K. A. G. MACNEIL AND J. C. J. THYNNE
2960 amount is very sensitive to the value of F assumed in obtaining the values of the equilibrium constants.
Acknowledgment. I am grateful to P. S. Hallman, D. D. Perrin, and D. Thacker for helpful discussions.
Negative Ion Formation in Carbon Disulfide and Carbonyl Sulfide by K. A. G. MacNeil and J. C. J. Thyme’ Chemistry Department, Edinburgh University, Edinburgh, Scotland
(Received February 17, 1069)
The formation of negative ions in carbon disulfide and carbonyl sulfide has been studied. At low electron energies ions of the formula 8,- (n 5 6) areformed,their formation is attributed to thermal decomposition of the sulfides on the filament. The principal ions in CSZ are S-,Sa-, C-, CS-,and CSZ-. In COS they are S-,SZ-, CS-,0-, and OS-. The onset and energy dependence of the ions from both systems are reported.
As part of a program concerned with negative ion formation at low electron energies2 we have examined carbonyl sulfide and carbon disulfide. Dillard and Franklin3 have studied negative ion-molecule reactions in these systems and Kraus4has reported the formation of the C-, S-, CS-, and CS2- ions in carbon disulfide. Experimental Section The mass spectra were obtained using a Bendix time-of-flight mass spectrometer, Model 3015. The electron energy was measured using a Solatron digital voltmeter, LM 1619. Use of two channels of the mass spectrometer analog output allowed two mass peaks (e.g., O-:SOz and S-:CS2) to be measured simultaneously on l-mV Kent potentiometric recorders. This was of especial value in energy-scale calibration procedures since no switching between peaks was necessary. The electron current was kept constant over the energy range studied; a very small electron current was used (average value -0.0025 PA) in order that the filament temperature be kept low so as to minimize effects due to thermal decomposition of the substances studied on the filament. Ion-source pressures were mm. usually about 5 X The ionization curves for the ions were normally measured between five and ten times. The electron energy scale was calibrated using the 0- ions from SO2 and also, for some of the early work on CSn, using the 0- ion from CO. Results and Discussion I n both carbon disulfide and carbonyl sulfide we observed that polysulfur ions S,- (n 5 6) were formed, particularly in the O-l-eV electron-energy region. The Journal of Physical Chemistry
Dillard and FranklinS have reported such ions to be formed by consecutive ion-molecule reactions, e.g.
s- + cos S2-
--t s2-
+ co
+ COS -+ Sa- + CO, etc.
Under our experimental conditions, where low ionmm), it is very source pressures are used (-5 X unlikely that such reactions contribute appreciably toward the formation of S-, ions where n > 2. We consider that, in our system, such ions arise via the thermal decomposition of the CS2 and COS on the tungsten filament. We have also observed, in the same energy region, ions such as C-, CS-, and CSZ- (from CS,) and C-, 0-, CS-, and OS- (from COS). No ions such as S7- and Ss- were found although we could have detected them at Se-:S7- ratios of -100. Our experimental setup requires, for constant electron current a t low energies, that the filament temperature increase; this causes more noticeable thermal decomposition, and hence S,- ion formation, a t low energies. It is likely, however, that some “background” ion formation is occurring a t all energies as evidenced by the fact that the ion currents never decrease completely t o zero. ( a ) 0- Ion Formation b y Sulfur Dioxide. Negative ion formation in sulfur dioxide has been examined by several workersa-6 and the appearance potential of (1) Address communications to Dr. J. C. J. Thynne, Chemistry Department, Edinburgh University, West Mains Road, Edinburgh, EH9 3JJ,Scotland. (2) (a) K.A. G. MacNeil and J. C. J. Thynne, Int. J . Mass Spectrom., 2, 1 (1969); (b) J. C. J. Thynne, Chem. Commun., 1075 (1968). (3) J. G.Dillard and J. L. Franklin, J . Chem. Phys., 48,2349 (1968). (4) K.Kraus, 2.Naturforsch., Ma, 1378 (1961).
NEGATIVE IONFORMATION IN CARBON DISULFIDE AND CARBONYL SULFIDE
2961
(i) CSZ-. A typical ionization curve for this ion is shown in Figure 2d. Kraus4 has suggested that a charge-transfer reaction involving CS - is responsible for ion formation
cs- + csz
4
3
5
I
I
I
I
6
7
8
9
10
Electron energy,V
Figure 1. Ionization efficiency curve for 0-ion formation by sulfur dioxide.
a
CSz
+e
---t
b
0-0
csz- + cs
Our experimental data for these two ions (Figure 2d) show them to have very similar energy dependences, both resonance peaks reaching their maxima a t 6.3 eV. This would appear to support such a charge-transfer reaction; however, examination of the ionization curves for C-, S-, Cs-, and Sz- [Figure 2a-d] indicates that all of the ions have onset energies at about 5.2 eV and reach maxima around 6.5 eV. This suggests that the ions have a common origin, and we tentatively conclude that their formation is the result of decomposition of CS2- formed in an excited metastable state in this energy region, the relevant reactions being
I
0
-
+S +c- + sz +8- + cs +sz- + c
(CS2-)* +CS-
(1)
(2) (3)
(4)
We have examined the pressure dependence of the v;
.-C
1
S-, CS-, and S2- ion currents a t 6.3 eV, where the CS2ion formation is a t a maximum. The CS- dependence
5c
5
.-e gf c
0
e
E
a 5
0 C
1oc
0
O O 0
0
5c
CpOO
C
5
6
7
8
5
6
7
8
9
1
0
Electron energy,V
Figure 2. Ionization efficiency curves for carbon disulfide: (a) S-; (b) SZ-; (c) C-; (d) CS-, open circles; CSZ-, full circles.
the 0- ion is sufficiently well established to be used to calibrate the energy scale. Our data are shown in Figure 1; both the onset energy and the resonance peak maximum were used as calibration points; our values indicate a difference of 0.80 eV between these energies, in agreement with previously reported values.af4 (b) I o n Formation in Carbon Disulfide. The relative intensities of the ions observed, a t 6 and 45 eV, respectively, are shown below. eV CScs Sa csz6 45
