Positive- and negative-ion formation due to the electron bombardment of germanium tetrafluoride

International Journal of Mass Spectrometry and Ion Physics Elsevier Publishing Compar;y, Amsterdam. Printed in the Netherlands

POSITIVE- AND BOMBARDMENT

NEGATIVE-ION FORMATION DUE TO THE ELECTRON OF GERMANIUM TETRAFLUORIDE

P. W. I-L4RLAND, S. CRADOCK AND J. C. J. THYNNE* Chemistry Department, Edinburgh Unirersity, Minburgh (Scotland) (Received

20 December

1971)

ABSTRACT

The formation of positive and negative ions resulting from the electron bombardment of germanium tetrafluoride has been studied as a function of electron energy. Appearance potentials have been measured for these ions and various ionisation processes suggested to explain their formation_ From these data bond dissociation energies of several species have been deduced and trends noted for the fluorides of carbon, silicon and germanium are compared with those reported for the analogous hydrides_ A parent negative ion is formed by germanium tetrafluoride as a result of the capture of zero-energy electrons, the electron attachment cross-section being estimated as l-5+0.3 x lo-l6 cm*. At ion source pressures greater than - lo- ’ torr, secondary negative ions such as GeF,-, Ge,F,-, Ge,Fa- are observed to be formed as a result of ionmolecule reactions. GeF,- is formed by the reaction GeF,-

-!-GeF,

A value of 2.9 x 10-l” of the reaction.

+ GeF,molec-’

+ GeF, . cm3 sec- ’ has been estimated for the rate constant

INTRODUCTION

The electron bombardment of a molecule may result in the formation positive and negative ions. The latter may be produced by (i) resonance attachment AB+e

of

+ AB-

(ii) dissociative resonance capture AB+e

+ A-+B

* To whom communications should be addressed; Industry, John Islip Street, London SWl, England. hf.

J. Mass Spectrom. Ion Phys., 10 (1972/73)

present

address:

Department

of Trade

and

169

(iii) ion-pair formation AB+e

---,A-+BL+e

These mechanisms operate at different electron energies, the resonance processes usually occurring in the O-10 eV energy region and the ion-pair processes at energies above this. We have examined positive- and negative&in formation in germanium tetrafluoride over a wide electron energy range. This compound was chosen so that information might be obtained regarding Ge-F bond strengths which might then be compared with those of the other Group IV tetrafluorides, and also because a preliminary study showed that several product ions were formed as a result of negative-ion-molecule reactions. In electron impact studies, when the electron source is a heated filament, uncertainties arise in the evaluation of experimental ionisation data because of the energy spread of the thermionically emitted electron beam. This is largely because the ionisation thresholds become smeared-out as a result of the high-energy tail of the electron energy distribution. Analytical methods have been developed to reduce this problem for both positive’ and negative2s3 ions and we have applied this technique to the negative ions formed at low electron energies by germanium tetrafiuoride.

EXPERIMEhTAL

The data were obtained using a Bendix time-of-flight mass spectrometer, model 3015. The pressure in the ion source was usually maintained below 2 x 10m6 torr except when ion-molecule reactions or very low abundance ions were under examination. The energy of the ionising electrons was read on a digital voltmeter and the spectra measured on two potentiometric recorders. Use of two channels of the mass spectrometer analog output scanners enabled two mass peaks (e.g. O-/SO, and F-/GeF,) to be monitored simultaneously. This was of special value during calibration of the electron energy scale since no switching between peaks was required; it was also of great assistance in ion-molecule studies since the reactant and product ions could be examined simultaneously. The appearance potential of the O- ion from suIphur dioxide was used as the reference for electron energy scale calibration4* 5. In both positive- and negative-ion studies, the electron current was maintained constant by automatic regulation over the entire energy range under investigation. Ionisation efficiency curves were normally measured three to five times. The experimental data were analysed using the deconvolution method described previously2; the electron energy distribution, which was required to be known for this analysis, was measured using the SF,- ion formed by sulphur 170

Int_ J. Moss

S’ectrom.

Zen Phys.,

!O (1972/73)

htzxafluoride6*‘. It was found that performing 15 smoothing and 20 unfolding iterations on the basic experimental data enabled satisfactory recovery of appearance potentials, resonance peak maxima and peak widths at half height to be mad:. For positive-ion studies the appearance potential of CO* at 14.01 eV8 was used to calibrate the electron energy scale, the method used for determining the appearance potentials being the semi-logarithmic plot technique. Germanium tetrafluoride was prepared by heating barium hexafIuorogerminate at 600 “C in a vacuum line; icfrared analysis of the vacuum-distilled germanium

tetrafluoride

revealed no trace of impurities.

RESULTS AND DISCUSSION

Positice-ion forn72tion The positive-ion mass spectrum of germanium tetrafluoride was measured at 70 eV; our results are shown in Table 1 together with similar spectra reported’ fcr the tetrafluorides of carbon and silicon. TABLE

1 CF,,SiF,

POSITIVE-ION~IASSSPE~IRAOF

IonlxFj

x=c

X=

r&*

49 0.5

17 7.2

49 17 8.4 40 128 1000 -

3s 12 42 39 6.9 1000 18

XFZ2+ XFBZ* X’ XF’ XF2’ XFa’ XFo-

GeFS

A&D

Si

~Ti'oev

X = Ge
Points of particular interest which emerge from Table 1 are (i) the absence or very low intensity of molecular ion formatlon, (ii) ions with an odd number of fluorine atoms are more abundant than those with an even number, (iii) the most probable dissociation process involves the loss of a fluorine atom to produce the decreases XF3 * ion, (iv) the abundance of almost all the ions (relative to m,+) in the sequence CF, > SiF4 z=-GeF,, (v) XF2 +-ion formation is only extensive in the case of the fluorocarbon, the intensity corresponding to CFz+ >> SiF,- > GeFzf, ( vi) t h e re1at ive ease of formation of doubly charged ioils, and (vii) the high abundance of XFz2+ compared with XF?‘. It is noteworthy that at 70 eV the intensity of both the GeFz2+ and SiFz2+ ions is greater than that of their Ink J. Mass Specrrom.

Ion Phys., 10 (1972/73)

171

respective singly charged ions; a similar observation in the case of the SF, ion formed from sulphur hexafluorideg (Le. SFa2’ > SF4+) received the explanation that two F-. ions were removed in the ionisation process thereby producing the doubly charged ion, i.e. SF6 + e ---, SF,‘+ i2F- +e. A similar explanation may be appropriate for the tetrafluorides of silicon and germanium. Appearance potential data We have measured the appearance potentials of the F+, Ge’, GeF’, GeF,’ and GeF, -f ions and our results, together with those (where available) for the comparable ions of CF.+ and SiFb, are shown in Table 2. TABLE

2

APPEARANCEPOTS;~TIALDATA

W)FORTHE

IonJXF4

x=

F’ X+ XF+ XF,+ XF3+ XF,+

35.5+ 1 27.5 & 1 29-S&0.3 21.8iO.3 15.9&0.2 -

a Data from ref. 9.

c=

POSITIVEIONSOF

X=

Sib

28.8&O-3 27.450.3 16.2&0.3 15.7&0.3

CF,,SIF, X=

AND

GeF,

Ge

33.0-_10.3 29.450.2 23.410.4 20.710.3 15.7&0_2 -

b Data from ref. 10.

