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Ultra-ionization potentials of mercury
ULTRA-IONIZATION
POTENTIALS
OF MERCURY.
BY
ERNEST ORLANDO LAWRENCE, Ph.D. National Research Fellow. INTRODUCTION.
I3r 13OMBarcmXG mercury vapor with electrons of definite velocities--produced by magnetic analysis of the electrons from a thermionic source--it has recently been shown that there are a succession of distinct types of electron inelastic impacts in mercury vapor above the ionization potential having the following critical potentials: Io.6, II.29, I I . 7 o , and I2.o6 volts, respectively. 1 Although a great many observations 2 in the past have been made on the ionization of mercury vapor by electron streams having thermal velocity distributions, there is no record of such ultra-ionization potentials. An estimate of the uncertainty introduced by a Maxwellian velocity distribution in the observation of ionization probabilities leads to the tentative conclusion that these critical potentials should have been observed. It was the purpose of this investigation to examine the apparent discrepancy existing between these recent experiments and the work of former investigators. It may be said at once that the experiment to be described shows that the ultra-ionization potentials are in evidence qualitatively when the vapor is bombarded with electron streams possessing Maxwellian velocity distributions, and, therefore, that the apparent discrepancy has been eliminated. THE EXPERIMENT.
Fig. I shows the experimental arrangement. Electrons from an approximately equipotential region' of an oxide-coated platinum filament were accelerated through an orifice .d into a region where another accelerating field caused a portion to pass into an ionization chamber ]. Positive ions produced by impacts of these electrons in the mercury vapor in the chamber were drawn to a grid G by a small field and to the wire collector /47 by a much larger field between G and I f . 1LAWRENCE,Phys. Rev., ~8, 947 (I926)See National Research Council Bulletin on " Crltieal Potentials, by K. T. Compton and F. L. Mohler for r~sum~ of results• 9I
92
E R N E S T ORLANDO L A W R E N C E .
[J. F. I.
The velocities of the electrons traversing the ionization chamber were varied by altering the difference of potential between the orifice A and the ionization chamber while the accelerating potential between the filament and -// was kept constant (usually about 9 volts). In this way the current into the ionization chamber did not greatly increase when the velocities of the electrons were made greater. The method of measurement of the positive-ion current was FIG. I.
1 -I'+
'1 i
i
+'1_
I i.
J
somewhat unusual. The collecting electrode l,//" was attached to the free quadrant of an electrometer E and the accumulation of a positive charge on the quadrant system was balanced by an equal and opposite current introduced by an attached India ink resistance R and a Leeds and Northrup potentiometer LNP, as is clearly shown in the diagram. In this way the electrometer served as a null instrument and the positive-ion currents were recorded as settings of the potentiometer. The system was evacuated by diffusion pumps below values measurable on a McLeod gauge. However, undoubtedly wax vapors were present as well as the mercury vapor at room temperature. Helmholtz coils served to vary the magnetic field in the region of ionization. RESULTS°
Typical experimental results are exhibited by Fig. 2. The ordinates record the observed ionization per unit electron current corresponding to electron accelerating voltages b e t w e e n / / and the ionization chamber given by the abscissas. Several distinct
July, 1927.] ULTRA-IONIZATION POTENTIALS OF MERCURY.
93
breaks in the curve are evident and if the most pronounced break is taken to correspond to a critical potential at tI.29 volts, the potentials at which ionization sets in, as well as the ultra-ionization potentials, are in agreement with the values lO.4, io.6, Ii.29, F1G. 2.
"X .5 7
b 6"
5
4
4
3
F
4 /I ,m_ _ 2
3
4
5
6
/Icceler~/2~ //o/Gage 8etm/een A//~d.[ I 1.7 volts, respectively. Within the experimental error these breaks are independent of the magnetic field in the region of ionization. DISCUSSION.
Thus it has been demonstrated that former critical potential methods involving Maxwellian velocity distributions are capable of detecting in a qualitative manner ultra-ionization potentials in mercurv and therefore that the apparent conflict in experimental
94
ERNEST ORLANDO LAWRENCE.
[J. F. I.
data is not real. That they have not been observed before perhaps is to be explained by the: fact that the data were taken at greater intervals than o.I volt and with much larger currents than the electron currents of about lO.9 ampere used in the present investigation. The method of magnetic analysis of the bombarding electrons involved two inherent possible causes for erroneous interpretation of experimental data which the present research has cleared up. In the first place, the magnetically resolved beams of electrons entering the ionization chamber were in all cases retarded from velocities above the ionization potential and therefore positive ions formed external to the chamber were drawn in and contributed to the observed ionization effects. On the other hand, in the present experiments the electron stream was accelerated to the ionization chamber and positive ions formed in the external region were accelerated away from the chamber. Thus, these experiments show that the ultra-ionization potentials are not a peculiar consequence of such diffusion of positive ions. Secondly, the presence of the magnetic field in the region of ionization was conceivably a factor in producing the critical potentials. The present research shows that the critical potentials are quite independent of such magnetic fields--for the ionization curve breaks have been observed not to change appreciably when the magnetic field was varied from o.5 to 3 gauss. It is an especial pleasure to record my indebtedness to Prof. W. F. G. Swann for his interest in this work. Sloane Laboratory, Yale University, February 5, 1927.
