Showers produced by the penetrating cosmic radiation 543 Frohlich, Heitler and Kemmer 1938 Proc. Roy. Soc. A, 166, 154. Heisenberg 1936 Z. Phys. 101 , 533. Heitler 1937 Proc. Roy. Soc. A, 161, 261. — 1938 Report of the Moscow Conference. Kemmer 1938 Proc. Roy. Soc. A, 166, 127. Nordheim 1937 Phys. Rev. 51, 1110. Pfotzer 1936 Z. Phys. 102 , 23, 41. Weisskopf 1937 Phys. Rev. 52, 295. The formation of helium molecules By F. L. A rnot , P h .D., Lecturer in Natural Philosophy and M arjorie B. M ’E wen , B.Sc., Demonstrator in Physics, The University, St Andrews {Communicated by H. S. Allen, F.R.S.—Received 11 March 1938) I ntroduction The occurrence of band spectra in the chemically monatomic gases leads to the belief that molecules are formed in the gas by the electrons or photons producing the spectra. Arnot and Milligan (1936) and Arnot and M’Ewen (1938) have shown that diatomic mercury molecules are formed by the attachment of excited atoms in P-states to each other and to normal atoms. Although it has been inferred from the band spectra found in helium that molecules are formed by some such attachment process, these have not previously been studied electrically, nor have the details of the attachment process been investigated.* No definite evidence has been put forward to show that band spectra occur in the other rare gases. This paper contains the results of an investigation made by the balanced space-charge method of the ionization produced in argon, neon and helium when these gases are subjected to bombardment by electrons whose energy was varied from a few volts below the first excitation potential to a few * In the early experiments on critical potentials ions were found to be produced in helium when the bombarding electrons had energies less than that corresponding to the atomic ionization potential. These effects were generally attributed to impurities in the helium used, but apparently never to the formation of helium molecules. on May 21, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from
Transcript
Showers produced by the penetrating cosmic radiation 543
Frohlich, H eitler and Kem m er 1938 Proc. Roy. Soc. A, 166, 154.Heisenberg 1936 Z. Phys. 101 , 533.H eitler 1937 Proc. Roy. Soc. A, 161, 261.
— 1938 Report of the Moscow Conference.K em m er 1938 Proc. Roy. Soc. A, 166, 127.Nordheim 1937 Phys. Rev. 51, 1110.Pfotzer 1936 Z . Phys. 102, 23, 41.W eisskopf 1937 Phys. Rev. 52, 295.
The formation of helium moleculesBy F. L. A r n o t , Ph .D., Lecturer in Natural Philosophya nd M a r jo r ie B. M ’E w e n , B.Sc., Demonstrator in Physics,
The University, St Andrews
{Communicated by H. S. Allen, F.R.S.—Received 11 March 1938)
I n tr o d u c tio n
The occurrence of band spectra in the chemically monatomic gases leads to the belief that molecules are formed in the gas by the electrons or photons producing the spectra. Arnot and Milligan (1936) and Arnot and M’Ewen (1938) have shown that diatomic mercury molecules are formed by the attachment of excited atoms in P-states to each other and to normal atoms. Although it has been inferred from the band spectra found in helium that molecules are formed by some such attachment process, these have not previously been studied electrically, nor have the details of the attachment process been investigated.* No definite evidence has been put forward to show that band spectra occur in the other rare gases.
This paper contains the results of an investigation made by the balanced space-charge method of the ionization produced in argon, neon and helium when these gases are subjected to bombardment by electrons whose energy was varied from a few volts below the first excitation potential to a few
* In the early experim ents on critical potentials ions were found to be produced in helium when the bom barding electrons had energies less than th a t corresponding to the atom ic ionization potential. These effects were generally a ttribu ted to im purities in the helium used, bu t apparently never to the formation of helium molecules.
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volts above the atomic ionization potential of the gas. Only atomic ionization has been found to occur in argon and neon, but molecular ions are found to be produced in helium when the energy of the electrons exceeds the resonance potential of the gas.