Ge’ No thermochemical data have been reported for germanium tetrafluoride and this hinders an analysis of our appearance potential data. It will however become apparent below that, if we make the assumption that no excess kinetic and/or excitation energy are involved in certain ionisation processes, then two indirect but consistent values for the heat of formation of germanium tetrafluoride AHF(GeF,) can be deduced which may be used in subsequent calculations.

GeF,+e

+ Ge*+-iFt2e

GeF, -+ Ge+4F

(1) (2)

If Ge* formation is assumed to take place via reaction (I) then, using the reported value” of 7.9 eV for the ionisation potential of germanium in conjunction with our value of 29.4kO.2 eV for A(G-e’), we may calculate the heat of formation of reaction (2) to be 21.5kO.2 eV and hence AH,“(GeF,) = -14.4f0.2 eV. Application of Stevenson’s rulel’ would predict that any excess excitation energy would be associated with the fluorine atoms and since the lowest available state (4P) is - 12.5 eV above the ground state then any excess energy must be in the form of translational energy and so our value for AH,“(GeF,) will represent a lower limit. 172

ht. J. Mass Spectrom. Ion Pllys., 10 (1972/73)

There is also the possibility that reaction (3) or (4) may be responsible for germanium-ion formation in which case values of - 16.2 or - 17.8 eV respectively would be appropriate for AHF(GeF,); although we cannot discount entirely these possibilities other data, discussed below, are more compatible with reaction (1) being responsible_ GeF,+e

+ Ge++2F+F,+2e

(3)

--) Ge’+2F,i2e

(4)

F+ The energetics of decomposition of reaction (2), as calculated above, indicate the mean C&-F bond ener,.g B(Ge-F) in germanium tetrafluoride to be 5.4&-Q.1 eV. A second value for AHF(GeF,) may be calculated from the appearance potential of the F* ion; in order to do this we would wish to make use of observations’ 3 that, for several inorganic molecules e.g., SF, and SF,, the mean bond energy frequently does not differ greatly from the individual bond energies. F+-ion formation was noted at 33.0+0_3 eV; this may occur by any of several reactions ccvered by eqn. (5). GeF,ie

--, F’+xF+GeF,_fxt,~+2e

(5)

If we use the relation (x-i- l)B(Ge-F)

< 33.0-1(F)

together with values of 17.4 eV for the ionisation potential of fluorine and 5.4 eV for a(Ge-F) we li..adthat x N 1.9. Since x must be integral then it is likely that x = 2 and so the reaction responsible for F- formation may be written GeF,+e

--, F’iGeF+2F+2e.

(6)

In the case of carbon tetrafluoride A(F’) = 35.3 eV; in vie-wof the similarity of this value and the calculated enthalpy requirement of 35.2 eV for reaction (7), F’ ion formation has been attributed9 to ion-pair formation, i.e. CF,+e

+ F-+F-+C+2F+e

(7)

The analogous ion-pair process for germanium tetrafluoride (reaction (8); would require that A(F+) - 36 eV and so may reasonably be discounted in this sy~-m. GeF,+e

-+ F++F-+Ge+2F+e

(8;

The bond dissociation energy D(Ge-F) has been reported14 to be 5.0 eV which leads to a value of -0.3 eV for A@(GeF). Substitution of this value in reaction (6) leads to a value of - 14.3 f0.3 eV for A@(GeF,) which is in excellent accord with the value of - 14.450.2 eV deduced from A(Ge’). ht. L Mass Spectrom. Ion Phys., 10 (1972173)

173

GeF’

GeF,+e

+ GeF++3Ff2e

GeF,

--, GeF+3F

(9) (10)

Using the values of - 14.3f0.2 eV and -0.3 eV deduced above for AHF(GeF,) and AHF(GeF) respectively we find the enthalpy of decomposition of reaction (10) to be 16.4 eV; since A(GeF+) = 23.420.4 eV our data lead to values of d 7.0 eV for the ionisation potential of germanium fluoride and of 6.7kO.7 eV for A@(GeF+). Johns and Barrowl have determined spectroscopically a value of 7.2+ 0.1 eV for I(GeF), a result which is in good accord with our value. The similarity of this result to the ionisation potential of the metal atom (7.9 eV) suggests that the electron is removed primarily from the germanium atom. The ionisation potentials of C, Si and Ge are 11.3,8-l and 7.9 eV respectively and a similar trend may be expected for the monofluorides of these atoms. O’Hare and Wahl l6 have reviewed the data available for I(GF); this covers the range 8.9-14 eV and they have concluded that a value of 9.2+0-S eV is likely to be accurate. The published values for I(SiF) are in much better accord, Johns and Barrow” having determined the value 7.26 eV spectroscopically and Hastie and Margrave’ ‘, by means of electron impact experiments, obtain I(SiF) = 7.3-tO.2 eV. Our value of < 7.0 eV for the ionisation potential of germanium monofluoride confirms the trend expected for these Group IVA monofluorides. GeF,+ It would seem probable that this ion is formed at 20.7&O-3 eV by reaction (11) so that A@(GeE2 ‘) d 4.8kO.3 eV. Reaction (12) requires 9.9+0.3 eV and hence the ionisation potential of germanium difluoride I(GeF,) may be estimated to be < 10.8 +0.6 eV.

GeF,-f-e + GeF,+ +2F+2e

(11)

GeF,

(12)

3 GeF,+2F

The ionisation potential of difluoromethylene has been reported” to be 11.7 f 0.1 eV so that, as might be expected, IT(CF,) > I(GeF,). The data for SiF, + formation from SiF, do not, on the surface, appear to conform with this pattern since, using the reported value lo of 27.5t0.5 eV for A(SiFzi) together with the thermochemical data at the end of this paper we may estimate that I(SiF,) d 15.1 eV if reaction (13) is responsible for SiF,’ formation. Measurement’ ’ of SiF,* formation from SiFz however suggests that I(SiF2) = 11.3 eV, a result which is more in keeping with the trend to be expected for the ionisation potentials of CF, and GeF2. It seems probable that some 3-4 eV of excess kinetic and/or excitation energy is associated with reaction (13). SiF,+e 174

+ SiFif +2F+2e

(13) Int.J. Mass Spectrom.

