NOTES
3!305
Table I : Liqutd-Liquid Solubilities,
2, and
Mole Fractions in Phases A and B a t 25’
7
1.
cc14
2.
GFie
1. csz 2. GFie
~
F _ -
ZA
QA
ZA
0 ,3026’0 0.698
0.157 0.843
0.140 Q ,860
0.042 0.958
-
0,0299 -1 .ID0 5 . 4 x 10-4
Experimental n-Perfluoroheptane was purified by the method described by (>lewand Reeves. Carbon tetrachloride and carbon disulfide were Spectro-quality reagents. They were distilled shortly before the experiments. The solubility of CiF16 in CS2 and in CCl, is very small, so we applied the method devised by Reeves and Hildebrand3 that measures the diminution in volume of the fluorocarbon after equilibrating with the other liquid, taking advantage of the fact that the latter wets glass preferentially. Calibrated tubes were sealed to bulbs of about 130cc. capacity. The volumes of these tubes, chosen after preliminary experiments, were 0.45 cc. for the CElz system and 2 cc. for the CC1, system. Measured volumes of the components were introduced into their respective vest,els, which were then sealed and rocked in a thermostat. After saturation was reached, thle remaining ciF16 was collected in the calibrated side tube and its volume read. Corrections were made for the CC14and CS2dissolved in the C7Fl6. Concordant figures for the solubility of CC14 in CiF16 were obtained by Hildebrand, Fisher, and Benesi4 and by Kyle and Reed.5
Results and Discussion Our results, together with those on CCl, in CiFltr from ref. 5 arid 6, are given in Table I, together with values of the difference between the solubility parameters that are calculated from the measurements by the equation used by Fujirihiro and Hildebrand 2
v1
x1A
VZ
=
62)’
(1)
The molal volumes of the components a t 25’ are: 225.6 cc. Equation CC14,97.1CC.;(282,60.7 cc.; C~FIB, 1 is obtained by combining the standard equation
RT In (azlxz)
=
v2(p12(61 - 6z)2
0O ,. 0 6 7
0’gg8] 0.002
61
-
8a
6n
61
3.08
8.6
5.5
4.48
10.0
5.5
-
The concordant values for 62 = 5.5 for CiF16 agree well within the usual limit of accuracy with the value 5.6 obtained for the system Brz CiF16 by Reeves and Hildebrand,3 calculated from the solubility of 13r2 in C7F16. These values of the solubility parameter of C7F16 are distinctly less than the value, 5.85, derivled from its energy of vaporization, a difference not unusual among systems departing appreciably from the simple model used in deriving eq. 2. However, the value here calculated agrees well with that found from the solubility of iodine, where & - 61 = 8.6, which with the standard value for I,, 14.1, gives 61 = 5.5. In this case, as in many others, a solubility parameter determined directly from solubility data is to be preferred, for practical purposes, over one based upon the heat of vaporization.6
+
Acknowledgment. This work has been supported by the Atomic Energy Commission. ~
-
~~
(2) D. N. Glew and L. W . Reeves, J . P h y s . Chem., 6 0 , 615 (1956). (3) L. W. Reeves and J. H . Hildebrand, ibid., 60, 949 (1956). (4) J. H. Hildebrand, B. B. Fisher, and H. A. Benesi, J . Am. Chem: Soc., 72, 4348 (1950). (5) B. B. Kyle and T. M. Reed, 111, ibid., 80, 6170 (1958). (6) For other examples, see R. L. Scott and J. H. Hildebrand, “Regular Solutions,” Prentice-Hall, Englewood Cliffs, N. J., 1962, pp. 145-147.
The Production of Molecular Beams in the Mass Spectrometer1
by Benjamin P. Burtt and Jay n!L Henis
xx2A) 2B ((OlB - (plA)(81 -
(PA
0.970
-
RT(LlnZ’Ii”++In-
B-
(2) with the corresponding equation for the other component.
