MULTIPLY-CHARGED ATOMS

advertisement
MULTIPLY-CHARGED ATOMS
B Y S I R J. J. THOMSON.
In the photographs of the positive rays (see, for example, those given in the
Phil. Mag. Aug. 1912) the mercury line is remarkable for the exceptionally small
displacement of the head of its parabola. Even when the electric and magnetic
fields are strong enough to produce deflexions of several millimetres in the heads of
the parabolas corresponding to the other elements, the head of the mercury parabola
is so little deflected that at first sight it seems to coincide with the origin. When,
however, the electric field used to deflect the particles is made very large, in our
experiments from 5000 to 10,000 volts per centimetre, the head of this parabola is
distinctly displaced, and on measuring the electrostatic displacement it is found to
be 1/8 of the normal displacement of the heads of the parabolas corresponding to the
other elements.
The displacement due to the electric field is inversely proportional to the kinetic
energy of the particle displaced, so that the atoms which produce the head of the
mercury parabola must have eight times the maximum amount of energy possessed
by the normal atoms. This could be accounted for if some of the mercury atoms in
the discharge-tube had lost 8 corpuscles, for then the energy communicated to the
atom by the electric field would be eight times the energy communicated to an atom
with the normal charge. Eight is a very large number of corpuscles to lose, much
larger than the number lost by the other elements which get multiple charges.
I had previously to these experiments never found a case in which this number
exceeded three; so that in my paper in the Phil. Mag. for August 1912, I suggested
another explanation of the behaviour of the mercury line. A study of the plates
taken with large electrostatic deflexions has revealed the existence of 7 parabolas
due to mercury, corresponding to the mercury atom with 1, 2, 3, 4, 5, 6, 7 charges
respectively, the parabola corresponding to 8 charges has not been detected, but as
the parabolas get in general fainter for each additional charge on the atom, it is
probably there but too faint to be detected. Fig. 1, taken from a photograph
when the gas in the tube was the residual gas left after exhaustion by the Gaede
pump, shows these lines very well. The measurements of m/e for these parabolas
gave the following values :
m/e
200
102
66-3
50-4
44
39-8
33-7
28-6
200/2
200/3
200/4
this not a mercury line but is due to C0 2 .
200/5
200/6
200/7
18—2
276
SIR J. J. THOMSON
It will be noticed that the heads of the parabolas corresponding to 1, 2, 3
charges respectively lie on a straight line passing through the origin, indicating that
the velocities of the particles producing the heads of these parabolas are all equal,
and therefore, since each particle is an atom of mercury, that the kinetic energy of
the particles at the heads of the parabolas is constant. This is what we should have
expected, for the heads of all the parabolas are due to particles which had originally
lost 8 corpuscles, the particle at the head of the parabola corresponding to one charge
has regained 7 of these after passing through the cathode, that at the head of the
parabola corresponding to two charges, six, and so on ; these particles when in the
discharge-tube were all in the same condition, and so acquired the same amount of
kinetic energy.
The question now arises as to how the mercury atom acquires these very various
charges. When a mercury atom is ionized, can it lose any number of corpuscles
from one to eight ? Taking for example a mercury atom with 5 positive charges,
has it got into this condition by losing 5 corpuscles when it was ionized, or did it
originally lose the maximum number 8 and regain 3 subsequently ? The photographs prove, I think, that the second supposition is the correct one, and that in the
discharge-tube there are two, and only two, kinds of ionization ; in one of these kinds
the mercury atom loses 1 corpuscle, while in the other kind it loses 8, and that there
are no indications of ionization of such a character as to deprive the mercury atom of
7, 6, 5, 4, 3, or 2 corpuscles.
The evidence for this is as follows : let us suppose for a moment that atoms with
any charge from 1 to 8 were produced by the ionization of the mercury atoms in the
discharge-tube. Consider now the parabola due to the mercury atom which, when
it passed through the electric and magnetic fields, had one positive charge. This
parabola would result from atoms of the following kinds:
1. Atoms which had lost 8 corpuscles on ionization and regained 7;
2. Atoms which had lost 7 and regained 6 ;
3. Atoms which had lost 6 and regained 5,
and so on, the eighth and last members of the series being atoms which had lost one
corpuscle on ionization and had not regained it.