F. L. Arnot and Marjorie B. M’Ewen
A ppa r a t u s
The apparatus used in this work and the experimental procedure adopted have been fully described in a previous paper on the formation of mercury molecules (Arnot and M’Ewen 1938). In the present work we used the second form of apparatus shown in fig. 1 b, as this was found in the work on mercury to be equally satisfactory and rather more sensitive than the form shown in fig. 1 a.The wiring diagram is shown in fig. la . All metal
0 1 2 31 I I - I
cm.
fT
F ig . 1. Apparatus and wiring diagram.
parts, including gauzes, filament leads and connecting wires, were made of nickel, spot-welded where necessary. The cylinders and were of identical construction except for the gauze window in Cv The caps on the ends of these cylinders were pressed out of one piece of nickel to give a perfect fit, and then were spot-welded to the cylinders. C2 was supported from C1 by quartz rods which insulated it from the rest of the apparatus. The filaments F2 and F3, which were connected in parallel, were co-axial with the cylinders C1 and C2, and their leads were insulated from the cylinders by short quartz tubes passing through the end-caps.
The apparatus was contained in a large pyrex tube fitted with a 5 cm. ground-glass joint, so that the whole apparatus could be withdrawn for
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adjustment and filament renewal. The joint was water-cooled and a low vapour pressure Apiezon grease was used on this and all other joints and taps in the vacuum system. The tube was evacuated through a mercury cut-off and a liquid-air trap. The gases used were stored in a glass bulb of about 1 1. capacity, and were admitted to the apparatus through a fine glass capillary leak and a liquid-air trap. The pressure of gas in the apparatus, which was read on a McLeod gauge, could be altered by varying the pressure in the bulb and by adjusting the mercury cut-off, of which the inner tube was drilled with a number of holes of varying diameter arranged in the form of a spiral round the tube. The rare gases were supplied by the British Oxygen Co. in glass containers as spectroscopically pure.
E x p e r im e n t a l pr o c e d u r e
After the usual baking-out, flashing and running-in of all filaments the current through the filaments F2 and F was set to give a saturated emission from each filament of about OT mA. The potential between and and their respective cylinders was then reduced to 2 V resulting in the emissions from F2 and F3being strongly space-charge limited to a value of about 0-01 mA. The emissions from and were then balanced by the bridge arrangement shown in fig. 1 which incorporated two 10,000 ohm resistance boxes and the galvanometer Mv In this way any fluctuations in the potentials of the cells supplying the heating current to F2 and F3 were eliminated. The emission from F3 alone was read by the galvanometer M2 so that its constancy during a run could be checked.
Electrons from the filament F1 are accelerated up to the gauze G1 by a potential V0 applied to the centre of the filament as shown in fig. 1, being read on a Weston standard voltmeter. When V0 is increased to such a value that ions are formed in the cylinder C\ their space-charge partially neutralizes the electron space-charge around F2. The consequent increase in the emission from F2 is read on the galvanometer Mv This method is known to be a very sensitive detector of positive ions, since it has been shown that a single positive ion neutralizes the space-charge of as many as 104 to 106 electrons, for the ions revolve in spiral orbits around the filament, making many loops before finally hitting the fine wire.
R esu lts
The results obtained at various pressures in argon, neon and helium are shown respectively in figs. 2, 3 and 4. These curves represent the change
The formation of helium molecules 545
Vol. CLXVI. A 35
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in the space-charge limited current from the filament measured by the galvanometer Mx, as the energy V0 of the electrons from F1 is increased. The curves in figs. 2 and 3 have all been displaced in a vertical direction in order to separate clearly the points. The lowest curves in figs. 3 and 4 represent runs taken in vacuum. For these runs the McLeod gauge registered a ' sticking vacuum” which is equivalent to a pressure of less than 10-6 mm. of Hg.
