PHOTOIONIZATION hiAss SPECTROMETRIC
STUDY O F
XeF2,XeF4,AND XeF6
1461
Photoionization Mass Spectrometric Study of XeF,, XeF,, and XeF,la by J . Berkowitz,* W . A . Chupka, P. M. Guyon, J. H. Holloway,lband R. Spohr Argonne hTational Laboratory, Argonne, Illinois 6'0459
(Received December 20, 2970)
Publication costs assisted by the U.S . Atomic Energy Commission
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The process XeFz hv 4Xe+ F- F has been observed, and from its threshold the value AHfoo(XeFz)= -28.0 f 0.5 kcal/mol has been derived. Subsequent studies of the thresholds for XeF2+from XeF4 and XeF4+ from XeFe then yield AHfoo(XeF4)= -57.6 2 kcal/mol and AHfoo(XeFe)= kcal/mol, respectively. The first ionization potentials of the compounds studied are: 12.35 f 0.01 eV for XeF2,12.65 f 0.1 eV for XeF4, and 12.19 f 0.02 eV for XeFB-all higher than that of atomic xenon and in good agreement with photoelectron spectroscopic values. The evidence presented indicates that the ground states of XeF4+ and x e F ~ have + some asymmetric character. The ionsXeF+,XeF3+,and XeF6+are found to be distinctly more stable to loss of a fluorine atom than are those containing an even number of fluorine atoms. The implications of these observations as regards the bonding in xenon-fluorine compounds are discussed. These studies give no evidence regarding the existence of low-lying excited states of XeFs; more extensive experiments involving long equilibration times will be required to decide this question.
*
I. Introduction
111. Sample Preparation
The very existence of noble gas fluorides has presented an exciting challenge to theoretical chemists. A number of molecular calculations2"tb have indeed been performed to explain the structure of these interesting chemical entities, but in order to test some of these calculations it is necessary to have more accurate data. For example, several independent experimental approaches3-5 aimed a t determining the heats of formation of XeF2,XeF4, and XeF6 have yielded significantly different results. The electron-impact mass spectrometric measurements6 are rather crude and cannot hope to achieve quantitatively conclusive numbers. A photoionization mass spectrometric study' of XeFz was able to deduce a fairly accurate first ionization potential of XeF2, but little else. Recently, some photoelectron-spectroscopic investigations8 of XeF2, together with vacuum-ultraviolet absorption morl
Xe+
+ F2+XeF+ + F
(7)
should be exothermic. Recent kinetic studiesz5of reaction 7 at this laboratory indicate that the cross section rises with increasing kinetic energy, a behavior normally associated with endothermic reactions. The (25)
J. Berkowitz and W. A . Chupka, Chem. Phys. Lett., 7, 447
(1970).
PHOTOIONIZATION MASSSPECTROMETRIC STUDYOF XeF2, XeF4, AND XeF6 interpretation of this behavior must somehow involve the mechanism of this reaction rather than its thermochemistry. The value for Do(XeF+) is not unreasonable when compared with those for the isoelectronic interhalogens. The direct isoelectronic analog is IF, whose dissociation energy has been somewhat uncertain. Of the two possible values (1.99 eV and 2.88 eV) that stem from observed predissociations, the higher value seems cur~ - e n t l y ~to “ ~be ’ distinctly favored. The value2*of Do for IC1 is 2.152 eV, and of the two permissible values2* (2.19 eV and 2.60 eV) for Do(FBr),the higher one is favored.,’ Hence, in this case of XeF+, as well as two others (the KrF+ ionz9and the ArF+ ionz6),the diatomic ions of the rare gas fluoride behave as pseudohalogens. However, as is discussed in ref 25, this is not the case for HeF+ and NeF+. As one now examines the ionization at shorter wavelengths in Figure 2, one finds that both the XeF2+ and XeF+ curves show several weak peaks, commencing a t ca. 935 8 and terminating a t the onset of a large increase in ionization at ca. 918 8. These are presumably Rydberg levels formed by excitation of an electron from the inner alg molecular orbital (notation of ref 9) or loa, (notation of ref 8). The convergence limit cannot be determi!ed accurately, but the onset of the steep rise a t 918 A = 13.5 eV corresponds very closely to the adiabatic ionization potential obtained in photoelectron spectroscopy (Brundle, et ala,* -13.5 eV; Brehm, el al., 13.58 f 0.05 eV). Both the XeF2+ and XeF+ acquire significant additional contributions to their intensities as the wavelength is scanned across this