Ion Phys., 10 (1972/73)

GeF4+e

---, GeF3++F+2e

(14)

We may identify reaction (14) as being responsible for GeF,‘-ion formation and since A(GeF3+) = 15.7&-0.2 eV, a value of d 0.610.3 eV may be deduced for AHf(GeF,‘). If we assume that the primary bond dissociation energy in germanium tetrafluoride is similar to the mean Ge-F bond, i.e. 5.420.1 eV, then a value of < 10.3 + 0.3 eV may be estimated for the ionisation potential of germanium trifluoride, I(GeF,). This may be compared with the value of 10.2+0.2 eV for I(SiFa), which may be deduced from an electron impact study of silicon tetrafluoride if the primary bond energy is assumed equal to 6.02 0.1 which may be calculated for the mean Si-F bond energy. The situation is less clear as to the ionisation potential of the trifluoromethyl radical; I(CF,) has been measured directly by electron impact measurements and a value of 10.1 +0-l eV reported by two groups of workers 1g*2‘_ It has however been suggestea’l, on the basis of reasonable values for the carbon-halogen bond strength in the trifIuoromethy1 halides, that this measured ionisation potential is too high by about 0.8 eV i.e. I(CF,) -9.3 eV. Recently, Lifshitz and Chupka ” have measured the ionisation potential of the trifluoromethyl radical by a photoionisation technique and have reported that = 9.25+0.04 eV. It appears that the difference between this value and the I&F,) directly measured value is largely due to the fact that the adiabatic ionisation potential of the radical IS much lower than the vertical ionisation potentiai, the configuration of the CF, r ion being very different from that of the radical. Clearly we are uncertain as to the configurations of GeF,and SiF3+ and therefore detailed comparison of the relative values of the reported ionisation potentials is not possible; comparison of the electron impact values (which however may be uncertain due to the possibility of fragments possessing excess kinetic or internal excitation energy), however, would suggest that the ionisation potentials of the trifluorides of C, Si and Ge are identical within experimental error. No molecular ion has been observed for CF, nor, in this work. for GeF,; the SiF,+.+ion however is formed abundantly’. Cradock23 has recently studied the photoelectron spectrum of germanium tetrafluoride and has measured a GeF,I

value of 16.1 eV for the fist vertical ionisation potential of the tetrafluoride; of 16.25 eV and

16-46

eV

have

been

similarly

measured

for

the

values

ionisation

potentials of CF, (ref. 24) and SiF, (ref. 25) respectively. GeF,+e

-+ GeF,+ +2e

(15) If Cradock’s data are used then we may estimate a value of > I-820.2 eV for A@‘(GeF4+); substitution of this value into reaction (16) together with our earlier value of 0.6 eV for AHF(GeF,+) indicates the reaction to be some 2 O-4eV exothermic, so it is to be expected that the parent ion wiil be unstable, decomposing Int. J. Mass Specrrom. Ion Whys., 10 (1972/73)

17s

preferentially to form thz GeF3 T ion. This prediction is in accord with our experimental observation that the GeF, + ion is not sufficiently stable (Le. lifetime c 2 psec) to be detected. GeF, + --, GeF,‘+F

(16)

Negatiue-ion formation

Negative-ion formation occurred extensively in germanium tetrafluoride and the mass spectrum measured at 70 eV and 1 x 10W6torr is shown in Table 3 together with similar spectra for carbon tetrafluoride and silicon tetrafluoridez6. TABLE

3

NEGATIVE-IONMASSSPECTRA

OF CF4,SiFj

lonIXFa

x=c

X

F-

1000

=

AND

Si

GeF,

AT 70eV

X=Ge

1000

1000

&-

2

t1.0

XF-

0

0

0

tl.O

XF, XFBXF,-

46

29

0


48-O 443

Of particular interest is the absence from the spectra of the tetrafluorides of the SiF- and GeF- ions although the CF- is formed, albeit in low abundance. O’Hare and Wahl16 using a Hartree-Fock approximation, have calculated the

electron aEnities of CF and SiF to be l.lkO.2 and 1.0+0.2 eV respectively; their prediction that the SiF- should be a stable negative ion encouraged us to make a very careful re-examination of the negative-ion spectrum of silicon tetrafluoride but we did not observe the ion even in very low abundance i.e., F-/SiF> 104. Our results suggest that the SiF- and GeF- ions are-unstable, probably decomposing to F- ions and the metal atoms since neither Si- nor Ge- were observed in the mass spectra. Another feature of the spectra is the formation of molecular ions by silicon tetraftuoride and germanium tetrafluoride, the abundance being in the sequence GeF4- > SiF,- (>> CF,- which was not observed). In a previous study of SiF, (ref. 26) the parent ion was not observed; in this present work, by using much higher ion source pressures and electron currents, a very low-intensity parent ion was observed. In all cases F- is the most abundant ion and the XF,- ion is formed quite extensively by all three molecules. At somewhat higher ion source pressures GeF5- and several ions containing two germanium atoms are formed with rather low intensity e.g. F-(1000): 176

Jnt.J. Mass Spectrom. Ion Phfs., 10

(1972/73)

GeF5- (6.2) : Ge,Fz(0.8) : Ge,F,(1.0) : Ge2F,- (0.2) : Ge,F,(< 0.1). to be a secondary ion; the verv Pressure dependence studies showed the GeF,Iow intensity of formation of ions such as Ge,F,did not allow pressure studies to be made. It was however apparent that such ions were formed at both high and low electron energies. No ions such as Si=F,- were formed by silicon tetrafluoride at high ion source pressures although the SiF5- ion was formed as a result of the ion-molecule reaction SiF3- -t-SiF, + SiF5- + SiF, .

(J7)

Evidence for the occurence of reaction (17) has been presented previo,uslyz6. Appearance potential data

At low electron energies the ions detected and shown to be formed by resonance capture processes were F-, Ft-, GeF2-, GeF3-, GeF,- and GeF,- all of which, except GeFs-, were shown by pressure dependence studies to be primary ions. At their respective resonance capture peak maxima the relative intensities of the ions F-, F2-. GeF2-, GeF,- and GeP;,- were in the ratio 1000 : 0.1 : 0.2 : 70 : < 0.1. Typical experimental results for some of these ions are shown in Figs. 1 and 3 and the results when deconvoluted

in Figs. 2 and 4.

Ion formation in germanium tetrafluoride is unusual in that the dissociative capture processes leading to their formation occur at comparatively high eIectron energies (-8 eV) although, in view of the bond strengths involved, ion formation might have been expected initially at -3 eV. Another interesting feature is that, with the exception of the molecular negative ion, GeF,-, all the ions studied were

Fig. 1. Isnisation efficiencycurves before deconvolutionfor F- (0) GeFo. ht. J. Mass Spectrom. Ion Phys., 10 (1972/73)

and F2- ( x ) formation by

177

*

!?

0 x 0

d

Q a

o

F-

X

5-

Q

Fig. 2. Ionisation efficiency curves after deconvolution for F-

(0)

and F2- (X ) formation by

GeF4.

Uncorrected electron energy Fig. 3. Ionisation efficiencycures mation by GeF,_

178

(eV)

before deconvolutionfor GeF2- (X 1 and GeFs-

ht. J.

Mass

Spectrum.

Ion Phys.,

10

(0)

for-

(1972/73)

00 0

*

xx 0 x

x

0

0 x

0

0

v)

_e

2-

c 3

x

>I

x

GeF2-

o

GeFT

0

D

k

.x

L _g

h

0

2

0 x CI

c

0

2

x

f

V

x 0

0

5

x

0

I

x

0

0 0

I(

x

0 0

x

0

L

0

6

,.

t

10

Uncorrected

11 electron

I

a.

