h1. S.B. lIuNsox, J. L. FRASKLIN, AND F. H. FIELD
1542
Vol. 67
A MA4SSSPECTROMETRIC STUDY OF HOMONUCLEAR AND HETERONUCLEAR RARE GAS MOLECULE IOSSl BY M. s. B.MUKXON, J. L. F R A ~ K L I N ,AND F. H. FIELD Research and Development, Humble Oil & Rejininy Company, Baytown, Texas Received February 18, 1965 The homonuclear and heteronuclear diatomic ions of all the rare gases except radon have been studied by mass spectrometry. Appearance potentials of all except the HeXe+ have been measured a t pressures where three-body processes could be neglected. All are found to result from chemi-ionization reactions, and our appearance potentials agree well with those of other workers where they are available. Most of the heteronuclear ions have appearance potentials in the ionization continuum of the partner of lower ionization potential. Thus most of the heteronuclear ions exist a t energies above one of their dissociation asymptotes, and presumably this is possible because of the non-crossing of potential energy curves. The appearance potentials of Hehr +, 17.9 e.v., and NeXe+, 16.0 e.v., occur below the lowest excited state of He and Xe, respectively, and can only be understood as resulting from reaction of a long-lived discrete excited state of argon or xenon existing in their respective ionization continua.
Introduction and Holstein to predict that Heilr+ and NeAr+ would not be table.^ The appearance potentials of the homonuclear rare Fuchs and Kaul, however, observed SeArf mass gas molecule ions were measured several years ago, but spectrometrically and obtained a value for the appearthe limits of precision were quite broad.2 Previously ance potential of 16.5 e.v.lo; they also observed the in this Laboratory we have studied gas phase reactions ArKr+ ion which was discussed in somewhat more of rare gas ions and excited atoms, including the kidetail by Kaul, Lauterbach, and Fuchs.l1 Kaul and netics of rare gas molecule ion f o r m a t i ~ nand , ~ ~we ~ felt Taubert also observed ArXef and KrXef and gave that a complement to our work would be a redeterminafor the appearance potentials of these ions indivalues tion of the appearance potentials with equipment capacating that they mere formed from excited states of the ble of a higher degree of precision than that previously rare gas atoms.5 Morris, however, was unable to find used. During the course of this work new measurements of these appearance potentials were r e p ~ r t e d , ~ the KrXe+ ion in gaseous discharge through a mixture of K r and Xe with a mass spectrometer probe.12 and good agreement exists between the values reported We have made a study of all the binary mixtures of here and the earlier values. rare gases, except Rn, and have observed mass spectroThe existence of several of the heteronuclear rare metrically all of the heteronuclear molecule ions. We gas molecule ions has been commented on in the literahave obtained the appearance potentials of all except ture. Oskam suggested the formation of HeKe+ by a HeXe+ and have studied the dependence of certain of three-body ion-molecule reaction in He-Tu'e mixtures them upon pressure. subjected to an electric discharge.'j Although he studied other rare gas mixtures as well, he made no Experimental mention of any other heteronuclear rare gas molecule The instrument is a 60' deflection, sector field type mass ions. Pahl and Weimer actually observed a secondspectrometer designed for high pressure operation.13 The aporder HeNe+ ion mass spectrometrically from the pearance potentials were measured with the long source, in which the length of the electron path is 20 mm., because the short positive column of a glow di~charge.~JThey argued source, perhaps because of potential penetrations, never gave that the energy of formation of HeKe+ must be less satisfactory operation for energy measurements. The short than 21.56 e.v. because they assumed that excited source (7 mm. electron path) proved quite adequate for the preshelium with energy higher than 21.56 e.v. would react sure studies, however. Many experiments were made to determine the adequacy of the by the Penning process
rather than by a process yielding Hen'ef. By contrast with the He-Ne system, for mixtures of He and Ar and Ne and Ar these workers did not find either the HeAr + or NeAr + ions.8 This apparent non-existence of HeAr+ and XeAr+ was explained on the basis of the non-overlapping of energies of excited states of Ar with those of He or Ne. Similar reasoning led Biondi (1) Supported in part by Project Squid under Contract No. Nonr3623 ( S-18) (2) J. A. Hornbeok and J. P. Rlolnar, Phys. Rev., 84, 621 (1951). (3) F. H. Field, H. N. Head, and J. L. Franklin, J . Am. Chem. Soc., 84, 1118 (1962). (4) (a) J. S. Dahler, J. L. Franklin, M. S. B. Munson, and F. H. Field, J. Chem. Phys., 36,3332 (1962); (b) M. 9. B. Munson, F. H. Field, and J. L. Franklin, ibid., 87, 1790 (1962). (5) W. Kaul and R. Taubert, Z. Naturfoorach., 17&, 88 (1962). (6) H. J. Oskam, Philips Res. Rept., 18, 401 (1958). (7) M. Pahl and U. Weimer, Natwwzss., 44, 487 (1957). (8) M. Pahl and U. Weimer, 2. Naturforsch., 12a, 926 (1957).
