small changes in r,, indicating that the equilibrium internuclear distance in NiAr+ is about 2.0~ IO-” CM. Since the experimental determination of 0: is more accurate than that of the isotope shift (although contingent upon the correct assignment of the limit), one might expect the second row of table 1 to more suitably represent the nature of the NiAr+ ground state.
The NiAr+ molecular ion has been probed spectroscopically for the first time. The nature of the bonding in this molecule appears to be dominated by simple charge-induced-dipole attractive forces. Two independent measurements (dissociation threshold and isotope shift) both indicate a very short equilibrium internuclear separation of 2.0x IO-’ cm and a fairly strong bond dissociation energy ( ~0.5 eV) for this molecule. The equilibrium vibrational force constant of the ground state of this ion is remarkably “tight” being 20% of that of Hz and 70% of that of
476
but electrostatic, i.e. ions electronic strucit appears likely that “thermometers” for
Acknowledgement Grateful acknowledgement is made to the ACSPRF, Finnigan MAT, and the University of Florida DSR for funding.
5. Conclusions
12. The present
action in NiAr+ is not covalent not specific to the details of the ture, only its charge. Therefore argon adducts may be used as metal cluster ions.
25 November 1988
data indicate
that the Ni-Ar
inter-
Hefe’rences [ I ] A.W. Castleman Jr. and R.G. Keesee, Chem. Rev. 86 ( 1986 589. [2] D. Lessen and P.J. Brucat, Chum. Phys. Letters 149 (1988 IO. [ 31 D. Lessen and P.J. Brucat, Gem. Phys. Letters 149 (1988
413.
[ 41 D. Lessen and P.J. Brucat, in preparation. [S] L.S. Zheng, P.J. Brucat, CL. Pettiette, S. Yang and R.E. Smalley, J. Chem. Phys. 83 ( 1985) 4273. [6] C.E. Moore, Atomic energy levels, NBS Circular
No. 467 (Natl. Bur. Std., Washington, 1952). [ 71 C.J.F. Bijttcher and P. Bordewijk, Theory of electric polarization, Vol. 2 (Elsewer, Amsterdam, IY78).
CHEMICAL
PHYSICS LETTERS
25 November
1988
ON THE NATURE OF NiAr’ Dan LESSEN and P.J. BRUCAT Department of Chemistry,
Universrly of FIorda,
Galnesvtllej FL 326 Il. USA
Received 28 August I988
N&r+ ions generated in a supersonic expansion of Nr+ with a 1% Ar/He carrter gas arc mtccted into a tandem mass spectrometer and photodissociated with visible tunable laser radiation. A photofragmentation feature observed at 17984 cm-’ is believed to correspond to the dissociation threshold of NiAr+ to excited state (ZF,,L) Ni+ and ground state Ar. This assignment ytelds a prediction of 0.55 eV for the binding energy (Di) of NiAr +. The “NiAr+ ib”NiAr+ isotope shift of this spectral feature indicates the vibrational frequency in the ground state of NiAr+ to be 235 -t 50 cm-‘. Thus NiAr+ is a surprisingly “stiff” and strongly bound diatomic, discussed in light of a simple electrostatic (charge-induced-dipole) model of the internuclear forces in this class of molecules
1. Introduction How is it possible to determine the degree of internal excitation of an isolated molecular ion passively and routinely? Any method capable of providing such information would facilitate the spectroscopic and chemical study of gas phase ions of al sorts. However, it is not easy to conceive of an easy way of accomplishing such a measurement of reactive species in intrinsically low abundance. Consider a molecular ion which is observed to undergo unimolecular decomposition, In this case the level of internal excitation of the species must be above the lowest dissociation limit of said molecule, i.e. the molecule is very hot. Utilizing this concept one can fabricate an analog to the ion under investigation, an ion with a very weakly bound adduct such as a rare gas atom. The new molecular complex is relatively unstable; decomposition by loss of the rare gas atom occurs at a much lower internal temperature than that necessary for the decomposition of the “bare” ion. This threshold decomposition temperature will be defined by total binding energy of the rare gas atom to the ion and the vibrational (and possibly electronic) partition function of the complex. If the binding of the rare gas adduct is weak enough and does not perturb the intrinsic nature of the ion to which it is bound, the presence of the rare 0 009-2614/88/$ (North-Holland
03.50 0 Elsevier Science Publishers Physics Publishing Division)
gas atom will indicate an upper limit to the temperature of the ion, making the atom a “hot” or “cold” thermometer. To utilize weakly bound rare gas adducts as “thermometers” for ions we must first understand the details of their binding to the ions of interest. Many ions with weakly bound adducts have been studied to shed light on the nature of ion solvation #I. In our laboratory transition metal cluster ions are of particular interest and generation of these ions in association with one [2] or more [3] rare gas atoms has recently been accomplished. As a benchmark for these studies, the study of rare gas atoms clustered around single transition metal atomic ions has been undertaken. The following is a description of the results obtained for one such ion, NiAr+.
2. Experimental A full description of the experimental apparatus and procedure used in this study will appear shortly [ 41. Briefly, transition metal atomic and cluster ions are generated in a laser-driven-plasma supersonicexpansion ion source. This source is similar to that used to produce positively and negatively charged w’ For an excellent review of these types of studies, see ref.
