Negative-ion formation by perfluoro-n-butane as the result of low energy electron impact

445 International JOWM! of Mass Spkrometry and Ion Physics, 11 (1973 j MS-454 @ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

NEGATIVE-ION FORMATION BY PERFLUORO-n-BUTANE’ RESULT OF LOW ENERGY ELECTRON IMPACT

AS THE

P. W. HARLAN-D AND 3. C. J. THYNNE* Chemistry Department, Edinbto-gh Uaicersity, EZnbargh (Scotland) (Received 15 January 1973)

ABSTRACT

The formation of negative ions by perfluoro-n-butane as a result of electron bomkrdment at low electron energies has been studied and various processes suggested to explain ion formation. At 0 eV a metastable molecular negative ion is formed, the autodetachment lifetime of which has been measured as 12.7 +0.3 psec. The electron attachment cross-section of perfluoro-n-butane has been estimated to be -10-l* cm2. Values of 2.2-tO.3, 2.2kO.2 and 3.2kO.3 eV have been estimated for the electron affinities of C&F,, CsF, and C,Fs respectively.

INTRODUCTION

As part of a programme concerned with the formation of negative ions at low electron energies1-4 we have examined perfluoro-n-butane. Negative ion formation by this molecule has been studied previously, althougfr not in detail’* 6. A preliminary examination of perfluoro-n-butane showed several interesting and novel features, including the formation of a metastable parent ion with an autodetachment lifetime amenable to determination by a time-of-flight technique. Accordingly we have investigated the ions formed as a result of low energy electron impact. In electron impact studies, when the electrons are emitted from a heated &unent, because of the energy spread of the ionising electrons, uncertainties may anse in the determination of the experimental ionisation efficiency curves, the appearance potentials becoming “smeared out”. To reduce the effect of this elecyrok energy distribution,_ analytical methods have been developed for positive’ and negative’ idn studies. l

Present address: Department df Trade z&i Industry, Shell Mex House, Strand, London WC2.

446 EXPERIMENTAL

The experiments were performed using a Bendix time-of-flight mass spectrometer, model 3015. Ion source pressures were usually maintained below 5 x 10w6 torr (<2x IO-+ torr for the lifetime studies). The electron energy was measured on a digital voltmeter. The electron current was held constant automatically over the electron energy range studied and ionisation efficiency curves were usually measured three to five times; the potentials at which the ions appeared were reproducible to &O-2 eV, often better. The electron ener,Fryscale was calibrated at 0 eV using the SF6- ion formed by sulphur hexafluoride’* lo and at higher energies using the appearance of the Oion from snlphnr dioxide at 4.2 eV 11-12 . Use of two channels of the mass spectrometer analog output scanners enabled two mass peaks (e.g. O-/SO2 and CFs-/ C,Fio) to be monitored simultaneously; this was of especial value in energy scale calibration since no switching between peaks was necessary. The experimental data for the ionisation efficiency curves were analysed using the deconvolution method described previouslyE. In general it was found that performing 15 smoothing and 20 unfolding iterations on the basic data enabled satisfactory determination of appearance potentials, resonance peak maxima and peak widths (at half peak height) to be made. The electron ener,v distribution, which was required to be known for the deconvolution procedure, was measured using the SF6 - ion formed by sulphur hexafluoride. Lifetime

measurements

The autodetachment lifetime of the ion was determined as follows. Negative ions, formed in the ion source as a result of electron capture; are accelerated through a variable electric field (usually 2-3 kV) into a field-free drift region, i.e. a fright tube (200 cm long)_ If, during their passage down this tube, some of the metastabIe ions undergo autodetachment (i.e. eject electrons) both the remaining ions and the neutral molecules formed will continue moving at the same velocity and they will reach the detector simultaneously. If, however, a retarding potential is applied some time after the ions leave the ion source and enter the drift zone, then the ion “packet” will be slowed down aud the neutrals will pass on unaffected. The detector will therefore respond to two “packets” separated in time, one being the neutral species resulting from ion decomposition and the other being undecomposed ions. Variation ol the ion flight time will cause the relative intensities of the two “packets” to vary and enable conclusions to be drawn regarding the lifetime of the negative ion state. The ion flight time was measured directly for the various ions by triggering the oscilloscope on the leading edge of the ion draw-out pulse and reading the flight time off the oscilloscope.