.
instrument for the measurement of appearance potentials a t high pressures (up t o P , = 100-150 K). While adequate techniques for appearance potential measurements a t analytical pressures ( P s < 0.1 F ) are well known, the possibility that difficulties would be encountered in operating a t very much higher pressures had to be investigated. Most effort was devoted to determining the known differences in appearance potentials of rare gapes under different instrumental conditions. The results were satisfactory, for a t low source pressures (P. < 1 U ) known differences were reproduced with an average error of 10.08 e.v., whereas a t pressures as high as 150 # the average error was f0.2 e.v. I n addition the appearance potential of An+ was independent of source pressure from 10-150 p . Measurements made with a Faraday CUP as an ion collector a t a high source pressure and with an electron multiplier a t a iower source pressure gave the same results. Furthermore, as long as the intensity of the ion under investigation was maintained at a reasonable value, the appearance potential was independent of the repeller field strength, electron drawout and electron collector potentials, electron current, magnetic (9) R I . A. Biondi and T. Holstein, Phys. Rev., 83, 962 (1951). (10) R. Fuchs and W. Kaul. Z. Naturforsch., 16% 108 (1960). (11) W. Kaul, U. Lauterbach, end R. Fuchs, Naturwiss., 47, 353 (1960). (12) D. Morris, Proc. Phys. Soc., 68, 11 (1965). (13) F. € Field, I. J . Am. Chem. Soo., 83, 1523 (1981).
July, 1963
HOMONUCLEAR AND HETERONUCLEAR RAREGASMOLECULE IONS
field collimating the electron beam, and the mass or appearance potential of the calibrating ions. The appearance potentials were determined by the vanishing current technique in which the slopes of the ionization efficiency curves of the calibrating ion and the ion in question were made equal over a 1-2-volt range just above the onset potential by appropriately adjusting the ion intensities. Since the ionization efficiency curves for the molecule ions and the primary calibrating ions have very different shapes, the best method of calibrating the electron energy scale is a matter of some concern. However, in view of the satisfactory results with knowns referred to in the preceding paragraph, we feel that the procedure chosen is reliable. For the appearance potentials reported in this paper the instrument was fitted with an electron multiplier, which sufficiently increased the sensitivity so that the pressures for all of the measurements were 1 . 3 e.v. Spectroscopic 3.1" 2.5b 2 16" Scattering 3 9d Calculated Pauling 0 05b Weinbaum .7" Moiseiwitsch .76' Csavinszky 840 Reagan, Browne, and Matsen 2 14h a G. Herzberg, "Spectra of Diatomic Molerules," in LLMolecular Spectra and Molecular Structurr," D. Van Nostrand Co., Inc., New York, N. Y., 1950, p. 536. See L. Pauling, J . Chem. Phys., 1, 56 (1933). E. A. Mason and J. T. Vanderslice, zbzd., 29, 361 (1958). W. H. Cramer and J. H. Simons, zbid., 26, 1272 (1957). e S. Weinbaum, ibzd., 3, 547 (1935). f B. L. Moisewitsch, Proc. Phgs. Soc., A69, 653 (1956). P. Csavinszky, J. Chem. Phys., 31, 178 (1959). P. 9.Reagan, J. C. Browne, and F. A. Matsen, J . Am. Chem. Xoc., 84, 2650 (1962). Q
Fig. 2.-Representative
plots of appearance potential data.
more reliable than the earlier values of Hornbeck and Molnar, but not necessarily more precise than the values of Kaul and co-workers. TABLE I APPEARANCE POTENTIALS OF HOMONUCLEAR RAREGAS MOLECULE IONS AP(Rz +), e.v.