B.V.
[ 11. 473
Volume152.number 6
transition metal cluster ions [ 51 but has been mod-
ified to optimize the internal cooling of the ions subsequent to their formation. The effective internal cooling of the cluster ions from this source is evidenced by the observation that trace amounts of argon in the ion source gas supply routinely generate [ 3 ] metal cluster ions with weakly bound charge-induced-dipole rare gas adducts (M,,~Ar,t ). Limitation of the atomic ion number density upstream of the expansion orifice by reduction of the laser vaporization energy makes it possible to generate almost exclusively MArT ions (0 $ ni 15 ) and in particular MAr’, the featured ion of this study. After generation and cooling the ions are entrained in a pulsed helium molecular beam, traverse two differentialpumping orifices, and pass collisionlessly into a specialized time-of-flight mass spectrometer. There, the ion ensemble is accelerated through 1.45 kV and focused down a 2.45 m flight tube and into the entrance aperture of a 127” electrostatic sector energy analyzer. Primary mass spectra are obtained from the time-of-arrival distribution detected at the exit aperture of the energy analyzer by a dual microchannel plate electron multiplier. Secondary or photofragment mass spectra are derived from the electrostatic analyzer transmission at the same time of arrival as the parent ion, but at a potential corresponding to the ratio of fragment to parent masses times the parent transmission potential. Photoexcitation of the ions is accomplished by coaxial Nd:YAG-pumped dye laser irradiation of the ion beam ( 5-25 ) x 1O-6 s prior to passage into the energy analyzer.
3. Results Fig. 1 shows the photofragmentation of NiAr+ +Ni++Ar as a function of dissociation laser frequency on the interval from 17400 to 18100 cm-‘. Ar+ is not observed as a photoproduct of NiAr+ at these photon energies as would be cxpccted from the large disparity in the ionization potential of the two atoms(IP(Ar)=15.755eV,IP(Ni)=7.633eV) [6]. Therefore, NiAr+ photofragmentation is monitored as the Ni+ fragment ion current transmitted by the electrostatic sector. Under the normal operating conditions of the mass spectrometer, a trace amount of Ni+ from NiAr+ is observed from collision-induced 474
I$ November 1988
CHEMICAL PHYSICS LETTERS
-I r
NiAr’+ ~~
hv
+
Ni j-t
Ar
E (3 c 0
Laser
Frequency
(wavenumbers)
Fig. 1. Relative photofragmentation (ion current) of NiAr*+Ni* +Ar as a function of laserfrequency over the interval 17400-1800 cm- ‘. A jump in the one-photon photofragmentation is observed at 17984 cm ‘, indicating a threshold for producing excited *F7,2 Ni+ ions. This establishes the binding energy of NiAr+ as 0.55 eV.
dissociative processes. The study of such collisioninduced processes involving metal-argon complex ions will be reported elsewhere. The data presented here are in the form such that the smatl CID contribution to the dissociation has been nullified. To within the signal to noise of the present data, the photodissociation action spectrum of NiAr+ in the region of 18000 cm ’ appears as a featureless edge, presumably indicating the onset of a photodissociation threshold, i.e. the point at which the laser photon has just enough energy to produce (excited state) products with zero kinetic energy, Therefore we ascribe the edge energy of 2.23 eV ( 17984 cm-’ ) as the sum of the binding energy of NiAr+ and some promotion energy in the isolated Ni+ ion. Internal electronic excitation of the Ar atom is not energetically possible. The absence of any vibrational or electronic hot band features associated with the threshold at 17984 cm- ’ implies extensive cooling of the NiAr+ emanating from the supersonic-expansion ion source. Laser fluence dependence of the dissociation yield at 18020 cm- ’ (above the dissociation edge) shows a linear fragmentation response over a range of 0.5 to > 8.0 mJ/pulse cmW2. This indicates that the photodissociation at this energy involves a simple one-
Volume 152, number 6
CHEMICAL PHYSICS LETTERS
photon absorption event, leaving no ambiguity as to the value of the excitation energy imparted to the NiAr+ by the laser. By premise, the absorption spectrum of NiAr+ must be similar to the absorption of the bare Ni+ ion. Consultation of the energy levels [4] of Ni’ shows that no electric dipole allowed transitions would be expected within the manifold of states arising from the 3d9 configuration since all these states have the same overall parity. The lowest electronic state corresponding to 3d84p configuration (and electric dipole connected to the ground state) is 6.39 eV above the 2DS,2 ground state of Ni+. The photodissociation of NiAr+ appears to occur through at least weakly allowed one-photon transition at 2.3 eV, where no isolated Ni+ transitions are expected. The presence of the polarized argon atom produces an electric field in the vicinity of the Ni+ and may effectively “mix” states of opposite parity in the isolated ion. The spherical symmetry of the atomic ion is broken, making J and L, the total orbital angular momentum of the electrons with and without spin, no longer strictly good quantum numbers. One possible assignment of the photodissociation feature in fig. 1 is the threshold for production of 2F 7,Z Ni+ and ‘S Ar in a transition that derives its nature from the forbidden 3ds4s 2F,,2+3d9 2DS,z transition in isolated NiC. If this is indeed the case, then the NiAr+ ground state binding energy (0: ) is the difference between the Ni+ 2F7,2+2Dg,2 transition energy of 1.68 eV (13550.3 cm-‘) [4] and the observed threshold energy of 2.23 eV, or 0.55 eV. The isotope shift between the s8Ni40Arf and 60Ni40Arf photodissociation features is 0.8 f 0.15 cm-’ with the heavier isotope being shifted to higher energy. This shift corresponds to the difference in the vibrational energy of the ground state of these two species. Presumably, this vibrational energy is zero point, suggesting the ground state vibrational frequency of NiAr+ is 235 250 cm-‘.