447 It became apparent during ‘ihe investigation that the horizontal and vertical deflection plates and the ion lens in the llight tube discriminated substaritially between the neutral and the parent ion and could result in quite erroneous lifetimes. As a consequence the focussing electrodes were maintained at the .drift-tube potential throughout the measurement-b and so no preliminary separation near the ion source was employed. The tim+of-flight adjust-lens (188 cm from the ion source) which was used to separate +_heneutral and the ion peaks, was shown to have a negligible focussing effect by comparing the separated ion currents (lens on) with the total ion current measured with the adjust-lens off. The sample was leaked into the spectrometer and the apparatus was adjusted so that the resolved charged and neutral peaks were scarmed repeztediy (usually about ten times) at 0 eV. Autodetachment may be assumed l3 to be a first-order process expressed by the relation: N-(t)

= IV&

exp (-t/T)

where N-(t) is the number of ions surviving autodetachment after time f, I?; is the original number of metastable ions undergoing acctleration = N-(tj-~-N’(tj, No(t) :‘sthe number of neutrals formed by autodetachment after time t and r is the mean lifetime of the negative ions. If no process other than autodetachment occurs, equal detection sensitivity of the neutral and the parent ion is assumed and there is no discrimination due to stray fields or focussing by the time-of-flight ion optics then the mean ion lifetime may be calculated from the equation: .

Measurements of N&, No(t) and t therefore yield the average lifetime of the metastable ion. The assumption that autodetachment is the exclusive process requires that the following conditions are satisfied: (i) colkonal detachment or charge transfer does not take place in the flight tube (ii) detachment by grids in the path of the ion beam is negligible.

Compton et al. l4 have shown such effects to be unimportant when the pressure in the flight rube region is lo-’ to lOA7 torr and our studies have been carried out in this pressure range. RESULTS

AND

DISCUSSION

Autodetachment

lifetime

Preliminary investigations showed that perfluoro-n-butane formed a metasta-

448 ble parent ion at 0 eV and also at 70 eV; in the latter case the ion was formed as

g result of secondary electron capture, the secondary electrons being formed during such positive iomsation processes as C,FIo +e --, CF3+ + C,F, +2e. C,Flo-*

+ C,F,,,+e

(1)

0u.r resdts for Tc4F10are shown in Table 1 together with values we have measured’ 5 for perfhxorocyclobutane and perfluorobutene-2. It is of interest that bor;h these latter molecules have longer autodetachment lifetimes than the butane although they have fewer degrees of freedom; the lifetime however is not simply a function of the number of degrees of freedom since it also depends on the vibrational frequencies, the electron af5nity and the magnitude- of the attachment cross-section. With this proviso our results may perhaps be taken to indicate’&& the perfluorocyclobutane ion is not so greatly strained (vis & vis the C4F10- ion) that it rapidly undergoes ring rupture and decomposition and it may be that the ring coders some extra stability on the ion. This tentative view perhaps receives some support from unpublished observations in this laboratory on n-C6F14 and c-C,F,, which indicate 7,C6F12/7n_CsF,4 - 3. TABLE 1 AUTODETACH&fENT

LIFETIMES

Ion

Lifetime (psec) ZkV 2.5 kV

3.0 kV

3.5 kV

12.4 30.7

13.0 29.2

31-7

15.1

13.8

15.1

C&-IO-* 2-c_& -* c-CqF8-*

(t)

AND

ELECl-RON

ATTACHMENT

CROSS-SECTIONS

(a)

-cw

Cross-section c (cm’)

-

12.7 30.6

-lo-‘* -lo-

15.1

14.7

-10-16

15

The elect of unsaturation is marked and more than offsets the decrease in ion lifetime which might be expected because of the fewer degrees of freedom of the olefin. That the olefin-ion is more stable than the alkane-ion is supported by our observations that C3F6 formed a low intensity parent ion -whereasno parent ion was detected for C3F8 9 i.e_ z,-,~~ < 2qec. n-C4FIo is, in fact, the ftrst member of the saturated perfluorocarbon series for which a metastable parent ion has been detected; it is of interest that, in the positive ion mass spectrum, the molecular ion C,FIoi is absent. Electron attachment cross-section We attempted to measure the relative attachment cross-sections of SF6 and C4F10 by admitting mixtures of SF6 and C,FIo. of known concentration to the ion source and measuring the parent ion currents. at 0 eV. C,F,,+e :

