Dissociative electron capture and dissociative ionization in perfluoropropane

Menrational Journal of Mass Spectrometry and Zon Physics

EisevierPublishingCompany,Amsterdam- Printedin the Netherlands

DISSOCIATIVE IONIZATION C. LlFSHlTZ

ELECTRON

CAPTURE

AND

211

DISSOCIATIVE

IN PERFLUOROPROPANE AND R. GRAJOWER

Department of Physica! Chemisfry, Hebrew Unicersity of Je.-usalem (Israel)

(Received April 16th,1969)

ABSTRACT

Ionization eficiency curves for positive ions and electron capture curves for negative ions in C,F, were determined by the retarding potential difference method. The appearance potentials of CF,+, CtF,*, C,F,+ and F- are in good agreement with computed values based on C-C and C-F bond energies and on the ionization potentials and electron affinities of the radicals involved. The eIectron affinities obtained for CF3, C,F, and C,F, were 2.0, 2.3 and 2.4 eV, respectively.

INTRODUCTION

The behaviour of perfluoropropane under electron impact has been studied by severai investigators’ - 3. There is, however, considerable disagreement among the various groups concerning the appearance potentials of positive, as well as of negative ions. The appearance potentials of the ‘positive ions may be correlated with estimated C-C and C-F bond energies4 and with the ionization potentials of the fluorocarbon radicals5y6. Negative ion appearance potential data may be employed to estimate C-F bond dissociation energies’. Appearance potentials of negative ions from C,F, have recently been employed to compute eIectron affinities of the fluorocarbon radicals, CF, and C,Fs8. These values are, therefore, of considerable interest. The behaviour under electron impact of C,F, is of further interest since, as is wel! known, neither the positive nor the negative parent ion is observedThe results which we present, were obtained by the RPD (Retarding Potential Difference) methodg, which gives a well defmed electron energy distribution. They are complicated however by the fact that perfluoropropane contains a considerable amount of internal vibrational energy at the normal operating temperatures of the mass spectrometerz the possible efI&t -of ~which will be disCussed.

ht. 3. Mass Spectrum. Ion Pkys., 3 (1969) 211-219

C. LIFSHITZ,

R. GRAJOWER

Fig_ 1. Ionization e‘5cicncy cu-rvefor Arc by the RPD method according to Fox et aLs.

ELEtiROF: ENERGYeV Fig. 2. Negative ion ~k-liierencccurrent for CFz- (C,F,) and SF,- (SF,) as a function of the electron ezkgy in a mkture of CzFs2 Nz and SF6 (at 1.0.3.7 and 02 mm Hg, r&pe&ively. in the inkqspstem). A.

DISSOCIATIVE

IONIZATION

IN

213

F’ERFLUOROPROPANE

The mass spectrometer used was the Atlas CH4 60” sector instrument with its “Fox” ion-source as described by Nounou lo. The choice of optimum operating conditions, to prevent “relaxation” of the electron energy dis:ribution, has been

described previously’l.

The energy scale for the positive ions was calibrated by

the Ar ionization e&iency curve (Fig. 1). The zero .of the enera scale Gor the negative ions was determined by either the SF6 or the nitrobenzenex2*13 electron capture curves. The ener,oy distribution (width of the SF,- curve at half height) was CL1 eV. The linearity of the enera scale has been questioned’“; we have looked into this by measuring the appearance potentials of the negative ions in a mixture of nitrogen, SF, and C,F,. Conditions were employed so as to produce the correct value’ ’ of 11.5 eV for the difference in energy between the maxima of the SF,peak at zero energy and the one due to the following reaction sequence:

lx

I

1

i

. CF; tC,F,l

.

Fox method

. _

. I

Fig. 3. Ionization

efficiency curve for CFJ’

(CsF$

by &he RPD

Znt. J_ Af-

method

according

Specrrom. Zen Ph&3

to Fox et aL9.