Department of Chemistry, Syracuse University, Syracuse, >Vew York 18310 (Received J u l y 17, 1964)
In the course of studies of negative ion-molecule reactions in N20 and CClBF a t relatively high pressure in the mass spectrometer, evidence has been obtained to indicate that many of the accelerated negative ions lose their charge by collision in the analyzer tube. V o l u m e 68, N u m b e r 19
December, 1964
NOTES
3906
The beam of particles impinging upon the collector plate contains more neutral a t o m than negative ions. The phenomenon of electron loss is not ne\\', for Thomson2 was one of the first to clarify some confusing observations made with early cathode ray tubes where the vacuum was poor. He showed that the charges on high-energy ions changed during their passage through the gas. Physicists, using special equipment, have studied the charge-changing cross section using positive or negative ions of various types with energies of from 0.2 to 8000 kev. An excellent review is available by Allison and Garcia-Xunoz. However, the phenomenon should be called to the attention of those chemists who use mass spectrometers a t relatively high pressure for the study of ion-molecule reactions (particularly where reactions of negative ions are concerned). Since most modern spectrometers have differential pumping and the ions make no collisions in the analyzer tube, and since they usually are equipped with secondary electron repeller plates at the collector end of the tube, the phenomenon would not normally be observed. At pressures of the order of 0.3 mm. in the ion source, the C1- peak in CC13F, for example, showed a valley on either side that dipped below zero current (ie., as if positive ions were being collected or as if the number of secondary electrons being ejected from the collector plate was greater than the number of C1- ions striking it). Curves of the form shown in Fig. 1 were obtained for 0 - from N 2 0 as well as for F- and C1- from CC13F. As the pressure is raised, many C1- ions, after passing through the magnetic field, lose their charge on collision but are scattered only slightly from the original path (those that lose charge before reaching the magnetic field are not detected). These neutral particles with energies of ,500 to 1000 v. eject secondary electrons from the grounded collector plate producing an apparent positive current. Application of a negative potential of 20 v. to the collector slit with respect to ground eliminates the effect and produces a normal peak with an intensity as much as 1000 times that obtained without the small electrostatic field. That the secondary electrons are not all produced upon collision of C1- ions with the collector plate and that C1 atoms must be present are clear from the following experiment. At low pressure, when the Clmakes no collisions in the analyzer tube, the ion current increases by about 28Oj, to a saturation value as the bias voltage on the collector slit is increased up to 20 v. I n other words, 100 C1- ions striking the collector plate produce about 22 secondary electrons. T h e Journal of Physical Chemistry
-.."c m
c
a a
0,
c o f! Z m
3.0
-
2.0.
c2
L 40
30
40
m/e P*l7mm. Biae=Ov.
30
m/e P=l4mm Blas=Ov.
P=lOmm. Bias=Ov.
P=lOmm. Bias = 2 O v .
Figure 1. Ion current for the C1- peak in CClaF a8 a function of reservoir pressure. I n graph Cg the collector slit has a negative bias of 20 v. with respect t o the collector plate. Note the change in scale in CZ.
C1+ ions are equally effective in producing secondary electrons (in that case the current decreases as the bias voltage is applied). At a high pressure in the analyzer tube (0.010 mni.), there will be almost no observed C1- peak. However, when the collector slit is biased to 20 v., a large peak is obtained and the increase in ion current cannot be accounted for by secondaries from the C1- ions alone. However, the increase is explained if the bias voltage collects secondary electrons produced by C1 atom collision with the plate. Assuming that the C1 produces the same number of secondary electrons as the C1- and Clf are observed to produce, it is possible to calculate the initial number of C1- ions that survive their passage through the magnetic field and the number of C1 atoms and C1- ions that eventually arrive at the plate. From these data, one can calculate the cross section u in the usual manner I = Ioe-""P" where N is molecules/cc. a t 1 mm., p is pressure in mm., 2 is the path length, and I and I , are, respectively, the ion current reaching the plate and the ion current entering the analyzer tube after passing through the magnetic field. For example, in CC1,F the electron detachment cross ~~
( 1 ) Supported in part by the U. S. Atomic Energy Commission.
(2) J. J. Thomson, "Rays of Positive Electricity," Longmans, Green and Co., London, 1921, pp. 134-142. (3) S. K. Allison and M . Garcia-Munoz, "Atomic and Molecular Processes," D. R. Bates, Ed., Academic Press, New York, N. Y . , 1962, pp. 722-782.