-SF,+e

+ C,FIo-

0

-+ SF~;

(3)

Because of the very low intensity -of the C,F,,ion curr&t we weir, unable to measure ac&ra~ely the cross-section of reaction (2):It was apparent &wever that bothSF6- andC4FIo- reached their maximum intensity at the same electro& energy ana we estimate that o~,/o~~~~, - 104. .Various .values have .been reported16-1g for the SF, attachment cross-section, varying from 3 x 10m2’ to 1.17 x 10Yf4 cm2. If we make &e arbitrary assumption of the latter value for osF6 then our results Sllg&St that flC4FID- IO-l* cm’. We know of no other value for this attachment cros§ion. Mass spectra al 70 eV In Table 2 the negative ion mass spectra of C2F6, C3F8 and II-C4FIo measured at 70 eV and an ion source pressure of ! x 10m6torr are shown. The most obvious feature of these spectra is that F- is the most abundant ion, all other ions being of much lower intensity. The abundance of the other ions increases as the series is ascended, the ratio of F-/total ion abundance decreasing as follows: 0.97 (C,F,), 0.95 (C,F,), 0.87 (C,F,,& Of interest also is the gradually increasing abundance of the moleculeminus-F ion as the homologous series is ascended. This is made clear by some TABLE

2

NEGATIVE-ION

MASS SPECTRA

OF PERFLUOROEmE,

PERFLUOROPROPANE

AT70eVANDlX10-6tOrr

loll

CFCZCFCSFZC2FCF,C3FCd=zCFsCzF3CaFaCzFsCaF5C3F6-

Cd=7Cd%C&s CAC,F,o--

CZ6

1000 16 5 2 1

12

0.1

Cd=6

n-C&o

to.1 lGO0 0.1 0.2 to.1 0.1 0.1 to.1

to.1 1000 0.9 1.1 (0.1 0.7 2.1 0.7 CO.1 0.3 23 0.3 to.1 1.14 to.1 (0.1 0.1 8.5 0.2 1.3
to.1 22 (0.1 to.1 19 to.1 0.3

AKD PERFLUORO-Il-BUTANE

450 observations C3F,

-/F-

116x

lo-

in this laboratory

= 0.3 x 1O-3

(C,Fs);

which show CzFs-/FC,F,-/F-

=

1.3 x lO-3

=

0.1 x lo-

(C,F&;

3

(C,F,);

C6F13-/F-

=

3 (C6%)-

It is apparent that C-c bond cleavage OC-YESreadily and the formation in moderate aburdan,e of ions such as CF,-, C2F,- and C,F,suggests that these

species have quite high electron affinities. CF, - is not the most abundant perfluoroalkyl ion in all cases; this is in marked contra-distinction to the positive ion spectra of these molecules where, in each case, CF3 * is the most abundant ion. Appearance potelztials low electron energies the following ions were observed to be formed by dissociative attachment processes: F-, CF3-. C,F,-, C3F7-, C,F,- and C,Flo-. At

At their respective resonance peak maxima these ions are formed in the ratios: 1000:4:103:13:1: tl. Bibby and Carter6 observed only the first four ions. Reese et aL5 observed all except C,F,,- ; it will be noticed that this ion is formed in very low abundance. Typical plots of negative ion current as a function of electron energy before

and after deconvolution are shown in Figs. 1 and 2; the appearance potentials are given in Table 3. The most conspicuous feature of the diagrams is the superposition of the capture pe hl:s which onset at 2.1+0.2 eV and reach their maxima at 3.0+0.1 eV; this is also possibly the case for those peaks maximising at 4.4kO.2 eV_ These coincident resonances may be interpreted in terms of the multichannel decomposition of a metastable molecular negative-icn state at this ener,7, i.e.