12 energy

t

13 (eV)

Fig. 4. Ionisation efficiency curves after deconvolution for GeF2mation by GeF&.

(x)

and GeF3-

(0)

for-

found to exhibit a vertical rise to their maximum cross-section at threshold. Our Teas0223&zr be)ieving t)2is are= (i) Both before and after unfolding our data the rise of the cross-section for each ion, from apparent onset to peak maximum, is as sharp as that obtained for the O- ion formed from carbon monoxide which is knownz7 to show the vertical onset effect. This is made clear in Fig. 5 where we have compared the ionisatiun eIEcieney curve for GeF,-/GeF, with that for O-jY33. The respective resconance peaks have bEEn 2y3zELii949 3~3222s3am.. k&2@ 22 zz2&2zzz~ An222.22* GeF,anset edge has been sWkd II~ - 32-32e-V_.I$ is &zr S&S $2~ SSSF~TprV&3~ for both ions are almost identical; the deviation from complete overlap on the high energy side of the maximum is due to the occurrence of a second resonance process in carbon monoxide at - 11 eV leading to O--ion formation which involves production of C(lD) as the neutral fragment. (ii) The GeF,- and O- capture peaks have similar widths; this observation is of importance in establishing vertical onset behaviour because, with a finite Znt.J. Mass Spectrom.

ZonPhys., 10 (1972/73)

179

electron energy distribution, a capture peak may rise as sharply as O-/CO and yet not have a vertical onset if the overall width of the peak is considerably less than that of the O-/CO ion. For peaks of similar width, a non-vertical onset process will always show a slower rise to maximum a=’ z5 qo 8 7o x Q s! L Q B

x

o-/co

o

GeF./GeF,

cross-section.

sl

P

6

:

Fig. 5. Comparison of the ionisation efficiency curves for O-/CO (x) and GeF,-/GeF, Energy scale for GeF3 - shifted by - I .32 eV to permit overlap of curves.

(0).

If all the ions show this vertical onset behaviour then two methods may be used to determine their threshold energies. In the first method the point of maximum gradient on the onset edge of the unfolded peak may be compared with that ;br O-/CO which is at 9.62 eV 27 _ The selection of this point is uncertain to 10.1 eV and this leads to a maximum uncertainty of +0.2 eV when calculating a difference. In the second method the fact that the leading edges of the ionisation curves may be superimposed is used and so the experimental energy shifts required to achieve this superimposition enables the onset energy to be determined; this second method is accurate to 30.05 eV_ The results obtained using both methods are shown in Taole 4 and it is clear that they give the same results within their respective experimental errors. TABLE 4 AppE.4uh’cE

POTE~I~ANDRESONAN~~P~KWIDTHATHALFHE~GHT

IONSFORMED

FROM

Ion

Appearance

Peak

poteniial

Superposition F-

8.60&0.05

FzGeFzGeF3GeF5GeFS-

8.7OiO.05 8.70f0.10 8.3O-r_O.O5 8.30&0.05

180

(In eV) FORTHE~~G~TIVE

GeF4

method

Maximum

gradient method

1.0~0.1 l.oio.l l.l&O.l 1.1+0.1 l.liO.1 0.5*0.1

8.5;0.2

I3.7+0.2 &S&O.2 8.4;0.2 8.4f0.2 Non-vertical onset 0 eV

Int. J.

Mass

width

at half height

Spectrom.

Ion Phys.,

10 (1972j73)

It should be noted that a vertical onset for an ion resulting from the bond cleavage of a diatomic molecule is an indication of a potential well in the molecular negative-ion state and hence of the formation of products with zero excess kinetic energy at onset (e.g. z7 O-/CO). This point is discussed in terms of polyatomic molecules below. This ion shows a single resonance peak onsetting at 8.6OfO.05 eY, the peak having a width of l.O+O.l eV at half height; the peak is slightly asymmetric on the high-ener,qy side.

F-

GeF,+e

+ F-+GeFs

(18)

+ F-iF+GeF,

(19)

-+ F-+2F+GeF

(20)

Using the reported value28 of 3.4 eV for the electron affinity of fluorine, E(F), together with the mean value of 5.4 + 0.1 eV for the Ge-F bond energies, reactions ( 18), (19) and (20) may be shown to have minimum enthalpy requirements of 2.OiO.1, 7.450.2 and 12.8-&0.3 respectively; reaction (20) may therefore be disregarded. We have noted above that vertical onset behaviour implies that the products of a diatomic molecule are formed with zero relative kinetic energy at onset. It is not clear if polyatomic molecules can be treated as “diatomic-like”

systems but some justification for such an assumption can be found in the work of Collins’g

who found that, for C-Br cleavage in a series of alkyl bromides RBr, the isotopic effect on the resonance peak parameters could only be reconciled by consideration of RBr as a diatomic system where R = C,H, to C,H,,. It is more likely however that in most polyatomic cases the excess ener,oy goes into translation and/or vibration; in support of the view that the preference is to lead to vibrational excrtation DeCorpo and Franklin3’ have measured the appearance potentials and kinetic energies of the ionic products resulting from severa- dissociative electron capture processes in polyatomic molecules using a Bendix time-of-flight mass spectrometer_ They reported the following correlation of excess energies at onset with the translational energies of the negative ions: cx = 0.42 = E’ NE,

IAl

where E* is the total excess energy of the system which may be determined from the appearance potential and the heat of reaction, Et the total translational ener,oy and N the number of degrees of freedom of the parent molecule. If we apply relation [A] to reaction (18), where E* = 6.6 eV, Er = 1.7 eV; this leaves 4.6 eV of excess ener,ay which may be distributed between the fragments as excitation energy. Since excited states of atomic negative ions are rare then we Int.

J. Mizss

Spectrom.

Ion Ph~s

, 10 (197Lj73)

181

may assume that the excess energy goes into the GeF, fragment as vibrational energy_ We have estimated above that D(GeF,-F) = 4.6 eV, since this is also the amount of excess vibrational energy it is likely that the GeF, fragment is highly vibrationally excited, possibly even decomposing further to GeFl and F atom fragments. Formation of this ion occurs initially at 8_70+0.05 eV; the linear dependence of the ion current upon ion source pressure indicates that it is formed by a rearrangement reaction such as (21) or (22) and not by an ion-molecule reaction such as (23) although the F- and F2- ions have similar energy profiles. F2-

GeF,+e F-+GeF,

+ F,-tGeF,

(21)

--, Fz-+GeF+F

(22)

--j Fz--tGeF,

(23) Using a value of 2.9-tO.2 eV for E(F,)31*32, reactions (21) and (22) may be shown to have minimum enthalpy requirements of 5.4kO.3 and 10.8+0.4 eV respectively. It is therefore probable that reaction (21) is responsible for F,--ion formation, some 3.3 eV of excess energy being released in the ionisation process. Application of relation [A] to this system yields a value of 2.3 eV for the total translational energy of the system and hence 1.0 eV of excess energy is distributed between the fragments as vibrational energy. We cannot make a definite attribution as to this excess energy but, on balance, consider it likely that it is the germanium difluoride which is vibrationally excited. Unfolding the FZ.--ion data reveals a second dissociative attachment process at 10.5 + 0.1 eV; this reaction has about one-seventh of the cross-section of reaction (21). This is in reasonable agreement with the value of 10.8+0.4 eV which may be calculated for A(F,-) on the basis of reaction (22); we therefore attribute ion formation at this ener,oy to this reaction, the products probably being formed with zero excess ener,oy. This ion occurs with a vertical onset at 8.70+0.10 is considered to be responsible for the appearance of the ion.