The formation of helium molecules 547
Electron energies in volts
F ig . 4. Showing onset of m olecular and atom ic ionization in helium.
The linear fall in the curves as V0 is increased up to the value at which ions begin to be formed has been investigated and discussed in our previous paper on the formation of mercury molecules (1938). The fall in the curves was there shown to be due to the fact that the emission from is not space-charge limited over its entire length. The temperature of the filament is highest in the centre, and the emission drawn from this portion is limited by space-charge; but the emission from the ends of the filament, which are cooled by the leads, is temperature-limited. Electrons from Fx can have no effect on the space-charge limited emission from the centre of F2, but they decrease the temperature-limited emission from the ends
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of F2 by virtue of their space charge. Now as is increased the number of electrons penetrating the gauze 0 1 and so affecting the emission from the ends of F2 is increased, thereby causing the emission from F2 to decrease as V0 is increased.
It will be seen from figs. 2 and 3 that in argon and neon ionization sets in at the atomic ionization potential for each pressure used. The spectroscopic value of the ionization potential for argon is 15-7 V, and for neon 21-5 V. The differences between these values and those given by the curves in figs. 2 and 3, amounting to 0*3 V for argon and 0-5 V for neon, are fully accounted for by contact potentials, for which no correction has been made in these figures. In argon and neon there is no evidence of ionization occurring below the atomic ionization potential even at high pressures.
The results for helium, which were taken after the argon results and before those for neon, show that ionization sets in well below the atomic ionization potential, the spectroscopic value of which is 24-5 V. Because of this the rise in the curves at the atomic ionization potential is not so sharp as in argon and neon, and consequently the correction for contact potentials to be applied to the voltage scale of fig. 4 cannot be determined as precisely as in the other two rare gases. However the rise in the three lower pressure curves appears to set in at about 23-5 V, which gives a correction of 1 V for contact potentials and energy spread in the electron beam. By subtracting the vacuum curve from the four pressure curves of fig. 4 and applying this correction of 1 V to the voltage scale we obtain the curves shown in fig. 5 a. These curves show that ionization sets in at 19-8 V.
The two lowest excitation potentials of the helium atom are 19-77 V for the transition 1 1S0 -» 2 3S and 20-55 V for 1 1S0 -> 2 1S0. Now, although the voltage scale correction applied to fig. 5a is a little indefinite it is quite definite enough to decide between these two excitation potentials. If ionization did not occur until the atom was excited to the singlet level the voltage correction would have to be 1-8 V instead of 1-0 V. The curves of fig. 4 show that it could not possibly be as large as this, for the uncertainty in the value 1-0 V adopted is not greater than + 0-2 V.
The conclusion we have reached is that ions are formed in helium when the bombarding electrons have sufficient energy to raise them to the first excited state above the ground state. We have now to decide whether these ions are atomic or molecular. A second collision of the excited atom with an electron, or its absorption of a photon of more than 4-8 V energy, could produce an atomic ion. However, the results for neon and argon
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show that the probability of this double absorption process is too small to produce ions in sufficient number to be detected. For the same reason the collision of two excited atoms, whereby one absorbs the energy of the other by a collision of the second kind resulting in an atomic ion and a normal atom, cannot occur with sufficient frequency to account for the ionization observed. Whatever the process, if the resulting ions were atomic
The formation of helium molecules 549
Electron energy in volts
F ig . 5. (a) Curves of fig. 4 corrected by subtraction of the vacuum curve and with voltage scale correction for contact potentials. Showing onset of molecular ionization a t the resonance potential, 19-77 V, of helium. ( Theoretical excitation function for the 2 3S sta te of 19-77 V energy.
then we should expect to obtain ions also in neon and argon at the resonance potentials of these atoms, 16-53 and 11*49 V respectively. We therefore conclude that the ions produced below the atomic ionization potential in helium are molecular ions.