C4F10 +e(2.1 eV) + [C,F,,

-*J --, F-,

CF,-,

C2F5-,

C3F7-.

etc;

The second set of coincident resonances onsetting at 3.8 f 0.2 eV may reflect the decomposition of a second such state at this energy. Similar suggestions of a multichannel decay of unstable excited states at - 6 eV have been made in the cases of

Electron

energy.

ev’ (corrected)

Fig. 1. Ionisation efficiency rxrves for n-C4Fxo . Experimental data for F- (crosses), CF3circks), C2Fs- (open circles), C3F,- (full triangles), C4F10- (open triangks).

(full

451

Fig. 2. Ionisation efficiency curves for n-C4F 1~. Deconvoluted data for F- (crosses), CFscircles), C2F5- (open circles), C3F7- (full triangles), C4FIo- [open triangles).

(full

tetrafiuoroethylene2 O and perfluoropropylenezl. Although a temporary negative ion state of C,Flo was detected at 0 eV and its autodetachment lifetime measured as described above, no such metastable parent ion was noted at 3-I or 3.8 eV; this failure to detect C,F,.-* suggests that the lifetimes of the unstable states at these energies must be < 2 psec. (i)

F-

It is apparent from Fig. 2 that two resonance processes are responsible for F- ion formation in the energy range below 6 eV, the first of which has the larger cross-section; only the lower energy process was observed by Bibby and Carter6. The C-F bond dissociation energies of a variety of molecules have been reported and many are in the 5.0-5.4 eV region. If reaction (4) is responsible for F- formation and D(F-C,F,) is -5-2 eV then, using the reported value of 3.4 eJ.’ for the TBLE

3

APPEARANCE

POTEhTIALS

(A),

RESONANCE

INTENSITIES

ZOII

A

n!f

F-

C&rC.&I -

2.0&0.3 3.8&O-2 2.3 fO.l 3.8iO.2 ,-dLO 2.1 io.1 3-S&0.2 2.1 f0.1 2.250.2

C4Fio-

O.Of0.1

0.9iO.l 4.4iO.I 0.910.1 3.210.1 4.660.1 1.2iO.f 5.8&-0.2 3.0FO.L 0.95-0.1 4.450.1 0.910.1 2.910.1 0.8+X1 -2.7 very low cross-section

CFJC2FS-

AT PEAK MAXIMA

PEAK MAXIMA

AND RELATIVE

2.9fO.l

(kf),

ANm PEAK HALF WIDTHS

(w)

(k

ev)

FOR THE NEG.4TIVE IONS FORMED BY PERFLUORO-Xl-BUTANE

W

Rel- Int.

PROCESS: C4Flo+e

1000 240 2 4 1 103 29 13 1
F- +CcF,

F-iC2F4+C2F5? CFS- -k&F, CFs- +CF3+C2F4 CFs-i-F-i-CsF6

CzFs - +C,Fs GFs--I-CF~+CFB CsF,- +CFB CbFs- 4-F GF~Q-

-s

452 electron af%inityof fluorine2’, we may estimate A(F-) - 1.8 eV. This is in reasonable accord with our experimental value of 2.OkO.l eV, and we therefore attribute F- ion formation, with little or no excess energy, to reaction (4). C,Fro-+ e + F-+CaF, + F-+F+C,F,

(4) (5)

(6) Reactions (5) and (6) ha ve estimated minimum enthalpy requirements of 3.5 and 3.2 eV respectively. These values are in fairly good agreement with our experimentally observed value of 3.8kO.2 eV. We cannot, however, decide which of these reactions is responsible for the increase in ion current noted at this energy. In general 3u.r studies of fluorocarbons have tended to suggest that C-C bond fission is preferred to C-F fission; we consider it likely therefore that reaction (6) occurs.