band. However, a t the adiabatic onset (14.00 f 0.05 eV, ref 8a; 14.06 f 0.05 eV, ref 8b) reported for the next band (presumably ejection of an electron from an elgmolecular orbital [ref 91 or 3ag [ref SI), the XeF2+ intensity has begun to decline, while the XeF+ begins to rise again at about the onset of this state and continues to rise through most of the wavelength region corresponding to the energy band of this state. For both the loa, state and the 3r, state we are well above the dissociation limit but a major contribution to parent ionization continues to be made by loa,, although not by 3a,. It is evident that a simple statistical theory cannot explain this behavior and perhaps should not be expected to for a triatomic molecule. As one proceeds to shorter wavelength, the next major contribution to ionization appears t? be the onset of significant Xe+ intensity, at ca. 810 A E 15.3 eV. This corresponds quite closely to the next observed adiabatic ionization potential (Brundle, et al., 15.25 f 0.05 eV; Brehm, et al., 15.40 0.05 eV) and is attributed to the ejection of an electron from the elu--(4r,) orbital. Slight changes in the XeF+ and XeF2+ intensities also occur in this region. This onset of Xe+ clearly corresponds to a process involving concomitant formation of two F atoms (rather than F,) since the
I
1465
XeF,’
Xe+ XeF+ 1
Figure 4. Photoionization mass spectrum of XeFa, obtained with the undispersed central-image light of the hydrogen lamp.
latter process would be expected to occur around 905 A. KO evidence for any significant contribution from the F2formation process exists in the Xe+ curve. On the basis of the previously determined threshold (11.481 eV) for reaction 4 and the electron affinity of F, one would anticipate this new Xe+ threshold (effectively the sum of reactions 1 and 2) t o occur at 14.93 eV. A glance a t the photoelectron spectrum of Brundle, et al., indicates that this corresponds to a deep valley in the ionization process. Very few, if any, ionic states are being made in this energy range; thus it is not surprising that the apparent threshold occurs at higher energy ( ~ 1 5 . 3eV) and hence this process is not a useful one for thermochemical purposes. Finally, at still shorter wavelengths, another significant process for production of Xef becomes evident, with an onset at ca. 735 8 (16.87 eV) which also agrees well with the photoelectron spectroscopic studies (Brundle, et al., 16.80 A 0.05 eV; Brehm, et al., 17.10 f 0.1 eV). The output of the helium continuum is too weak in the 600-620-A region to enable us to reach any significant conclusion regarding the next ionic state, which has tentatively been placed at ca. 20-22 eV in the photoelectron-spectroscopic work.* B. XeF4. The mass spectrum of photoionized XeF4, obtained at central image with the hydrogen many-line continuum, is depicted in Figure 4. There is no measurable XeF,+ intensity with mass higher than XeF4 ( L e . , no XeFBimpurity) and the XeFz+ intensity is so weak that it may be attributable to fragmentation of XeF4; the appearance potential of XeF2 (to be discussed later in this subsection) bears this out. (26) R. A. Durie, Can. J . Phys., 44, 337 (1966). (27) E. H. Wiebenga, E. E. Havinga, and K. H . Boswijk, Adoan. Inorg. Radiochern., 3, 133 (1961). (28) G. Herzberg, “Molecular Spectra and Molecular Structure. 11. Spectra of Diatomic Molecules,” Van Kostrand, Princeton, N. J., 1950. (29) J. Berkowitz, J. H. Holloway, and W. A . Chupka, “Photoionization Mass Spectrometric Study of KrFz,” unpublished work. T h e Journal o j Physical Chemistry, Vol. 76, N o . 10, I971
1466
BERKOWITZ, CHUPKA,GUYON,HOLLOWAY, AND SPOHR
The Xe+ has some contribution from a very small xenon impurity. No ion-pair formation was detected for this system. I n Figure 5 , the parent XeF4+ion intensity is shown as a function of incident wavelength. This intensity rises rather slowly from a threshold at ca. 980 8 (12.65 eV) to a maximum intensity at ca. 950-952 8 (13.02 eV). This range of energy is much larger than can be accounted for by monochromator slit width (ca. 0.8 8) or Boltzmann distribution (average vibrational energy22 a t 300°K = 0.09 eV). Brundle, et ai!.,30 report similar values (12.72 and 13.06 eV) from their photoelectron spectrum. Therefore, it appears as if the 1
I
Xe F4’
I
I
I
I
700
800
900
I
a
x tal Figure 5. Photoionization efficiency curves of (a) XeF4+, (b) XeFQ+, (c) XeFz+, (d) XeF+, and (e) X e + from XeF4.
