NEGATIVE IONFORMATION BY BFa
AND
2257
PF3
Negative Ion Formation by Boron Trifluoride and Phosphorus Trifluoride by K. A. G. MacNeil and J. C. J. Thynnel Chemistry Department, Edinburgh University, Edinburgh, Scotland
(Received October 1, 1969)
Negative ion formation as a result of the electron bombardment of boron trifluoride and phosphorus trifluoride has been studied as a function of the electron energy. Vertical onset behavior is a feature of the ionization processes for several ions. A value of 1.9 x 10-9 molecule-1 cma sec-1 has been estimated for the rate constant F, and a value of - 15.5 eV has been estimated for the of the ion-molecule reaction Fz- BF, + BFdheat of formation of the BFI- ion.
+
As part of a continuing program2 concerned with negative ion formation as the result of electron bombardment of molecules, we have studied boron trifluoride and phosphorus trifluoride. Boron trifluoride has been studied previously3 but only the F- ion was identified. Preliminary studies of B B and PF3 suggested that the ionization eficiency curves for several of the negative ions formed showed near-vertical onsets, the ion currents rising almost to a maximum value a t the threshold. Such behavior has been noted for the 0- ion formed by CO but not for ions formed by polyatomic molecule^.^ I n electron impact studies, when the electrons are emitted from a heated filament, because of the energy spread of the electron beam uncertainties arise in the determination of the appearance potentials of ions. This is in part due to the smearing-out effect of the high-energy tail of the electron energy distribution. Analytical methods have been developed to reduce this effect for positive5seand negative? ions, and we have applied this technique to the ions formed in this study.
Experimental Section The experiments were performed using a Bendix time-of-flight mass spectrometer, Model 3015. The pressure in the ion source was usually maintained below mm except when secondary ion formation 5 X was studied, in which case pressures of up to 8 X mm were used. The energy of the ionizing electrons was read on a digital voltmeter, and the spectra were recorded on two l-mV potentiometric recorders. The electron current was maintained constant by automatic regulation over the whole energy range studied. Ionization curves were usually measured five times, the appearance potentials being reproducible to *0.1 eV. The appearance potential of the 0ion from CO was used as the reference for energy scale calibration since it has an appearance potential4 (9.65 eV) which is close to the values obtained in this work for several ions. The experimental data were treated by the deconvolution method described p r e v i ~ u s l y ;the ~ electron energy distribution (which was required to be known
+
which would have a minimum appearance potential of 10.88 eV. If the first resonance peak is assumed to tail off steadily to 12 eV, we may make a rough estimate that the second process is about 0.05 times as intense as the first. Chantry4 reports that q / a Z = 21, where u1 and u2 are the cross sections for reactions 1 and 2. ( b ) Boron TriJIuoride. The following negative ions were observed: F-, Fz-, BFz-, and BF4-. Examination of the dependence of ion current upon ion source pressure showed that BF4- was a secondary ion and the other ions were primary. At their capture peak maxima the primary ions have the following relative intensities F-:Fz-:BFz-
=
1000:27:-3.3 eV, no ions are observed until -10 eV. Also, the ionization curves for F- and Fz- rise sharply from their thresholds. Near-vertical onset behavior has been noted for the 0- ions formed by GO4 and K0l1 The Journal of Physical Chemistry, Vol. 743No. 11, 1970
.--
0
A
A
0 C
0
a
1
,
,
0.:
AA
9.0
9.5 10.0 ”9:;
915 160 165
1110
Electron energy scale, eV, uncorrected.
Figure 2. Comparison of the initial portions of the ionization efficiency curves for O-/CO (open circles) and Fz-/BF3 (open triangles) and for F-/BFa (full triangles). Energy scales for Fz- and F- shifted by -0.85 and -1.05 eV, respectively, to permit overlap of appearance potentials.