GeF, -

GeF,+e

-+ GeF,-+2F

eV; reaction (24) (24)

No value has been reported for the electron afiinity of germanium difluoride and therefore the minimum energy requirement of reaction (24) cannot be calculated. Using the thermochemical data listed at the end of this article we may caiculate from reaction (24) a value of Z 1.3 + 0.3 eV for the electron affinity of germanium difmoride and also AH,“(GeF,-) = -7.2kO.3 eV. This value for the electron affinity may be compared with the value of < 1.6+ 0.1 eV for E(CF,) calculated from a study of CF,’ loa * formation from perfluorocyclobutane33; no value has been reported for E(SiF2). 182

Int. .T. Mass Spectrom. Ion PhEs., 10 (1972/73)

GeF3-

This

ion has an appearance potential formation may be attributed to the reaction

GeF,+e

-+ GeF,-+F.

of

8.3040.05

eV and ion (25)

Lack of thermochemical data precludes a detailed analysis of reaction (25); if however the primary bond energy is assumed to be equal to the mean Ge-F bond energy, i.e. 5.4 +O. I eV, our data would suggest that at least 2.9 eV of excess energy are associated with the ionisation process. Values of 2.050.2 eV and 3.4 eV have been reported 34 for E(CF,) and E(S1F3) respectively. If we assume that E(GeF3) -3 eV (a not unreasonable assumption in view of the high abundance of the GeF,- ion and the reported electron affinities of CF, and SiFs) then reaction (25) will have minimum enthalpy requirements of -2-4 eV which suggests that the fragments contain some 6 eV excess energy. Using relation [Aj we may calculate that Zt = 1.6 eV so that 4.4 eV of excess vibronic energy are distributed between the GeF,and F fragments. Since the first excited state of fluorine (“P) is at - 12.5 eV, we conclude that the GeF3- ion must be formed in an excited state. Using the above estimate of 3 eV for E(GeF3) we

may calculate a value of - 12.7 eV for A@(GeF,-). GeF4GeF,+e

+ GeF,-

(26)

This ion, which must be formed by the associative electron capture reaction (26), was detected initially at -0 eV; it was also observed at high electron energies (-20-100 eV) when it was formed as a result of secondary electron capture, the secondary eIectrons being produced by such positive ionisation processes as reaction (27): GeF,+e

---,GeF,‘+-Ft2e

(27)

Such behaviour has been noted previously for nolecules such as SF, (ref. 35), SF, (ref_ 35) WF, (ref. 36), and CFJCOCF3 (ref. 37). When GeF, was studied at electron energies -0 eV fairly abundant parention formation was observed; admission of small quantities of sulphur hexafluoride to the ion source considerably reduced the intensity of the molecular negative ion, suggesting that the attachment cross-section for reaction (28) was much greater than that for reaction (26). SF,;e

+ SF,-

(28)

The experimental problems and difficulties inherent in the measurement of

ionisation cross-sections of reactions such as (26) have been well-documented by Kieffer and Dunn3 8. In this work we have attempted to measure the reZalil;ecrosssections for SF6 and GeF,; this has obviated the need for knowledge of the Int. J. Mass Specfrom. Ion Phys., 10 (1972173)

183

pressure of the gases in the ion source (only their relative proportions

need be known), of the total ionising electron current and of various ion source and drift tube parameters which are required to be known if measurements are to be made on one gas alone. We have however assumed that the GeF,- and SF,- ions have the same collection efficiency but feel that in view of the nearness of masses of the ions (149 and 146 amu) and hence the similarity of their kinetic energies this assumption regarding multipiier response is not unreasonable. No attempt was made to measure the total ion current at any energy, although at 0 eV the parent ion is responsible for virtualiy all of the ion current.

o

SF6-

x

GeF,-

. x

I 0

I

1

Uncorrected

I

2 electron

wea

i energy

(eV)

Fig. 6. Ionisation efficiency curves for SFsto the same intensity at maximum.

(e)

and GeF,-

(X ); the curves have been normahsed

In Fig. 6 we show typical data (not deconvoluted) for GeF,and SFBformation obtained using a mixture of GeF, and SF,. “Negative” voltages were obtained by the introduction of a 3 V dry cell into the electron energy circuit. The GeF,- and SF6- ionisation curves have been normalised to the same peak height; it is apparent that they have very similar electron energy dependences, both reaching a maximum value at the same electron energy. Because of this similar energy dependence, the relative peak heights may be used to evaluate the relative 184

Int. 6. Mass Spectrom.

Ion Phy.s., 10 (1972/73)

attachment cross-sections for reactions (26) and (28). If 1x- is the X--ion current, (X) the ion source pressure of X and ox the electron attachment cross-section of X, then %F.s -c-s

r,F,-

GGeFs

‘GeFL-

(Ge&) Wed

-

Our results indicate the csF,J~o~r, = 78 f I5 (the error limits represent the mean deviation of several repeat determinations); if a value is known for GsF6we may calculate the attachment cross-section of germanium tetrafluoride. Various values have been reported 3g-44 for the sulphur hexafluoride attachment crosssection varying from 3 x lo-‘* to 1.17 x lo-‘” cm’. We shall assume the latter value to evaluate hoer, but wish to point out that this is an arbitrary assumption although the value seems to have been very carefully determined34. Using this result we find that cGeF4 = 1.5+0.3 x lo-l6 cm2. There is no other result for o&r?? with which our result may be compared. The molecular negative ion formed by silicon tetrafluoride was examined but the electron attachment cross-section was too iow to be measured with any accuracy; it was however at least three orders of magnitude less than that for germanium tetrafluoride, Le. csiF4 - 1O-19 cm2_ Carbon tetratluoride does not form a stable parent negative ion so that clearly the ease of parent-ion formation increases markedly as Group IVA is descended and as more unoccupied orbitals in the molecule become available to the incident electron. We also attempted to measure the autodetachment lifetime of the metastable GeF,- ion using the time-of-flight technique we have employed3’ for such molecule ions as SF,-, SF,- and C,F,,-. GeF4- + GeF,+e It was found, however, that although the neutral fragment was observed, the low attachment cross-section of GeF, and the complication introduced by the presence of several germanium isotopes made accurate evaluation of the neutral fragment extremely uncertain. We may therefore conclude only that the autodetachment lifetime rocr4 > 2 psec; a Shih COnChSiOn may be drawn for ~siF~_The autodetachment lifetime of sulphur tetrafluoride has been reported35 EO be 16.3+0.3 psec. The lifetime of CF,- must be c 2 psec since no CF,- ion was observed in our study of carbon tetrafluorrde. GeF5The pressure dependence of this ion showed it to be a secondary species and so probably formed by an ion-molecule reaction, possible reactions responsible being: F,- + GeF,