Rz These data Lit. He2 23.3 =I= 0.1 23 4,"23.2' Ne:! 20.9 f . 2 20.9" Ar2+ 14.7 .1 15.l,* 15.1" Krg 13.0 f .1 13 2," 13.2" Xes+ 112f 1 11.6" a Reference 5. W. Kaul and R. Fuchs, Z. Naturforsch., 15a, 326 (1960). Reference 1. +
-
+
Table I1 shows values for the dissociation energy G f Hez+from various sources, including several calculated values. The mass spectrometric value represents the difference between the ionization potential of He and the appearance potential of He2+. The value listed by Herzberg is considered as uncertain, and Pauling cites the other spectroscopic value in his early paper. The two values from the scattering of He+ by He were obtained from the data of Cramer and Simons. Biondi and Holsteingobserved in the spectra of helium after a discharge, presumably resulting from He* produced by the recombination of He2+with electrons, no states of energy less than ca. 23.1 e.v. This observation may be construed as giving about 1.5e.v. for the dissociation energy of He2+. The other experimental data do not appear to be sufficiently well established to be of concern; it is only the recent calculation of Reagan, Browne, and Xlatsen, which sets a lower limit for D,(He2+) greater than our value, that is disquieting. The zero point vibrational energy is unlikely to be greater than 0.2 e.v., which is not sufficient to account for the discrepancy. Subtracting about 0.2 e.v. from their value of 2.14will correspond to an excited state of He of 22.6 e.17. for the lowest energy state which could be involved in
the formation of He2+, which is close to the 33Slevel a t 22.7e.v. Our value of 23.3 e.v. for Dois nearest the 3lP level at 23.1 e.v., the highest level for principal quantum number 3. However, one should perhaps not expect the 3lP or any lP level to be the major source of He2+ since, according to Fig. 1, He2+has a maximum at about 30-36 e.v., but Gabriel and HeddIe report for all lP states a maximum in excitation cross section a t about 100 e.v.I7 If we assume that a t these pressures the radiation to the ground state is completely trapped then the lifetime sec. The lifetimes of the of the 3IP state is 7.4 X other states of principal quantum number 3, all of sec.; lower energy than the 3IP, are 3S, 3.6 X IS,8.4 X sec.; 3P,9.7 X lo-* sec.; 3D, 1.4 X lo-* sec.; and ID, 1.5 X see. These values were calculated by Gabriel and Heddle17 and agree with the less complete set of experimental values of Heron, AlcWhirter, and Rhoderick.18 The lifetimes of the different states do not appear to be sufficiently different that one would not expect reaction from all. Excitation cross sections do not appear to be available below about 30 e.v. but extrapolation of the data of Gabriel and Heddle suggests that the cross sections for all of these states should be comparable in magnitude. Since we observe no variation of the appearance potential of He2+ with pressure, we do not think that there will be a contribution of lower energy states to the Hez+ intensity which we are not observing a t present because of low intensity. We have no explanation for the discrepancy between our value for Do(He2+) and the calculated one. One may assume that perhaps the 31P atomic state forms Hea+ in an excited level, but there is still no explanation of why the lower states do not react. Table I11 shows the dissociation energies of the other homonuclear rare gas ions. For Ne2+ there is an indication that D0(Ne2+)S 0.85e.v. from the spectral lines observed by Biondi and H ~ l s t e i n . ~Mason and Vanderslice estimate De(;\r'ez+)as between 0.33 and 0.71 e.v. (17) A. H. Gabriel and D. W. 0. Heddle, Proc. Roy. SOC.(London), 8268, 124 (1960). (18) S.Heron, R,W . P, McWhirter, and E. H. Rboderiek, i b z d . , 8 2 9 4 , 565 (1056).
HOMONUCLEAR AND HETERONUCLEAR RAREGASMOLECULE IONS
July, 1963
1545
from scattering data of Ne+ by Ne.1g There is also a value of 0.0035 e.v. calculated for De(Arz+)from ion scattering data,zowhich is much too low. TABLE I11 ENERGIES
nrRSoCIATION
D(XZ+)D(Xd,
D(Xz), X
kcal.&
He Ne Ar Kr Xe F C1 Br I 0 H
.. . ...
D(Xt’), kcal.
kcal.