Franck-Condon factors are not favorable at the adiabatic ionization potential. The XeF3+ fragment intensity, also shown in Figure 5 , has an apparent threshold at ca. 906 (12.91 eV). The photoelectron spectrum of Brundle, et aZ.,30 does not show the onset of another state until ca. 13.3 eV. Hence the dissociative ionization must somehow involve the ionic ground state of XeF4+. The ab initio calculations of Basch, et u Z . , ~ ~ on XeF4 show the uppermost occupied orbital to be loax, (the calculated Koopman’s theorem ionization potential = 12.6 eV) and the next deeper orbital to be 5azu( ~ 1 5 . 1eV). Since both of these orbitals are nondegenerate, ionization from these orbitals should not give rise to Jahn-Teller splitting. Yet one must acThe Journal of Physical Chemistry, Vol. 76, No. 10,1971
count for the apparent distortion in the XeF4+,which can so readily yield XeF3+ F, and also for the weak Franck-Condon probability a t the adiabatic ionization limit. Within the framework of simple molecular orbital theory, ionization from an alg orbital (totally symmetric) should only yield concomitant vibrational excitation which is also totally symmetric. This cannot account for the above phenomena. It appears as if there must be an interaction that distorts the symmetry of XeF4+ in its ground state. Configuration mixing of alg with the 5aZu can presumably give this kind of dishave presumably not solved tortion. Basch, et the configuration-interaction problem. An alternative approach that can rationalize the low-energy fragmentation of XeF4+ proceeds as follows. The doubly degenerate asymmetric stretching mode V6 of XeF4 has been reported by Claassen, et to have a Vibrational frequency of 586 cm-l. At room temperature, the first excited state of this normal mode represents 12% of the molecular concentration. The act of ionization would approximately preserve this concentration and also permit the total wave function (electronic and vibrational) to be asymmetric. The fragmentation could then proceed without violating symmetry restrictions. The higher vibrational levels of an antisymmetric vibration are symmetrjc for even values of the vibrational quantum number. Therefore, it is possible in principle to excite antisymmetric modes by A U K = 0, f2, f4. . . . However, the transition with AVK = 0 is by far the most intense.32 Therefore, while this represents a mechanism for breaking the symmetry, it does not seem likely that it can account for the relatively strong XeF3+ intensity just beyond threshold. It may be useful to describe this ionic ground state in a hybrid of valence-bond and molecular-orbital terminology. C o ~ l s o nhas ~ ~ given a qualitative picture of bonding in the xenon fluorides, in which about one unit of electronic charge is donated by the central Xe and apportioned among the appropriate number of fluorine ligands. The bonding between each fluorine and xenon is then primarily Coulombic. If an electron primarily localized on one of the fluorjnes is removed, the bonding of that particular fluorine to the rest of the structure would be drastically weakened. We note that from the description of their uppermost l ca. 90% fluorine orbital (loa,,) by Basch, et ~ l . , it~ has contribution and 10% Xe. Hence, removal of an electron from this orbital is effectively removing a fluorine electron, and an appropriate description may be
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(30) C. R. Brundle, G . R. Jones, and H. Basch, submitted for publication in J. Chem. Phys. (31) H. Basch, J. Moskowitz, C. Hollister, and D. Hankins, u n p u b lished work. (32) G. Herzberg, “Molecular Spectra and Molecular Structure. 111. Electronic Spectra and Electronic Structure of Polyatomic Molecules,” Van Nostrand, Princeton, N. J., 1966, pp 150-153. (33) C. A. Coulson, J. Chem. Soc., 1442 (1964).