4 GeF,-iF

(29)

GeFt - + GeF, -+ GeF5- + GeF

(39

GeF3- -i-GeF, -+ GeF5- + GeF2

(311

GeF,- + GeF, -+ GeF,- -i-GeF,

(32)

Znt. J. Mass Spectrom. Ion Phys_, 10 (1972173)

185

9

8 I

? P

5

.

o

GeG-

x

GeF5-

D

P

P

9

a

a

I

9

0

OS il

9

6

70

9

Uncorrected

11

electron

12 energy

% *v

Ql.,

(ev)

Fig. 7. Comparison of the ionisation efficiency curves for GeFshave been normalised to the same intensity at maximum.

(0)

and GeFs-

(x);

the curves

Comparison of the electron energy profiles of these primary ions with GeF,showed only the GeF3- and GeF, - ions to have identical profiles (see Fig. 7) which indicated GeF,- to be the reactant ion. Reaction (31) is therefore considered to be responsible for the formation of GeF,-. In the case of silicon tetrafluoride, SiF5- was observed26 to be formed by the analogous ion-molecuIe reaction SiF,-

-t SiF, --+ SiF,’

+ SiFz .

(33)

We have estimated the rate constant for reaction (31 j by comparing the primary and secondary ion current ratio,-, Isec/~~rim,for reactions (31) and (34), when similar pressures of methane and germanium tcirafluoride were maintained in the ion source. CH,‘+CH,

-+ CH,‘tCH3

(34)

Assuming, under these conditions, I ~e~s-lke~a-

&I

z CH5’&Hs’

= k34’

then, using a value4’ of 1.2 x 10mg moIec- l cm3 see- l for ks4, our data yield a value of 2.9 x lo- lo molec- ’ cm3 set-’ fork,, . There have been few thermal rate constants reported for negative ion-molecule reactions but these values include cm3 see-’ fork,, (ref. 26) and 19 x lo-” molec-’ cm3 see-’ 2 1 x 10-lomolec-’ for k,, (ref. 46) m * reaction (35) below. These reactions were also studied using time-of-flight mass spectrometry and the average reactant ion kinetic energy in all cases was probably ~0.3 eV. F,‘+BF, 186

+ BF,-+F

(35) Ink J. Mass Spectrom. Ion P&s-, 10 (1972/73)

A value for AHB(GeFS-) may be deduced from the fact that since the ion is stable, AH 36 > 0 and hence AHF(GeF,-) < - 16.9 eV. GeF, - + F- + GeFs

(36)

Another limit for AHF(GeF,-) may be determined by using the requirement that for an ion-molecule reaction to be observed it must not be endothermic; application of this condition to reaction (31) indicates the AHF(GeF,-) < -21.1 eV. Similar calculations based upon reactions (33) and (35) result in values of 4H~(BF,-) ,< -24 eV_ < - 15.5 eV and AHf(SiF,-) Our observation that the GeF, - ion is formed is of particular interest for the following reason: the isolation of the SrFs- ion was demonstrated4’ by the formation of tetraphenylarsonium pentafluorosilicate in the reaction of tetraphenylarsonium chloride and silicon dioxide in aqueous methanohc hydrogen fluoride; the existence of this ion was confIrmed by our earlier electron impact study”? Clark and Dixon4’ also suggested, on the basis of infrared spectra, that use of germanium dioxide may have yielded a compound containing the GeF,ion but no more definite evidence for the formation of this ion could be obtained. Our study of reaction (31) has confirmed their suggestion that their compound probably did contain the GeFS- ion. Ge, F,Following our observation that ions such as Ge,F,were formed, a search for these and similar ions was carried out at low electron energies. Mass analysis at zero electron energy showed the ions Ge,F,and Ge,F, - to be formed in the same region as GeF4-. These ions were probably formed by ion-molecule reactions with GeF,as the reactant ion; possible reactions responsible will be discussed below. Using very high trap currents (-0.5 PA) and ion source pressures in the region of 5 x 10s5 torr, Ge,F,-, Ge,F,and Ge2Fs- were detected, their maximum intensity of formation corresponding to the ener,7 region of the maximum formation of the GeF,and GeFs - ions. This suggests that ion-molecille reactions involving these as reactant ions may well be responsible for the formation of these complex ions. The following reactions are of the type envisaged: GeF,-

GeF,-

+ GeF,

--, Ge,FI-

-i-6F (F,?)

(37)

+ Ge,F,-

t4F

(38)

i- GeF, --, Ge,F,-

+ 3F

--, Ge,F,-+F GeF,-

+ GeF,

+ Ge,F,-

(F2?)

(39) WI

+ F

(411

It will be noted that reaction (41) involves the reaction of the secondary ion GeF,with the tetrafluoride which emphasises the likelihood that these reactions have quite large cross-sections. Znt. J. Mass Spectrom.

Zon Pizys., 10 (1972/73)

187

The formation via ion-molecule reactions of species such as Ge,F,-, Ge2F,- and Ge2F,- enables values representing upper limits to be calculated for the heats of formation of these ions. From reactions (34), (40) and (41) we have estimated that AH,“(Ge,F,-) < -29 eV, AHF(Ge,F6-) < -28 eV and AH~(Ge,Fs-) ,( -36 eV. As regards the structure of these ions, germanium compounds containing two germanium atoms have been isolated by Margrave48; a GeF,/GeF, adduct, Ge,F,, thought to originate from the complex (GeF,),GeF,, was isolated as a white solid. Raman investigations of the white solid failed to reveal any Ge-Ge frequencies. In the light of these observations we envisage fluorine atom bridged structures for the digermanium-containing ions, namely:

ft is clear that three, four and five coordination of the germanium is maintained in reactions (39), (40) and (41) respectively. The detection of these ions together with GeF,- underlines the usefulness of the”-application of negative-ion mass spectrometry to inorganic compounds since the ðod gives direct evidence of the existence of anionic species. It should be remembered of course that formation of a complex ion in the gas phase does not necessarily imply the existence of the ion in the solid state because the factors affecting the stability of the ion in the two states are different; for example, in the solid the lattice energy term is an important parameter.

Bond dissociation energies GeF

The

structures of the outer electron

shells, in accordance

with

Mul-

positive

and

electron,

it is

liken’s scheme4’, are denoted by . . . (za)’

(ya)2

(xc+

(w)4

(2m)

2.i7

The most probable structures for the ground states of the negative ions and their electronic states are therefore respectiveljr, . . * (zo)2

(J-a)2 {xcr)2

(w)”

. _ . (ZG)’

(Y0)2

(W7C)4

lz

and (XG)”

(&Zj’

3Z

Since formation of GeFi intolves the removal of an antibonding to be predicted that D(GeF+) > D(GeF). 188

ht.