MO configurationd Xa -t x2+
-
+30 (uu*ls)2 (uu*ls) 30 (1.30 e.v.) 16 (0 69 e.v.) 16 ( ~ ~ * 2+ p (uu*2p) )~ .. . 25 (1 08 e.v.) 25 ... 23(1.00e.v.) 23 ... 21 (0 91 e.v.) 21 37 76b 39 (?rg*2p)4-,(rrg*2p)3 58 97c 39 46 64” 18 36 54“ 18 117 149“ 31 (rs*2p)2 -+ (?re*2p) 104 61” -43 (ugls)2 4 (ugls) a Taken from the compilation in S. IV. Benson, “Foundations of Chemical Kinetics,” McGraw-Hill Book Co., New York, N. Y., 1960, p. 662. R. P. Iczkowski and J. L. Margrave, J. Chcm. Phys., 30, 403 (1959). Calculated from A P data in F. H. Field and J. L. Franklin, “Electron Impact Phenomena,” Academic Press, New Yorli, E.Y., 1957. From W. Kauzmann, “Quantum Chemistry,” Academic Press, New York, X. Y., 1957.
Table 111 also contains dissociation energies of other compounds for comparison with the dissociation energies of the rare gas molecule ions. It may be seen tha,t the rare gas ion dissociation energies are about half those of the halogen molecules and may be considered as showing the same trend as the halogens, although the variation from Nez+ to Xe2+ is about the same as the experimental error. I n the examples listed, except Hz, the molecule ion has a larger dissociation energy than the molecule. These are cases for which, according to the simple n/rO treatment, the molecule ions have one less occupied an tibonding molecular orbital than the molecules. From these data we obtain an average increase of 26 kcal. in bond strength caused by removing an antibonding electron, which may be compared with a decrease of 43 kcal. in going from Hz to H2+, removing a bonding electron. If we note that Fz- is isoelectronic with Nez+ with similar comparisons for the other halogen molecule negative ions, and assume that D(R2+)E D ( X z - ) for isoelectronic pairs, then from
EA(Xr) = D(Xz-) - AH,(X-) - A H f ( X ) we can obtain estimates of the electron affinities of the halogen molecules. From our data we find 63 kcal./ mole for the electron affinity of F?, which compares favorably with a recent estimate of 69 kcal./mole.21 Similarly for Clz we obtain 55 kcal./mole which niay be compared with the tentative value of 39 kcal./mole given by Pritchard.22 We estimate EA(Br,) = 51 and EA&) = 45 kcal./mole, but we know of no data for comparison. Heteronuclear Molecule Ions.-Figure 3 shows representative ionization efficiency curves for two heteronuclear rare gas molecule ions. Relative intensities are used so that both may easily be plotted on the same (19) E. A . Mason and J. T. Vanderslice, J . Chem. Phvs., 30, 599 (1959). (20) R. D. Cloney, E. A. Mason, and J. T. Vanderslice, zbzd., 36, 1103 (1962). (21) R. MI. Reese, V. H. Dibeler, and J. L. Franklin, h d . , -29, 880 (1958). ( 2 2 ) H. 0. Pritohard, Chem. Reu., 62, 629 (1953),
o OO
Fig. 3.-Ionization
ELECTRON 10 . ENERGY (UNCORRECTED), 2 e.v.
0
eficiency curves for heteronuclrar rare gas molecuk ions.
scale, and each shows the maximum that is characteristic of products of reactions of electronically excited atoms and molecules. Ionization efficiency curves similar to these t ~ 7 owere observed for all of the heteronuclear rare gas molecule ions. We were able to obtain appearance potentials for all of the heteronuclear ions except HeXef, but the existence of the ion itself was observed. The intensity of HeXe+ was too low and there was too much interference from the high mass wing of the Xe+ peaks a t high pressures to permit a reliable measurement of the appearance potential. The appearance potentials are listed in Table IV with the available literature data. For three of the four previously reported ions the agreement is good. However, for Her\‘e+ our value is higher than that of Kaul and Taubert by a n amount greater than the experimental uncertainty. Our value of 23.4 e.v. is essentially the same as that for Hez+so that the lowest states involved in Hez+ formation also appear to react to form Hen‘ef. A careful search was made for a lower energy process for the formation of HeSe+, particularly around 20 e.v., but none was observed. TABLE IV APPEARANCE POTENTIALS OF HETERONUCLEAR RAREGAS MOLECULE Ioxs PAP
(e.v.)
Ion
These data
Llt.