(1969) 21 i-213

214

C. LIFSHITZ,

N2

+erat

ethemnl+

-+

R. GRAJOWER

~~(C3~ul~fethtmO~

SF, --, SF,-.

(0

An example for the CF,-

ion is shown in Fig. 2. A Cary model 31 vibrating reed electrometer was connected to the secondary electron multiplier to measure ion currents. Electron capture curves of low-yield negative ions. were determined by connecting a suitable condenser to the reed and employiig the rate-of-charge method. The perfiuoropropane used was from Matheson with a stated minimum purity of 99.0 “/,.

RESULTS A?iD DISCWSSXOX (a)

Posititie

ions

The ionization efkiency curve for CF3+ ob%ned by the Fox method9 is shown in Fig, 3, The ionization efhciency curves for all the major positive ions from C3Fs obtained by the method of Cloutier and !WIZ’~ are shown in Fig. 4. The two methods gave similar resuits. The curves demonstrate Iong “tails” and the onset energies for all the .fragment.sare very much !ower than all the previously published electron impact v&es. Part of the tail might be due to “hot bands”_ The vibrational energy distzibutions of C,F, at 300” K (room temperature) and

Fig. 4. Ionization efikkncy curv~es for the rnaiwrposi;ive ions from CSFs by the R4D method, zwrding to Cfoutier and SchiE16. The zero of the difference current scale for CYa* is shifted upwards for $arity. &U. i, &iii

&GWO~.

IGfX

Phys.,,

3 (1969) 211-219

DISSOCIATIVE

IONIZATION

215

IN PERFLUOROPROPANE

VIBRATiONAL

ENERGY

CV

Fig. 5. Computed internal vibrational ener,q distribution for neutral C3F8 molecules. TABLE

1

APPEARANCE KOR

CFJf CzF*+ CzFs+ C&T’

POTEXEUS

FOX POSiTIVE IONS (ev)

Ekperirnentai onsef energies

Compured Ref-19’

Ref. 20 E RefT

A.P.

14.4

13.61+_0.04

13.22 13.32

14.6 13.6(?) 14.7

13.310.1 11.7 10.7 14.0~0.1

15.4

15.9

15.4$0.1

Ref.

13.410.1 i3.5iO.l 13.910.1

14.65 15.25

15.3

14.13io.10

15.7+.0_1

16.5

17.1

15.88~0_10

s RPD method. b Ordinary e!ectroc

I ’

Ref.3’

Present results -

impact.

c P~otoionization. Inr_ J_ _Mass Spectrom.

Zoon F&s.,

3 (1969)

211-219

216

C. LIFSHITZ,

500” K (the normal operating temperature of the ion-source)

R. GRAJOWER

are shown in Fig_ 5,

from the known vibrational frequencies of C,Fs”, using the approximation formula for the density of vibrational states due to W-bitten and Rabinovitch”. It is difficuh to estimate the contribution of the vibrational energy of the neutral molecule to the decomposition reactions of the moIecule-ion. Since no parent ion is observed, the decompositions might take Flace by direct transitions to repulsive states of the molecule-ion, in which case only part of the thermal vibrational tiner&eyof the neutral molecule will contribute_ The “onset” energies for positive ions are compared with literature values in Table 1. Each of the v.aIues is an average obtained from several ionization efficiency cures of the fragment ion involved- There is fairly good agreement between the present values and those of Greenhaus”, who employed the RPD method, and with recent photo-ionization values”‘. Appearan~z potentials were computed for the processes which are probably involved in forming the ions. These calculations are based on unexcited reactants and products. IcZnown and estimated heats of formation of fluorocarbons” w-ere employed as w&i as estimated bond energiess*21 and known ionization potentials. For example thz appearance potential of CF3’ can be computed for the process: These were computed

e-!-&F,

---,CFsi

+C,F,tZe

(2)

according to: AJ?(CF3’)

= fi(C-C)+LP.(CF,)

(3)