J. Mass Spectrom.

Ion Phys., 10 (1972/73)

A Hess cycle enables the following relationship to be deduced: D(GeF’)-D(GeF)

= I(Ge)-I(GeF)

= 7.9 eV1l and I(GeF)

= 7.2 eV” so that D(GeF‘)--D(GeF) = 0.7 eV, a result which accords with the prediction above. Comparable vaIues for D(CF+)--D(CF) and D(SiF’)--D(SiF) are 2.1 eV and 0.8 eV respectively; the decrease in this difference becomes less pronounced along the Group IV series e.g. D(SnF+)-D(SnF) = 0.3 and D(PbF+)-D(PbF) - 0 eV, indicating that the antibonding effect of the remcved electron decreases along the series. The above differences enable the followng values (in et’) to be deduced for D(XF’): 7_5(CF’), 6_2(SiF+), 5_7(GeF’), 4.2(SnFf) and -3(PbFL)_ The electron captured by germanium monofluoride would be expected to go into an antibonding orbital so that the GeF- bond is likely to be weakened compared with the GeF bond; our failure to detect the GeF- ion suggests that this is so. A thermochemical cycle shows that: f(Ge)

D(GeF)-D(GeF-)

= E(F)-E(GeF)

If we assume the non-observation of GeF- indicates that E(GeF) < 0 eV then D(GeF)-D(GeF-) > 3.4 eV; a similar conclusion may be drawn for the SiFion, i.e. D(SiF)-D(SiF-) > 3.4 eV so that D(GeF-) < 1.6 eV and D(SiF-) < 2.0 eV. The CF- is a stable negative ion and, using O’Hare’s calc&ated value16 of 1.1 eV fcr E(CF), then D(CF)--D(CF-) = 2.3 eV and hence D(CF-) = 3.2 eV. The sequence D(GeF+) > D(GeF) > D(GeF-) is in accord with the trends cored for the analogous CF and SiF species and illustrates the destabilising effect of antibonding electrons. TABLE BOND

5

DISSOCIATION

ENERGIES

Bond

x=c

XF XFXFXF-F XFZXFzXF2-F XFxT XFSXFS-F XF4+ XF,-

5.5

= Meanbond int. 1. Mass

7.5 3.2 5.2 2.7 2 3.4 4.3 5.9 2.9 5.4 to -

(in

ev)

x

OF THE FLUORIDES

=

5.4 6.2 G2.0 6.4 2.4 <30 5.1 6.2 5.1 6.0” 1.1 -

Sl

OF

C, SI

AND

Ge

X=Ge

5.0 5.7
energies. Spectrom.

Ion PAYS_, 10 (1972173)

189

It is of interest to compare data for the isoelectronic species GeF and AsO; the bond dissociation energies of both species are almost identical since D(As--0) = 4.9 +O.l eV14. A much bigger difference is noted between D(GeF+)and D(AsO+), the latter value being 7.7+ 1 eV ’ 4. A similar trend, i.e. the difference between the bond strengths of ions is greater than the corresponding difference between the neutrals, is also observed for the pairs of isoelectronic species NO and CF and PO and SiF, the differences being 8.0 and I .OeV for the first pair and N 3 and 1.4 eV for the second pair. This species is bent; one possible structure for the outer electron shell GeFz written in chemical terms is

. . . (F 2~)~ (Ge 4s)’ (G GeF)4 (F 2p,)* (Ge 4p)‘, where F 2pILrepresents only the F 2p orbitals orthogonal to tire F-Ge bond direction. The most probable structures for the ground states of the positive and negative ions would therefore involve (i) the removal of a F 2p, electron, probably not one which was involved in 7~bonding to the empty Ge 4p orbital, and (ii) the addition of an electron to the Ge 4p orbital which may be slightly antibonding if z delocalisation occurs. These considerations would lead us to predict that D(GeF2 +) - D(GeF,) and D(GeF,) > D(GeF,-). Alternatively, the G GeF (a,) orbital may involve principally the Ge 4s orbital, and we may obtain the following electronic structure for GeF, . . _ (F

2s)” (0 GeF)” (F 2~~;)’ (Ge 4~(a,))~

(Ge 4p(b,))‘.

Now removal of an electron occurs from the Ge 4p (al) orbital which is mixed with G GeF (a,) and is thus formally G bonding. Addition of an electron again reduces the z-accepting capacity of Ge 4p (6,) and so we predict that D(GeF2) > D(GeF, + ) and D(GeF2)

> D(GeF,-).

D(GeF2 ‘) - D(GeF2)

=

I(GeF) - Z(GeF,)

This relation, together with values of 7.2 and 10.8 eV for Z(GeF) and Z(GeF2) respectively, indicates that D(GeF,*)-D(GeF,) = -3.6 eV, a result which confirms that it is a bonding electron which is removed during ionisation. D(GeF-F) may be estimated to be 6.4+0.2 eV from D(Ge-F) and AH,“(GeF,) so that D(GeF2+) = 2.8 eV. Comparable values for D(XF,‘)-D(XF,) are -2.5 eV (X = C) and -4.0 eV (X = Si); these differences lead to values of 2.7 eV and 2.4 eV for D(CF2+) and D(SiF,+) respectively. Within experimental eror all these values are identical and it may therefore be concluded that the weakening effect on the bond due to electron removal does not diminish along the series. Our experimental results for D(GeF,+) and D(GeF2-) are compatible with those predicted on the basis of the second possible structure for germanium

190

Int. J. Mass Spectrom. Ion Phys., 10 (1972/73)

difluoride. Confhmation of the second picture of the electronic structure comes from our value of < 10.8 eY for the ionisation potential of the difluoride which is quite inappropriate for a F 2pz electron. The bond strength D(GeF2-) may be calculated from the relation D(GeF,)

- D(GeF,-)

= E(F) - E(GeF,).