HeSeT HeAr + HeKr + NeAr + NeKr + KeXe + ArKr + ArXe + KrXe
23 4 i 0 . 1 17 9 5 . 3 19 9 5 . I 168h 1 166f 1 1 6 0 3 ~3 140h 1 1353Z 1 1235 1
22 6’
+
16
13 56 12 25
The heteronuclear ions, with the possible exception of ArKr+, KrXe+, and HeAr+, appear to be formed primarily from excited states of the gas of higher ionization potential which lie above the ionization potential of the other rare gas. For ArKr+ and perhaps KrXe+ the values of the appearance potentials are sufficiently close to the lower ionization potential that it is possible that these ions may be formed from states of either rare gas immediately below the ionization potentials of Kr and Xe, respectively, as well as from states of Ar and Kr above the ionization limits of Kr and Xe.
~
1546
M. S.B.
Fig. $.-Ionization
>fUSSON,
J. L. FRASKLIX, AND F. H. FIELD
efficiency curves for heteronuclear molecule ions.
I \
Fig. 5.-Schematic
representation for non-crossing rule for states of RiR2*.
The 17.9 e.v. value for AP(HeAr+) cannot be the result of a reaction of ai1 excited state of He, since there is no known state a t that energy. The ionization efficiency curve for HeAr+ is somewhat different from the others, in that there is a low voltage tail, giving the appearaiice potential of 17.9 e.v., but there is a relatively well defined break a t about 20 e.v., corresponding in energy to the metastable helium states. Figure 4 shows the ionization efficiency curves for HeAr+, HeKr+, and SeAr+ for a short range above the threshold, indicating this difference. From the relative slopes of these two sections of the curve one is inclined to think that most of the Hehr+ was formed by states of He above 19.9 e.v. There is some recent evidence for the existence of . ~other ~ states of Ar of energy as high as 18 e . ~ In atoms for which states above the ionization limit have been observed, both sharp and broad spectral lines emanating from these states have been noted.24 The broad lines are associated with states for which autoionization may occur, and may have lifetimes as short ( 2 3 ) F. J. Comes and W. Lessman, 2. a~atur/orsch.,16a, 1396 (1961). who also list earljer references. (24) H. E. W h i t e "Intioduction to Atomic Spectra," McGraw-Hill Book Co., New York, N. Y., 1934, pp. 394-398.
Vol. 67
as sec. Kormal radiative lifetimes may be associated with the sharp lines, which are perhaps long enough for chemi-ionization reactions to occur. Thus it is possible that states of Ar above the ionization limit can produce the HeAr+ ion. HeKr+ has an appearance potential corresponding to the helium metastable levels, and NeAr-., KeKrf, and NeXe + have appearance potentials which correspond well to the neon metastable levels. The NeXe+ appearance potential is below the value of the Ne metastable levels by an amount greater than the expected precision and may perhaps have contributions from a t present unknown states of Xe in the ionization continuum in a manner analogous to HeAr +. The stability of these ions, which is a t first surprising since dissociation to the ion of lower ionization potential is strongly exothermic, is probably the result of the noncrossing (or pseudo-crossing) of attractive and repulsive curves.25 Essentially the same treatment is given for some dipositive diatomic ions in treatments by Bates and Carson for Nz++ and 0 2 + + 26 and by Fraga and Ransil for He2++.27 The shape of the potential energy curves is indicated in Fig. 5 and shows the activation energy for dissociation to the more stable products. We assume that our appearance potential corresponds to atomic states near the potential minimum for the lower curve, but it is not possible to obtain a value for this dissociation energy from our appearance potential measurements. One would also expect some higher energy atomic states of the gas of higher ionization potential to contribute to heteronuclear molecule ion formation according to the upper curve. No attractive curve results from the lower state of Rl+ and R2, for if this were the case the appearance potential of the heteronuclear molecule ion would be lower than the lower of the two ionization potentials. From the pressure studies there was no noticeable charge exchange between He+ and Ne or Ar or between Ar+ and K r since the quantities (Ne+)(He)/(He+)(Ne), (Ar f)(He)/(He+)(Ar), and (Kr +) (Ar)/(Ar +) (Kr), which represent the ratios of ionization cross sections, were independent of pressure to within = k l O ~ o . The data of Stedeford and Hasted on charge exchange give Q(He+, Ke) = 2 X 10-17 a t 100 e.v. and &(He+, Ar) = 1 X 10-16 cm.2 at about 250 e.v.28 Cross sections a t the lower energies of our mass spectrometer source will be even smaller so that the contribution from charge exchange should be less than our experimeiital error. The formation of hr*, K r f , or Ke+ by the Penning process, reaction 1, is negligible compared to the formation of the ions by direct ionization through electron impact. The relative intensity of the homonuclear rare gas molecule ion of higher appearance potential was always decreased by the addition of the gas of lower ionization potential as the result of the Penning process and heteronuclear molecule ion formation. There was a small apparent decrease in the ratios (Net+)/(Ne+) and (Arz+>/(Ar+) with the addition of helium which was probably of instrumental origin. A larger decrease (25) See, for example, C. A. Coulson, "Valence," Clarendon Press, Oxford, 1952, p. 65. (26) D. R. Bates and T. R. Carson, Proc. Phys.SOC. (London), A68, 1139 (1955). (27) S. Fraga and
B. J. Ransil, J. Chem. Phye., 87, 1112 (1962). (28) J. B. H. Stedeford and J. B. Hasted, Proc. Roy. SOC.(London),
A227, 466 (1955).