PHOTOIONIZATION MASSSPECTROMETRIC STUDY OF XeF2, XeF4, AND XeF,
The four equivalent resonance structures of this type could be combined to give a wave function of alp symmetry. It should be noted that this behavior (forming a fragment ion very near to threshold) is not a rare anomaly, It will be encountered again (section IV C) when we discuss XeF6 and probably occurs for most hexafluorides, for alkali halidesj3*and for CF4.36 The explanation given above (removal of an electron localized on the halogen, which then destroys the Coulombic bond of that halogen to the remaining positive entity) may not be appropriate for CF4, but it offers a plausible interpretation for the other ionic systems. Returning now to the XeF4+ spectrum, we only briefly point out that there appears to ,be evidence for autoionization, particularly at -951 A, where a dip occurs in the ionization yield. This dip is presumably a form of the Fano profile36that characterizes the interaction of a quasidiscrete state with the underlying continuum. The peak Brundle, Jones, and Basch observed at 13.38 eV has no obvious correlation with a noticeable increase in ionization, either in XeF4+ or XeF3+. On the other hand, the XeF3+intensity does begin rising at ca. 910 8 (-13.6 eV) and reaches a local maximum at ca. 879 A (14.1 eV), which corresponds to a valley in the spectrum of Brundle, et aL30 The next feature, a very broad maxipum in the XeF3+ intensity, has an onset at cab 844 A (14.6 eV) and reaches a maximum a t ca. 693 A (17.99 eV), and again does not bear close resemblance to the spectrum of Brundle, et al. Meanwhile, in the 820-828-8 region, XeFz+ has begun to appear (Figure 5). The increase in ionization at this energy may roughly correlate with the very broad band (maximum at 15.18 eV) in the spectrum of Brundle, et al. An accurate determination of this threshold is quite useful, since it enables us to set a good lower limit and to obtain an approximate value for AHf"o(XeF4). We extrapolate the long linear region of the XeFz+curve near threshold to the background level and subtract the average inoternal energy.21 The extrapolated threshold is 821.5 A (15.092 eV), the average rotational energy is 0.039 eV, and the average vibrational energyz2 is 0.090 eV. Therefore, a fairly rigorous upper limit to this threshold is 15.22 eV. By combining this with the previously determined AH foe(XeF2) and ionization potential of XeFz and with Do(Fz), we deduce AHfoo(XeF4)3 -57.6 kcal/mol ( i e . , more positive). From the photoelectron spectrum of Brundle, et al., in this energy region, we see that ionic states are being populated, and a significant activation
1467
barrier is not likely for this dissociation. Hence, we feel that the true 300°K threshold is not likely t o be more than 0,l eV lower than 15.09 eV, and therefore AHfOo(XeF4) = -57.6 f 2 kcal/mol. The linearly extrapolated onset for XeF+ occurs at 790.5 (15.68 eV). The value corrected to 0°K is 15.81 eV. From this result, we can deduce another (somewhat less accurate) value for AHfoo(XeFr)by making use of the previously measured AH Oo(XeF+), reported in section 111 A, and Do(Fz). The errors in the limiting values for the onset of XeF+ from XeFz and of XeF+ from XeF4 tend t o cancel. The result, AHf"o(XeF4) = -58.6 kcal/mol, is in excellent agreement with that based on the XeFz+ threshold. The error in the latter determination is estimated to be 3 kcal/mol. Because of a slight xenon impurity from decomposition of XeF4,no meaningful threshold could be deduced for Xe+. Our best value AHfoo(XeF4)= -57.6 f 2 kcal/mol is in good agreement with the calorimetric value of Stein and Plurien3 [-60 lrcal/mol when corrected for the newer AH,"o(HF)] but distinctly diverges from the value of ref 4 (- 50.2 kcal/mol). Errors are not clearly assessed in ref 3 and 4, but the impression given is that they are 6 1 kcal/mol. We can now estimate a lower limit to AHfo0(XeF3+). The linearly extrapolated threshold is 955.5 8 ; corrected to O " K , this yields 13.10 eV for the appearance potential of XeF3+ from XeF4,and hence AHfoo(XeFa+) 2 226.3 kcal/mol. For the dissociation energy for XeF4++ XeF3+ F, me estimate 13.10 eV - 12.65 eV = 0.45 eV. C. XeF6. The photoionization mass spectrum of XeFB obtained at central image with the helium Hopfield continuum is shown in Figure 6. Two points are noteworthy: the XeF6+ parent ion is barely detectable, and the xenon impurity is small. The XeF6+ is clearly the major ion, but it is necessary to make ap-
+
xr+ A
11,
XeF4+
?igure 6. Photoionization mass spectrum of XeF,, obtained with the undispersed central-image light of the helium lamp. (34) J. Berkowitz, J. Chem. Phys., 50, 3503 (1969). (35) T. A. Walter, C. Lifshite, W. A. Chupka, and J. Berkowita, ibid., 51, 3531 (1969). (36) U. Fano, P h y s . Rev., 124, 1866 (1961). T h e Journal of Physical Chemistry, Val. 76,No. IO, 1071
1468
BERKOWITZ, CHUPKA,GUYON,HOLLOWAY, AND SFOHR look
I
[
I
I . !