NOTES
section a t an ion energy of 1.0 ke.v. for F- is 2.5 X lo-’’ cm.2 while that for C1- is 5.5 X lo-’’ cm.2. These are rough values since the pressure of CC&F in the analyzcr tube was known only approximately. The ratio of the two cross sections, 2.2, has greater validity and it can be compared to the ratio of 2.0 for C1- and F- in k r y p t ~ n . ~ The cross section for electron detachment for 0- in KzO is found to be about 2.2 X 1O-I’ cm.2. Of the three ions at 1.0 kev., 0- was most effective in producing secondary electrons, possibly because of its higher velocity. One 0- ion produced 0.55 electron, on the average; one F- produced 0.46, and one C1produced 0.22. Kerwin and hIcGowan5 have pointed out how a mass spectrometer can be used to study charge exchange of positive ions a t high pressure, but they did not use the secondary electron emission as a tool. With equipment designed specifically for this purpose, it should be possible to use this method to obtain accura,te values for cross sections for charge exchange. Electron loss from negative ions produces a beam of neutral particles which might also be useful to studies of the chemistry of high energy molecular beams using the proper negative ion as a precursor. Equipment could be patterned after some of that used for the charge-changing cross section measurements described by A l l i ~ o n but , ~ ions could be fed into it from a masa spectrometer. In this way, the proper ion can he selected and a gas may be used that is easy to handle. For example, the CC13F is easier to use as a source of C1- or F-- than would be Clz or F2 or the corresponding hydrogen halides. Kegative ions would seem preferable to positive ions for the production of inolecular beanis for two reasons. First, the production of a neutral atom from a positive ion is a resonant process, and the electron must be captured into a particular state of the atom. The stripping of an electron from a negative ion should be more probable. Secondly, many negative ions can be produced in large amounts by very low energy electrons. Only one or perhaps two types of ions will be produced, and these usually are in their ground state. To produce positive ions, greater ionizing energies are required, and a multitude of positive fragments is produced. The latter may have to be separated out, and the excited states of some ions may introduce complications. I n CC13F, for example, C1- and F- are produced by a resonant process a t 2.5 and 5.0 e.v., respectively.6 0- is produced efficiently from K 2 0a t 2.2 e.v.’
3907
Acknowledgment. The writers are indebted to Professor Herbert Berry of the physics department of Syracuse University for his helpful comments and suggestions.
-
(4) 3. B. Hasted, Proc. Rou. SOC.(London), A212, 235 (1952). (5) L. Kerwin and W. McGowan, Can. J . Phgs., 41, 316 (1963). (6) Unpublished work from this laboratory; manuscript in prepa~a-
tion. (7) R. K . Curran and IR. E. Fox, J . Chem. Phys., 34, 1590 (1961).
Paramagnetic Resonance Study of Fermi Level Motion and Defect Formation in HighResistivity Cadmium Sulfide Crystals
by G. A. Somorjai and R. S. Title ZBiM Watson Research Center, Yorktown Heights, New York (Received August 97,1964)
In this Note we wish to report how paramagnetic resonance measurements may be used to monitor Fermi level motion and the kinetics of defect formation in high-resistivity crystals and illustrate the technique with measurements on high-resistivity, sulfur-dopcd CdS crystals. Electrical measurements in crystals with resistivities greater than about los ohm-em. are difficult to make because of the low current levels involved. Changes in the resistivity, however, imply a change in the position of the Fermi level that can drastically affect other physical and chemical properties such as photoconductivity’ or evaporation. A high resistivity implies that the Fermi level IS close to the center of the band gap. A defect whose energy is near the center of the band gap will gain or lose electrons as the Fermi level passes through the level. If any of the charged states of the defect are paramagnetic, e.p.r. techniques can be used to monitor the motion of the Fermi level in the vicinity of the defect. We have found iron to be a suitable defect to monitor the motion of the Fermi level in sulfur-doped, high-resistivity CdS. Iron is present in the crystals (supplied by Eagle-Picher Co. , Miami, Okla.) wle studied in concentrations of rv5 X 1016/cm.3. In untreated crystals no paramagnetic resonaiice spectrum was observed a t 77°K. In crystals fired in a sulfur ~~~
(1) K. H. Bube, “Photoconductivity of Solids,” John Wiley and Sons, Inc., New York, N. Y., 1960. (2) G. A. Soniorjai, Proceedings of the International Conference or1 the Physics and Chemistry of Solid Surfaces, Providence, R. I., 1964.
Volume 68, Number 1.2 December, 1964.