Helium molecules have been known since 1913 when the helium band spectrum was discovered independently by Curtis (1913) and by Goldstein (1913). During the last twenty-five years the band spectrum has been very fully investigated and has proved of supreme importance in testing the quantum theory of molecules developed by Heitler and London, Hund, Mulliken
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and others. A table of the energy states of the helium molecule is given by Jevons (1932). No band spectra have been observed in argon, and although a number of bands in the red in neon were reported by Dhavale (1930) this observation has not been confirmed. There is thus no definite evidence from band spectroscopy that molecules are formed in argon and neon, a negative result which is supported by our curves given in figs. 2 and 3.
Our results show that helium molecules are formed by the collision of an excited atom in the 2 3S state with either a normal atom or another excited atom in the 2 3S state. The collision with a normal atom is far more probable than a collision with a second excited atom, and, since the former process provides sufficient energy to ionize the molecule, it is probably the effective process. We thus have
He(ls2,*S0) + H- e'{ls2s,3S) -> He+(lt§o'22p(r, 2AJ + e,
in which the first term represents the normal atom in its ground state 1 1S0 and the second term represents the excited atom in the metastable state 2 3S of 19-77 V energy. The ionization potential of the molecule obtained from the band spectrum limit is 18-58 V (Jevons 1932), leaving 1-19 V energy to be carried away in kinetic form by the electron.
The theoretical excitation function for the metastable state occurring in the above process has been determined by Massey and Mohr (1931), and is shown in fig. 56 . We see that the probability of excitation to this state from the ground state is practically constant over the range of energy from 22 to 24 V. Now the lowest curve of fig. 5 representing the probability of ionization at a pressure of 5-6 x 10~2 mm., is similar in form to the excitation function up to the atomic ionization potential, 24-5 V, and is practically flat between 22 and 24 V. The higher pressure curves in fig. 5 differ more in form from that of the excitation function the higher the pressure. This indicates that at low pressure the molecular ions are formed mainly from atoms in the metastable state 2 3S, while at higher pressures atoms in higher excited states also form molecular ions when they collide with normal atoms. Since these higher states are not metastable a higher pressure is required in order that an effective collision leading to attachment should occur before the atom radiates.
We have shown in our previous paper on the formation of mercury molecules (1938) that excited atoms in states other than P-states do not apparently form ionized mercury molecules by attachment even though they have more than sufficient energy to do so. The helium atom is similar to the mercury atom in that it has two outer electrons, but differs
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from the mercury atom in that its outer shell is complete. This difference apparently allows the formation of ionized helium molcules from excited atoms in S-states.
Sum m ary
An investigation of the formation of ionized molecules in argon, neon and helium has been made by the balanced space-charge method which had previously been used by the authors to study the formation of mercury molecules. No evidence of molecular ionization was found in argon and neon. In helium molecular ionization sets in at the resonance potential. The results show that these molecular ions are formed by the attachment of metastable atoms to normal atoms according to the process
He(ls2, 1S0) + H e'(ls25, 3S) -> He+( lso ^ c r , ITJ + e.
The appearance potential of the molecular ions is the energy of the 2 3S state, 19*77 V, which is 1*19 V greater than the ionization potential of the helium molecule.
Attention is drawn to the fact that these molecular ions are formed from excited atoms in S-states, whereas we have shown that mercury molecular ions are apparently formed only by excited atoms in P-states.
R e f e r e n c e s
A rnot and Milligan 1936 Proc. Roy. Soc. A, 153, 359.Arnot and M’Ewen 1938 Proc. Roy. Soc. A, 165, 133.Curtis 1913 Proc. Roy. Soc. A , 89, 146.Dhavale 1930 Nature, Pond., 125, 276.Goldstein 1913 Verh. dtsch. phys. Ges. 15, 10.Jevons 1932 “ R eport on B and Spectra of Diatom ic Molecules” , pp. 270—1.
Camb. Univ. Press.Massey and Mohr 1931 Proc. Roy. Soc. 132, 605.
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