,
,
I
**.,\
****.... X e F6+
t
,,
,
,
,
*
i
c
'E-
l i
b.
XeF,'
'a 0.
501 **.
I - .
I
1
.
i
0.
* t
\
lot
Xes,
Figure 7. Photoionization efficiency curve of XeFs +,plot,ted on (a) linear and (b) semilogarithmic coordinates. The photoionization efficiency curve of XeFj+ is also shown in (a).
pearance-potential measurements on the other fragments to demonstrate that they do not arise from XeF4 and XeF2 present as impurities. No ion-pair process could be detected. In order to perform meaningful measurements on the parent ion XeFe+, it was necessary to use 1-mm entrance and exit slits on the monochromator, together with a hydrogen-lamp light source. In Figure 7 the results in the threshold region are shown on both linear and semilogarithmic coordinates. The latter display is now more useful, since the exponential Boltzmann tail should be linear on such a plot, whereas it is somewhat more difficult to perform the subtraction of hot bands for complex molecules on the linear plot. A more accurate approach would be to determine the density of vibronic states populated at the temperature of the experiment, as was shown21 for SS+ and SB+. However, there is insufficient information regarding the possibility of low-lying electronic states in neutral XeF6, and the vibrational spectrum is also not adequately established. Hence, the departure from linearity on a semilogarithmic plot (at 1019 A) is a useful lower limit to the first ionizatjon potential; the curvature between 1016 and 1019 A may reflect the densityof-states factor,21 and hence an upper energy limit is T h e Journal of Physical Chemistry, Vol. 76,No. 10, 1971
probably 1016 A. We therefore set the first ionization potential of XeF6 at 12.19 0.02 eV. The threshold region of XeF5+ was examined under the same conditions as for (described above) ; the results are also displayed in Figure 7 . We note a linear rise in the threshold region on linear coordinates, which is treated by extrapolating this linear region to the background level and subtracting the internal thermal energy.21 The slit width of the monochromator, although contributing 8.3 FWHM in this experiment, should not affect the extrapolated threshold of a linearly rising function.21 From the linearly extrapolated threshold (1003 A = 12.36 eV) and the internal thermal energys7 (-0.20 eV), we deduce 12.56 eV as the 0°K appearance potential of XeF5+,and hence the bond energy for the dissociation XeF6+ XeF5+ E' is 0.37 i 0.05 eV. This value is very similar to the values 0.45 and -0.55 eV found for removing the , corresponding first fluorine from XeF4+ and XeF2+, respectively. The trend is these bond energies is also implied in Coulson's analysis,azsince the partial negative charge on the fluorine (and hence the strength of the ionic bond) diminishes in the order XeFz > XeF4 > XeF6. It is appropriate at this point to compare the photoelectron spectrum of Brundle et U Z . , ~ O and the ab initio calculations of Basch, et u L . , ~ ~for XeFe because the next fragment (XeF4+) occurs at significantly higher energy, where successively higher ionic states are involved. Brundle, et al., find one relatively narrow peak ( ~ 0 . 4 5eV FWHM) corresponding to the removal of the most loosely bound electron in XeFe. The threshold for removal of this electron (the adiabatic first ionization potential) appears to be in fair agreement with our value (12.19 eV), although a small xenon atomic impurity in their measurement may interfere with a more accurate determination of this threshold. The vertical ionization potential, according to Brundle, et al., is 12.51 eV. Although XeFa has 15 vibrational degrees of freedom whereas XeF4 has only 9, this peak in XeF6 is not appreciably wider than the corresponding one in XeF4. The next (very broad) peak in the photoelectron spectrum of XeF6 has an onset at ca. 14 eV and a maximum at ca. 15.2 eV. Hence, the parent XeF6+ and first fragment XeF5+, and only these ionic
+
(37) Since the vibrational spectrum of XeFa is not well established, have estimated the internal (vibrational rotational) energy of this molecule a t 300°K in two somewhat independent ways. The vibrational spectrum of MoFa is rather well established [D. W. Osborne, F. Schreiner, J. G. Malm, H. Selig, and L. Rochester, J . Chem. Phys., 44, 2802 (1968)l. Its mass is not very different from that of XeFa, and its stretching frequencies are comparable with (though slightly higher than) the stretching frequencies of XeF6 heretofore observed. Taking the value of Ha' oo - HOofrom Osborne, et aE., and subtracting 3/2kT (for translation) and kT (for the C p Cv correction), we obtain 4.3 kcal/mol. Alternatively, H. Kim, H. H. Classen, and E. Pearson Ilnorg. Chem., 7, 816 (19SS)l have crudely estimated 200 cm-1 for all three triply degenerate bending modes and 600 cm -1 for the stretching fundamental of an octahedral XeFe. From these we compute 4.7 kcal/mol ( ~ 0 . 2 0eV) for the sum of vibrational and rotational energy. we
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1'HOTOl ONIZATION
MASSSPECTROMETRIC STUDY
OF
1469
XeF2, XeF4, AND XeF6
species, correlate with the removal of an electron from the uppermost populated molecular orbital. According to Basch, et C L Z . , ~ ~ this uppermost occupied orbital is the totally symmetric and nondegenerate 8~~1,.This orbital has properties similar to those of the 10algencountered in XeF4; it has a 78% fluorine contribution. The arguments advanced for XeF4 would then also be applicable here, i.e., removal of an electron localized on a fluorine would disrupt the ionic bond between that fluorine atom and the residual structure, and the ground state of XeFti+ may be distorted by mixing with either of the degenerate orbitals eg, tip, or tzu.