POTENTIALS
OF MERCURY.
BY
ERNEST ORLANDO LAWRENCE, Ph.D. National Research Fellow. INTRODUCTION.
I3r 13OMBarcmXG mercury vapor with electrons of definite velocities--produced by magnetic analysis of the electrons from a thermionic source--it has recently been shown that there are a succession of distinct types of electron inelastic impacts in mercury vapor above the ionization potential having the following critical potentials: Io.6, II.29, I I . 7 o , and I2.o6 volts, respectively. 1 Although a great many observations 2 in the past have been made on the ionization of mercury vapor by electron streams having thermal velocity distributions, there is no record of such ultra-ionization potentials. An estimate of the uncertainty introduced by a Maxwellian velocity distribution in the observation of ionization probabilities leads to the tentative conclusion that these critical potentials should have been observed. It was the purpose of this investigation to examine the apparent discrepancy existing between these recent experiments and the work of former investigators. It may be said at once that the experiment to be described shows that the ultra-ionization potentials are in evidence qualitatively when the vapor is bombarded with electron streams possessing Maxwellian velocity distributions, and, therefore, that the apparent discrepancy has been eliminated. THE EXPERIMENT.
Fig. I shows the experimental arrangement. Electrons from an approximately equipotential region' of an oxide-coated platinum filament were accelerated through an orifice .d into a region where another accelerating field caused a portion to pass into an ionization chamber ]. Positive ions produced by impacts of these electrons in the mercury vapor in the chamber were drawn to a grid G by a small field and to the wire collector /47 by a much larger field between G and I f . 1LAWRENCE,Phys. Rev., ~8, 947 (I926)See National Research Council Bulletin on " Crltieal Potentials, by K. T. Compton and F. L. Mohler for r~sum~ of results• 9I
92
E R N E S T ORLANDO L A W R E N C E .
[J. F. I.
The velocities of the electrons traversing the ionization chamber were varied by altering the difference of potential between the orifice A and the ionization chamber while the accelerating potential between the filament and -// was kept constant (usually about 9 volts). In this way the current into the ionization chamber did not greatly increase when the velocities of the electrons were made greater. The method of measurement of the positive-ion current was FIG. I.
1 -I'+
'1 i
i
+'1_
I i.
J
somewhat unusual. The collecting electrode l,//" was attached to the free quadrant of an electrometer E and the accumulation of a positive charge on the quadrant system was balanced by an equal and opposite current introduced by an attached India ink resistance R and a Leeds and Northrup potentiometer LNP, as is clearly shown in the diagram. In this way the electrometer served as a null instrument and the positive-ion currents were recorded as settings of the potentiometer. The system was evacuated by diffusion pumps below values measurable on a McLeod gauge. However, undoubtedly wax vapors were present as well as the mercury vapor at room temperature. Helmholtz coils served to vary the magnetic field in the region of ionization. RESULTS°
Typical experimental results are exhibited by Fig. 2. The ordinates record the observed ionization per unit electron current corresponding to electron accelerating voltages b e t w e e n / / and the ionization chamber given by the abscissas. Several distinct
July, 1927.] ULTRA-IONIZATION POTENTIALS OF MERCURY.
93
breaks in the curve are evident and if the most pronounced break is taken to correspond to a critical potential at tI.29 volts, the potentials at which ionization sets in, as well as the ultra-ionization potentials, are in agreement with the values lO.4, io.6, Ii.29, F1G. 2.
"X .5 7
b 6"
5
4
4
3
F
4 /I ,m_ _ 2
3
4
5
6
/Icceler~/2~ //o/Gage 8etm/een A//~d.[ I 1.7 volts, respectively. Within the experimental error these breaks are independent of the magnetic field in the region of ionization. DISCUSSION.
Thus it has been demonstrated that former critical potential methods involving Maxwellian velocity distributions are capable of detecting in a qualitative manner ultra-ionization potentials in mercurv and therefore that the apparent conflict in experimental
94
ERNEST ORLANDO LAWRENCE.
[J. F. I.
data is not real. That they have not been observed before perhaps is to be explained by the: fact that the data were taken at greater intervals than o.I volt and with much larger currents than the electron currents of about lO.9 ampere used in the present investigation. The method of magnetic analysis of the bombarding electrons involved two inherent possible causes for erroneous interpretation of experimental data which the present research has cleared up. In the first place, the magnetically resolved beams of electrons entering the ionization chamber were in all cases retarded from velocities above the ionization potential and therefore positive ions formed external to the chamber were drawn in and contributed to the observed ionization effects. On the other hand, in the present experiments the electron stream was accelerated to the ionization chamber and positive ions formed in the external region were accelerated away from the chamber. Thus, these experiments show that the ultra-ionization potentials are not a peculiar consequence of such diffusion of positive ions. Secondly, the presence of the magnetic field in the region of ionization was conceivably a factor in producing the critical potentials. The present research shows that the critical potentials are quite independent of such magnetic fields--for the ionization curve breaks have been observed not to change appreciably when the magnetic field was varied from o.5 to 3 gauss. It is an especial pleasure to record my indebtedness to Prof. W. F. G. Swann for his interest in this work. Sloane Laboratory, Yale University, February 5, 1927.