It is apparent that, for the Fz-and 0- ions, complete overlap of the energy profiles for both ions is obtained when the energy scale is shifted slightly (-1 eV). The F- ion curve however, although it rises quite rapidly from onset to a maximum value, does so rather slower than the corresponding rise for the 0- ion (1.3 eV cf. 0.7 eV), and there is no close overlap of the ionization curves. We therefore consider it likely that the ionization curve for Fz-, but probably not for F-, has a vertical onset edge. (1) F-. A typical ionization efficiency curve for this ion before and after performing 15 smoothing and 20 unfolding iterations is shown in Figures 3a and b. A value of 10.7 f 0.1 eV is obtained for the appearance potential of the ion; this may be compared with the value of 11.4 f 0.2 eV reported for A(F-) by Marriott and Craggs. Possible reactions to explain ion formation are
NEGATIVE IONFORMATION BY BFa BF3
+e
AND
2259
PFa
+ BFz --% F- + B F + F F-
Using known thermochemical data (see section d), together with a value of 3.45 eV for the electron affinity of fluorine, E(F),12 then the minimum enthalpy 0.2 and requirements for reactions 3 and 4 are 3.3 7.9 i 0.2 eV, respectively. It is therefore apparent that reactions 3 and 4 would have 7.4 0.3 and f 0.3 eV, respectively, of kinetic and/or excitation 2.8 energy distributed among the fragments. I n view of these excess energies it would seem more reasonable (since D(BF-F) < 7.4 eV) to suggest that F- ion formation occurs via reaction 4 but, for reasons to be mentioned in the section below on Fz- ion formation, reaction 3 is not completely ruled out. (6) Fz-. Typical data for this ion are shown in Figures 3a and b. A vertical threshold is obtained a t 10.5 f 0.1 eV, and ion formation is assigned to the reaction
* *
BF3
+ e A F2- + B F
If a value of 3.0 eV is used for E(F2),l8then the minimum appearance potential for reaction 5 is 6.7 f. 0.2 eV; this suggests that about 3.8 i 0.3 eV of excess energy is associated with reaction 5 although we are not able to assign it to kinetic or excitation energy. I n Figure 3b, the unfolded F2- ionization curve shows b
. e
50
e
I
I
.
*' *
I
*
an indication of a second ionization process on the tail of the first; this process has a maximum value a t about 11.5 eV. It is unlikely that reaction 6 is responsible since this would require an appearance potential of -15 eV. BFa
+e
F2-
+B +F
Examination of the pressure dependence of F2- ion formation indicated that a t the first peak maximum (10.5 eV) the ion was formed by a primary process. At 11.5 eV however the pressure exponent was 1.7 0.1, suggesting that a secondary process was contributing towards ion formation in the region of the second peak. This, together with the similarity of the peak maximum energies for F- and the second F2- peak, suggested the possibility of an ion-molecule reaction with F- as the reactant ion, e.g.
*
F-
+ BFa
Fz-
+ BF2 *
Reaction 7 is endothermic by 5.6 0.2 eV; if reaction 3 is responsible for F- ion formation and the 7.4 & 0.3-eV excess energy is all in the form of kinetic energy, then 5.4 0.2 eV of this energy may be partitioned to the F- ion. It therefore appears that reaction 7 is possible; however, it is still an unlikely reaction since, in actual fact, about 2 eV more energy is required in the center-of-mass for the reaction to become thermoneutral. We are therefore unable to suggest a likely process for the F2- ion formation observed a t 11.5 eV. (9) BF2-. Basic and deconvoluted data for this ion are shown in Figures 4a and b; the quality of the data are not good because of the extremely low ion intensity and the structure on the leading edge of the curve was not consistently reproducible. It is apparent that ion formation occurs a t -7.6 eV, the ion current increasing slowly to reach a maximum a t 11.5 eV; beyond this energy the decrease in ion current is very sharp being almost vertical. Thermochemical calculations show that the minimum energy for reaction 8, i.e., where the electron is not captured but causes dissociation of the molecule, to be
*
BF3
+ e-%-
BF + 2 F
+e
*
X
O'
*.
'.
4 l'd; -5; 1'2 1; Electron energy, eV, corrected. *-*
Figure 3.(a) Ionization efficiency curves for BFs. (b) Deconvoluted curves for BFs; F- (crosses), Fz-(full circles).
114
11.4 0.2 eV. We have not observed such a sharp decrease in ion formation in any other system we have studied. We tentatively suggest that, a t 11.5 eV, the potential energy curves corresponding to reactions 7 and 8 cross and, above this energy, transitions on to the surface leading to decomposition of the molecule and not ion formation are favored. (4) BFd-. This ion is a secondary species. I n Figure 5 we have compared the normalized ionization (12) R. S. Berry and C. W. Riemann, J . Chem. Phys., 38, 1540 (1963). (13) R. M. Reese, V. H. Dibeler, and J. L. Franklin, ibid., 29, 880 (1958). The Journal of Physical Chemistry, Vol. 7.4) No. 1 1 , 1970
2260
K , A. G. MACNEIL AND J. C. J. THYNNE
.
N '