HOMONUCLEAR AND HETEBONUCLElR RAREGAS MOLECULE IONS
July, 1963
in (Krz+)/(Kr*) was observed with the addition of Ar which may be instrumental or which may indicate that some of the higher excited states of K r are reacting to form ArKr +. As an idea of the relative intensities of the ions found in the systems, for (He)/(Ne) = 1 a t about 27 e.v. and P E 120p, (4He+) = 1.3 X lo5, (4He~+)= 1.5 X lo3, (20Ne+)= 8.9 X lo5, (4He20Ne+)= 1.9 X lo3, and (20r\rez+)= 3.7 >(:lo3,all ion intensities in the same arbitrary units. F0.r (He)/(Ar) = 1 a t 26 e.v. and P E 100 p, ( 4 ~ e + ) =- 3.5 x 104, (411e%+) = 1.4 x 1 0 2 , (40Ar+) = 1.9 X lo6, (*He40,4r+) = 8.4 X lo2, and (40Ar2+) = 9.3 X lo3. For (Ar)/(Kr) = 1 at P E 100 p and 20 c.v., (40Ar+)= 8.7 X lo5, (40Ar2+)= 2.3 X lo3, (84Kr+)= 7.0 X lo5, (40Ars4Kr+) = 2.3 X lo3, and (64Kr2+)=: 2.1 X lo3. The intensities of heteronuclear molecule ions are less than those of the homonuclear ions which can be made from the components, but the difference is usually less than the order of magnitude indicated by Pahl and Weimer for HeNe+.5 Rough pressure studies on all of the heteronuclear rare gas molecule ions indicated an approximately first-order dependence on each gas. More careful studies on He-Ke, He-Ar, and Ar-Kr mixtures gave a good first-order dependence on each rare gas at low pressures. KO third-order processes were detected. Although all three reactions HeKe+
Fig. 6.-Ratios
+ Ke = Ye2++ He (or Ne+ + Ke + He) (2)
HeAr+
+ Ar --
Ar2-l-
+ He (or Ar+ + Ar + He)
(3)
ArKr+
+ Kr = Krp + Ar (or Kr + Kr + Ar) +
Rz* = Rz + h~
k5
(10)
+ h~
ke
(11)
=
R1
Making the usual steady-state approximation on R *, we obtain for low pressures (Ri)z+
(4)
Ice
of heteronuelear and homonuclear rnoleelile ions as a function of pressure.
R1*
+
are exothermic, evidence for reaction 3 only was found. The ratios (€Ie?uTe+)/(Hez+) and (ArKr+)/(Arz+) are first order in Ke and K r pressure, respectively, and inversely proportional to He and Ar pressure, respectively. The ratio (HeAr+)/(Hez+) is inversely proportional to He pressure, but is proportional to Ar pressure only a t very low pressures. These data are shown in Fig. 6 and indicate a reaction of Ar removing HeAr+ from the system. From studies of the field strength dependence of the heteronuclear rare gas molecule ions we find that the ions have only excited atom reactants, in agreement with the observations of Kaul and T a ~ b e r t . ~ In a manner analogous to that of our two previous studies on excited stdate reactions of rare gases3.4 we postulate the following mechanism, in which R1 is t,he rare gas of higher ionization potential
=
+ R1= (R&+ + e R2*+ RP (R&+ + e
RELATIVE R.WE CONSTANTS FOR MOLECULE IONS RI
R2
He
Ne
kl/ka
=
(8)
k4
(9)
a(RdQ a(Rd
1.80 1.37 1 52 1.521 -,
1.62 He
Ar
Ar
I