4. Discussion Since there is little likelihood of any formal charge residing on the Ar atom in NiAr+, a good approximation to the nature of this molecule may be derived from a picture of an almost unperturbed Ni+ ion with
25 November I98 8
a polarized Ar atom bound to it by simple chargeinduced-dipole forces. Postulating the functional form of the repulsive interaction between the Ni+ and the Ar electron clouds, we may guess a singleparameter adjustable interaction potential of the form U(r)=ar-‘2-tq2ar-4, fq’Cr=
1.195X 10e3’ eV cm4,
where I is the Ni+-Ar internuclear separation, N is the polarizability [ 71 of argon, and a is an adjustable parameter. This approximate potential curve assumes a rotationless diatomic. Table 1 shows the application of this hypothetical electrostatic potential described above to the experimental data determined in this study. The columns indicate values of w,, the vibrational frequency of NiAr+ in its ground electronic state; r,, the ground state equilibrium Ni-Ar internuclear distance: O!, the energy required to dissociate NiAr+ in its zeropoint vibrational level to 2D5,2 Nit + ‘S Ar, and k, the vibrational force constant at r,. In the first row of this table, the experimental vibrational frequency is used to determine the force constant (in the harmonic oscillator approximation) and the second derivative of the potential surface at r,. This determines a and all other listed molecular parameters. In the second row of table 1, the experimentally determined dissociation energy, 0: =0.55 eV, is used to determine a and a second (and independent) set of derived molecular parameters. Comparison of the vibrational frequency determined from the assignment of the observed dissociation feature as production of ‘FTi2 Ni++ ‘S Ar leads to a predicted vibrational frequency of 289 cm- ’ in the context of our electrostatic model which is in remarkable agreement with the vibrational frequency of 235 + 50 cm-’ determined from an independent isotope shift measurement. This lends credence both to the threshold assignment and to the Table 1 Estimated molecular constants for NiAr+ W, (cm-‘)
@ (eV)
r, (10-8cm)
k
-235 289
0.416 0.550
2.074 1.935
71 117
( lo3 erg/cm’)
475
volume 152,number 6
CHEMICALPHYSICSLETTERS
validity of the model potential. Moreover, the vibrational frequency is very sensitive to small changes in r,, indicating that the equilibrium internuclear distance in NiAr+ is about 2.0~ IO-” CM. Since the experimental determination of 0: is more accurate than that of the isotope shift (although contingent upon the correct assignment of the limit), one might expect the second row of table 1 to more suitably represent the nature of the NiAr+ ground state.
The NiAr+ molecular ion has been probed spectroscopically for the first time. The nature of the bonding in this molecule appears to be dominated by simple charge-induced-dipole attractive forces. Two independent measurements (dissociation threshold and isotope shift) both indicate a very short equilibrium internuclear separation of 2.0x IO-’ cm and a fairly strong bond dissociation energy ( ~0.5 eV) for this molecule. The equilibrium vibrational force constant of the ground state of this ion is remarkably “tight” being 20% of that of Hz and 70% of that of
476
but electrostatic, i.e. ions electronic strucit appears likely that “thermometers” for
Acknowledgement Grateful acknowledgement is made to the ACSPRF, Finnigan MAT, and the University of Florida DSR for funding.
5. Conclusions
12. The present
action in NiAr+ is not covalent not specific to the details of the ture, only its charge. Therefore argon adducts may be used as metal cluster ions.
25 November 1988
data indicate
that the Ni-Ar
inter-
Hefe’rences [ I ] A.W. Castleman Jr. and R.G. Keesee, Chem. Rev. 86 ( 1986 589. [2] D. Lessen and P.J. Brucat, Chum. Phys. Letters 149 (1988 IO. [ 31 D. Lessen and P.J. Brucat, Gem. Phys. Letters 149 (1988
413.
[ 41 D. Lessen and P.J. Brucat, in preparation. [S] L.S. Zheng, P.J. Brucat, CL. Pettiette, S. Yang and R.E. Smalley, J. Chem. Phys. 83 ( 1985) 4273. [6] C.E. Moore, Atomic energy levels, NBS Circular
No. 467 (Natl. Bur. Std., Washington, 1952). [ 71 C.J.F. Bijttcher and P. Bordewijk, Theory of electric polarization, Vol. 2 (Elsewer, Amsterdam, IY78).