(ii> CF3Figure 1 shows that the CF, - ionisation efficiency curve is broad and there are indications of overlapping resonances; the deconvoluted curve reveals three resonance processes the second of which has the largest cross-section. C,F,,+e

+ CF,-+C,F,

(7)

CF3 - + CF, + C,F,

(8)

+ CF, - + CF2 + C,F,

(9)

+

--, CF, -+F+C3F6

(10)

If the initial onset at 2.3 +O.l eV corresponds to the occurrence of reaction (7) then, using the reported value of 2.0&O. 1 eV for the electron affinity of CF323, the CF,-C3F, bond dissociation energy may be estimated to be < 4.3 20.2 eV. This is in good agreement with values which have been determined for C-C bond energies of other perfluorocarbons by electron impact methods e.g. CF,-CF, d 4.2 eV’, CF,-C,F, G 4.4 eVzl. These values may be compared with values of - 3.7 eV for these C-C bond energies which may be calculated from thermochemical data24. It may be that there is some OS-O.7 eV release of excess energy during such reactions as (7) or that the thermochemical data are somewhat in error. Reactions (8) and (9) have minimum enthalpy requirements of 3.4 and 4.0 eV respectively; it is likely that reaction (8) is responsible for the ion formation process at 3.8 eV. The onset of the third resonance process is difficult to determine since it is a low inten&= process on the tail of the largest of the resonance peaks; the appearance potential must be less than 5.6 eV. (the position of the minimum between the two peaks); we estimate it as probably - 5.0 eV. Reaction (10) has an estimated appearance potential of 4.5 eV. It is possible therefore that either

453 reaction (9) or (10) may be responsible for the resonance prop-s; excess energy would be involved in the case of reaction (9).

some I eV of

(iii) C,F, -

If we assume that D(CF3-C3F,) N D(C2F,-C2F,) then, if the C,F,appearance potential at 2.1 +O.l eV corres_uondsto ion formation by reaction (1 l), the electron afhnity of the C,F, radical may be estimated to be < 2.2+0.3 cV. This may be compared to values of 2.1-2.4 eV which have been reported in the literature23. C4F10+e --, C2F,-+C,F,

01)

--, C2F, - + CF2 -!-CF,

(12)

--, C2F,-+FiC:F,

(13)

Reactions (12) and (13) have minimum enthalpy requirements of 3.8 and 4.6 eV respectively; it would therefore appear that reaction (12) is responsible for C2F,formation at 3.8 eV. (l-v) C3F7-A single sharp resonance peak onsetting at 2.15 0.1 eV is observed for this ion. If reactions (7) and (14) do not involve excess kinetic and/or excitation ener,oy tne expression A(CF,-)+E(CF,)

= A(C3F7-)+E(C3F7)

may be used to estimate that E(C,F,) = 2.2F0.2 eV. This may be compared with reported literature values which are in the range of 2.0-2.4 eVz3. C,F1,,+e

---, C,F,-+CF,

(14)

This ion was formed in very low abundance, the onset energy being 2.2 + 0.2 eV. if the ion is formed by reaction (15) (the complementary process to reaction (4)), we may estimate that E(C4F9) = 3.2i0.3 eV. C,Flote

---,C&F,-+F

(15)

The following values (in eV) for heats of formation have been used in our calculations: F = 0.8 (ref. 29); CF = 2.5 (ref. 25); CF? = - 1.8 &-O-l (ref. 25); CF3 = -4.9iO. 1 (ref. 26); C2Fz = - 6.750.1 (ref. 29); C?F, = - 9.2 (ref_ 37); C3F6= - 11.3(ref: 27); CF,CF= CFCF3 = - 16_6(ref.28), C,,F,,!‘= -21_9(ref. 27).

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2 3. C. J. T~yrrr~~ J. Phys. Chem,

Znt. .T. Mass Spectrom. ion Phys., 2 (1969) l_

73 (1969) 1586.

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J. L. FRANKLIN, J. G. DILLARD, H. M. ROSENSIOCK, J. T. HERRON, K. DRAXL _~ND F. H. FIELD, NSRDS-NBS, 26 (June 1969).

M. FARBER, M. A. FRISCH AND H. C. Ko. Trans. Faraday Sot., 65 (1969) 3202. 26 J. W. CLIMBER AND E. WHITTLE, Trans. Faraday Sot., 63 (1967) 1394. 27 W. P. D. BRY~, J. Polymer Sci., 56 (1962) 277. 28 S. W. BENSON AX? H. E. O‘NEAL, NSRDS-NBS 21 (Feb. 1970). 29 JANAF Themochemicai Tables, Dow Chemical Co., Midland, Michigan, 1961. 25