Using the value of b 1.3kO.3 eV we have deduced for E(GeFz), together with a D(GeF=-) < 2-l eV and hence value of 3.4 eV for E(F)28, we find D(GeF2)D(GeF2-) 3 4.3 eV. The SiF=- ion was not observed in the negative-ion mass spectrum of SiF, ,- if we assume that E(SiF2) < 0 we find D(SiF2)- D(SiF2-) > 3.4 eV and hence D(SiF*-) c 3.0 eV. The CF,- ion is stable and a value of < 1&ieV has been deduced for E(CF,) f rom a study of ion formation by perfluorocyclobutane33; use of this value leads to D(CF,)-D(CF,-) 2 1.8 eV and consequently D(CF2-) >, 3.4 eV. The results for the three species CF,, SiF, and GeF2 are shown in Table 5 and indicate the following sequence to apply D(XF,)

> D(XF,

-)

> D(XF2+)

The difference, albeit small between D(XF2-) and D(XF2’), suggests that the removal of a c electron from a bonding orbital has a more marked effect on bond strength than does the capture of an electron into a II: antibonding orbital. The outer shell of this 25electron system will be the Ge 4p= orbital GeF, and consequently the GeF,’ and GeF3- ions may be written . . . (Ge 4~~)’ and . . . (Ge 4p,)* respectively. It is therefore to be expected that formation of the GeF,ion will be accompanied by a slight weakening of the 7r bond and of the GeF3+ by a slight strengthening of the bond. The issue may be complicated however since, if GeF3 is pyramidal, mixing of the germanium 4s and 4p= orbitals may have an effect on the c bonding also. D(GeF,‘)-D(GeF,)

= I(GeF,)-I(GeF,)

Our data for I(GeF,) and I(GeF,) indicate that D(GeF,+)-D(GeF,) = 0.5 eV and hence D(GeF,‘) = 5. I eV. This slight strengthening of the bond is in accord with the prediction above. We also tentatively suggest that since the effect is quite small, there has been no effect on c bonding due to mixing; this might indicate a planar structure for the GeF, + ion which would be in accord with similar suggestions for the CF, + ion. Since CF, is pyramidal this decrease might possibly indicate that the trifiuorides become progressively nearer to planarity as the series is descended. Comparable values for D(XF,‘)-D(XF,) are 1.6 eV (X = C) and 1.1 eV (X = Si) so that the difference decreases along the series. These values = 5.9 eV and D(SiF3*) = 6.2 eV. iead to D(CF,*) D(GeF,)-D(GeF3-) ht.

= E(F)-E(GeF,)

J. Mass Spectrom. Ion Phys., 10 (1972173)

191

Using the value of 3 eV we have assumed for E(GeF,) we tid that, as predicted, D(GeF3) > D(GeF,-) and D(GeF,-) = 4.2 eV. Similar expressions lead to values of 5.1 eV and 2.9 eV for D(SiF,-) and D(CF3-) respectively. The results for the three species CF 3, SiF3, and GeF, indicate t-hefollowing

sequence to be valid D(XF,*)

> D(XF,)

> D(XF,-).

A chemical description of the electronic structure of germanium tetraGeF4 fluoride may be written . . . (F 2de))C

(F 2~,(&jj~

(F 2~,(h))~

(Ge We))“.

This suggests that formation of GeF,might be accompanied by weakened G bonding whereas formation of the GeF,+ would lead to a slight weakening in the n bonding; we should therefore predict that GeF,+ would be more stable than GeF4-. This prediction is not in accord with our experimental observations which show GeF,- to be a metastable ion whereas there was no indication of GeF,’ formation. An alternative suggestion which might account for the failure to observe a GeF, * ion invokes Jahn-Teller distortion and is as follows: the highest occupied orbital in the tetrafluoride is probably a t, combination of F 2p, orbitals which cannot enter into 7~interactions with Ge 4d orbitals. The GeF,’ ion should there-

fore have a triply degenerate ground state which is not allowed; Jahn-Teller distortion would remove this degeneracy. If this occurred by a lengthening of one GeF, bond (retaining Cfv symmetry and transferring the “vacancy” into the a1 component of the now-split t1 level) a satisfactory mechanism for Ge-F band cleavage would exist. As the “retreating” fluorine atom has no 2p, level of the a, symmetry species the “vacancy” must remain with the GeF3 group leading to the production of FiGeF3’ (i.e. reaction (16)). Jahn-Teller distortion should or”course apply to GeF&- as well as to GeF,’ ; since however the former ion is found (although we observe it to be metastable) it would seem the difference is a matter of degree, Le. of the lifetime of the relevant ionic species. D(GeF,*)--D(GeF,j

= I(GeF,)-1(GeF,)

Using the value we have estimated for I(GeF,) we find that D(GeF,‘)D(GeF,) = -5.8 eV. Ionisation clearly causes considerable bond weakening and since D(GeF,-F) N 5.4 eV then D(GeF,‘) N -0.4 eV. It is therefore to be predicted that the parent molecular ion is unstable; this is confirmed by our experimental work. Values of - 1.4F0.3 eV and 1.1+0.4 eV may be calculated for D(CF4+) and D(SiF,+) respectively which is in line with the observation that only silicon tetrafluoride forms a stable parent ion. D(GeF,)-D(GeF,‘) 192

= E(F)-E(GeF,) Int. .l. Mass Spectrom. Ion Phys., 10 (1972/73)

No value has been reported for E(GeF,); the electron afEnity of sulphilr tetrafluoride is 1.2 eV5*. These two tetrafluorides have similar electron attachment cross-sectionsg5 and autodetachment lifetimes both of which parameters reflect the electron affinity of a species. If we assume therefore that E(GeF,) - 1 eV then D(GeF,)-D(GeF,-) - 2.4 eV and hence D(GeF,-) - 3.0 eV. The formation of GeF,- and SiF,- but not of CF, - is of interest and may be explained in terms of the availability of valence she11d orbitals in the former cases. Conzparisotz of bond dissociation energies qffluorides

and hydrides

A complete comparison of the bond dissociation energies of the species XF,,+, XF,- and XF,, with those of the corresponding hydrides XI&’ etc. carnot be made because of lack of experimental data for some species. Certain sequences and patterns however may be deduced and rhe following comments made on the respective fluorides and hydrides.

Comment

Sequence D(SiH-) D&H-) D(SiH,+)

> D(SiH) > D(SiH’) > D(CH) - D(CH’) > D(SiH,) > D@HZ-)

I

contrary behaviour to th& noted for corresponding

I

fluorides

f

For X = C, Si and Ge D(XH2+)

> D(XH,)

J

D(XF,) > D(XF,+) D(XF3+) > D(XF,)

1

similar to fluorides

In addition, within each particular series the following sequence of ion fragment bond dissociation energies for hydrides and fluorides may be noted.

Fluorides

HJldrides CH2--

> CH3+ > CH+

SiH,-

> SiH2+ > SiH’

GeH,’

7GeHzt-

7

GeH+

CH4+

CF’

7

SiH4’

SiF’

> SiF3-

7 7

GeH,+

GeF-

CF3’

7

GeF3’

7

CFI*

> SiF,-

7

CF4+

> SiF,’

> GeF2+ > GeF4’

The most notable difference is between the XF’ and XH+ ions which have very strong and relatively weak bond dissociation energies respectively. Int. J. Mass Spectrom.

Ion Php., 10 (1972/73)

193

Thermochemical data

The following values (in eV) for heats of formation at 298 K have been used in

our

CF,

=

calculations: C = 7.4, F = -55.0+0.1, CF, = -9.610.1, -X1&0-4, SiF, = -11.6f0.1, SiF, GeF, -0.3+0.1, GeF, = -5.9+0.1, AI1 values are taken either from ref. 51

0.8, CF = 2.9+0.1, CF, = -l-5+0.1, Si = 4.620.1, SiF = O.Of0.1, SiF, = = -16.8+0.1, Ge = 3.9f0.1, GeF = = -9.7+0.3 and GeF, = -14.3t_O.3_ or estimated in this paper.

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