Goodman38believes that low-lying excited states of XeF6 can exist by population of the next higher (%I,) orbital. The calculation of Basch, et ai.,indicates that this tlu lies -10.6 eV above their uppermost populated orbital (8alg). It is, of course, recognized that promotion of an electron from 8al, to tlu would result in new interactions, i.e. electron repulsion and spin-orbit interactions. Xevertheless, the calculated energy of this tl, state lies so much higher than Sal, that even taking into account the configuration interaction of singly excited configurations, the energy of this excited state is still likely to be quite large compared with IcT, and hence one would not expect any appreciable population of excited states on the basis of this calculation. It has already been noted that in the photoelectron spectrum of the ionic ground state appears no broader than the corresponding XeF4 states. If there were several low-lying ionic states near the ground state, one might expect a significant broadening. Hence, one is forced to conclude that neither the photoelectron spectrum nor the photoionization spectrum provides evidence for low-lying excited states in XeF6. These experiments also do not shed any light on the much discussed, but still uncertain, symmetry of XeF6 in its neutral ground state. Although the evidence points to distortion in the ionic ground state, it is not likely to be of Jahn-Teller-type, since the highest degenerate orbital (5eg) lies some 7 eV deeper than the uppermost 8a1g, according to Basch, et aL31 The next increase in ionization in going to higher energy (Figure 8) occurs for XeF6+at ca. 875 A (-14.17 eV). This correlates well with the onset of a new ionic state in the photoelectron spectrum (-14 eV). The XeF4+ intensity rises from this new threshold, more or less linearly, for the remainder of the energy range covered (to ca. 20 eV). The photoelectron spectrum also shows a very broad ionization region that extends to ca. 20 eV and contains unresolved peaks at ca. 15.2, 16.0, and 17.65 eV. This implies that at the threshold for XeF4+ from XeFs, there should be many states available, and hence this latter threshold should be reliable. From Figure 8, we obtain the value 801.5 A (15.47 eV) as an extrapolated linear threshold for the process
XeF6
+ hv -+XeF4+ + 2F
(8)
When correctedz1 for the estimated internal energy (0.20 eV), this becomes 15.67 f 0.05 eV. One can now complete a cycle employing the previously determined AHyoO(XeF4)= -57.6 f 2 kcal/mol, the ionization potential of XeF4 = 12.65 eV, and the dissociation energy of Fz = 1.586 eV. The result of this cycle is AHloo(XeFe) = -90.6 kcal/mol. Probably the major source of uncertainty in this calculation is the ionization potential of XeF4 (section IV B) because the adiabatic and vertical ionization potentials are so widely separated that it is difficult to deduce the precise adiabatic value. It is almost certainly 612.9 eV, however, so this uncertainty is 0.25 eV. The next largest source of error is in the true threshold for process 8. An upper limit to the threshold wavelength would appear from Figure 8 to be 810 A = 15.3 eV. Both of these possible errors would act in the same direction and would reduce AHyoo(XeFti) to --81.1 kcal/mol. The thermal energy correction may have been estimated too low, however, if XeF6 turns out to have many lowfrequency modes of ~ i b r a t i o n . ~ 'This could make