We estimated the carbon-carbon bond energy [D(C-C)] in perfluoropropane to be 93 kcai/moIe iby analogy with the carbon-bon bond energy in perfluoroethane 21 (the C-C bond ener_gy in C,F, should be a few kcal higher than I)(CF,-C,H,> = 89-5 kcal/moIe”)_ The value used for the ionization potential of CF, fLP.(CF,) = 9.25 eV] is the adiabatic ionization potential of the CF3 radica16. Surprisingly, there is r^airlygood awment between the present experimental onset energies and the computed appearance potentials, for CiTst, C2Fs+ and &F,? The C2Fa+ ion is however formed by the process: e-+&F,

--* C;F,‘fCF,-t2e

(4)

with a considerable amount of excess ener,oy_This might be the case if the minimum energy necessary for ionization of C,Fs is 13.4tO.l GV.

We have observed the folIowing negative ions at low energies: F-, Fz-, CF,-, C+F,--, CIF,- and C,F,-. Despite the search for it, the C,F,ion has not been observed previously 1g. The maximum intensity of C,F, - is in fact smaller by a
Mass

Spec&z.-ion

P&s.,

3 -(l%?)

21 l-219

DISSOCIATIVE

IONIZATION I

I

217

IN PERFLUOROPROPANE I

eV

Fig. 6. Dissociative electron capture curyes for the major negative ions from C,Fs.

5

Fig. 7. De&&d

onset region of F-(C3Fs)

curve on a semi-Iogatithmic scale. Int. 1. Mass Spectrom. Ion Phys., 3 (1969) 211-219

C. LIFSHITZ,

218

R. GR AJOWER

F-, CF,-, C,F,and CsF,- o bt ained by the RPD method are given in Fig. 6. The question of the contribution of the internal thermal energy of the neutral molecule to the onset energies rises again. The onset region of each of these ions was determined carefully and the one for I?- is shown on a semilogarithmic scale in Fig. 7. A linear section can be drawn through the points in the onset region of Fig. 7 and the departure from linearity obtained this way in a number of experiments for F- gave 1.35f0.1 eV. The appearance potentials for the major negative ions are compared with previous literature vahres in Table 2. The present values are somewhat iower than previous results. The experimental value for F- is however in very good apement with the value calculated for the following process: e + C,F, TABLE

+ i-C3F, ;F-

2

APPE4RAXCE

POTi2xiAm

FOR NECATWE

Ion

Presenr rest&s

FCPsC=F,-

1.3550.1 2.0 io.1 1.7 io.1 2.4 $0.1

C3F7-

(5)

a

IOSS

(ev)

Ref- I b

Refi 19”

Computed

E-8 2.2 2.1

1.63~0.09 2.07to.w 1.99&O-09

1.32

-= RPD method_ b Ordirtsry ekctron impact.

according to: A.P.(F-) = B[(CF,)LCF-F]-eIectron afhnity (Fj = 110.0/23.053 (ref 21)-3.448 (ref. 24) = 1.32 eV. Assuming that the other negative ions are formed at onset energies without excess ener_gy, one computes the following electron affinities: E(CF3) = 2.0 eV, E(C,F,) = 2.3 eV and EfCsF,) = 2.4 eV. The first two vahres are in good a,-ment with the ones obtained recentfy’ from appearance potentials of negative ions in C;F6_

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DISSOCIATIVE

IONIZATION

IN PERFLUOROPROPANE

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Negatice

Ian Formation

b-v Eiectron

Zmpaci

in some

993.

Perfluoroafkanes,

University Microfilms, Ann Arbor, Mich., 1965. 20 C. J. KOL’TXRY, Nazi. Bur. Std. J. Res., 72A (1968) 479. 21 J. HEUXXEX, Gas Phase Oxidation of Perhalocarbons, ir? Advances Interscience, New York. 22

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in Photochemistry,

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1540.

Specrrom.

Ion Phys-,

3

(1969) 211-219