The parabola on the photographic plate would be due to the superposition of
the 8 parabolas due to these types of atoms.
If 8d is the horizontal distance from the vertical axis of the head of the parabola
due to the atom which lost one corpuscle on ionization, the horizontal distances of
the heads of the parabolas due to atoms of the 1st, 2nd, 3rd types will be respectively
8d/8, Sd/7, Sd/6, 8d/5, 8d/4, 8d/3, Sd/2, Sd. Thus up to the horizontal distance 8d/7
there would be only one parabola, at 8cü/7 another parabola would be added, this
would produce an abrupt increase in the intensity of the photograph, there would be
another abrupt increase at Sd/6, another at 8d/5, and so on. Thus the intensity of
the parabola on the photograph would not be continuous, there would be places
where the intensity was suddenly increased giving a beaded appearance to the
photograph. The abrupt increase at 8d is very marked on this parabola, the others
are not visible ; but as the intensity of this parabola is very great it might be
thought that they escaped detection owing to the breadth of the parabola. Let us
MULTIPLY-CHARGED ATOMS
277
therefore consider one of the finer parabolas, say, that due to the atom with four
charges, to which this objection does not apply.
This parabola might arise from atoms which had lost 8 charges on ionization and
regained 4, from those which had lost 7 and regained 3, and so on, the last being
atoms which had lost 4 and not regained any. Then if d has the same meaning as
before, the horizontal distances from the vertical axis of the heads of the parabolas
would be 4 x 8d/8, 4 x 8d/7, 4 x 8d/6, 4 x 8d/5, 4 x 8d/4 ; there would, therefore, be
abrupt increases in intensity at S2d/7, 32d/6, S2d/5, 32d/4. The photographs show,
however, that the intensity is perfectly continuous, and not one of these abrupt
increases is to be seen. We conclude, therefore, that there are no atoms which begin
with 7, 6, 5, 4, 3, 2 charges, and that in this case there are two and only two types of
ionization, in the one type an atom loses a single corpuscle, in the other it loses 8.
This result suggests that ionization takes place in the discharge-tube in two
ways. In the first method the ionizing agents are the rapidly moving corpuscles
which constitute the cathode rays, these very small particles penetrate into the atom
and come into collision with the corpuscles inside it individually, the collision in
favourable cases causing the corpuscle struck to escape from the atom ; this type of
ionization results in the atom losing a single charge. In the other type of ionization
we suppose that the mercury atom is struck by a rapidly moving atom and not by a
corpuscle ; after the collision the mercury atom starts off with a very considerable
velocity, which at first is not shared by the corpuscles inside it. The tendency of the
corpuscles to leave the atom depends only upon the relative velocity of the atom and
the corpuscles inside it, so that the ionizing effect produced by the collision is the
same as if the atom were at rest, and all the corpuscles were moving with the velocity
acquired by the atom in the collision. Thus if there were eight corpuscles in the
mercury atom connected with about the same firmness to the atom, the result of the
atom acquiring a high velocity in a collision might be the detachment of the set of
eight leaving the atom with a charge of 8 units of positive electricit}5-. We see in
this way how the cathode particles might produce one type of ionization resulting in
singly charged atoms, while the atoms forming the positive rays might produce
another type of ionization resulting in multiply-charged atoms.
All the elements I have examined give multiply-charged positive atoms with
the exception of hydrogen, on which I have never observed more than one charge ; in
no other case, however, have I observed charges approaching that possessed by
mercury. The majority of the elements seem to acquire only two charges ; this is
the number acquired by helium, and this case is interesting since in the vacuum-tube
the helium atom occurs with both single and double charges, whilst as an a particle
it always seems to have two charges, suggesting that the process by which the a
particle acquires its charge is analogous to the process by which multiply-charged
atoms are produced in the discharge-tube.
I have observed nitrogen atoms with three charges, but the parabola due to the
triply-charged atom is exceedingly faint. Argon shows triply-charged atoms very
distinctly as can be seen from fig. 2, where the parabolas I, II, III due to Arg + , Arg + + ,
Arg + + + are all very distinct ; the Arg + + parabola has probably a parabola due to
neon superposed on it.
This plate shows the helium line, and thus incidentally gives us an estimate of
the sensitiveness of this method of detecting small quantities of a gas. The volume
278
SIR J. J. THOMSON
of the discharge-tube was about two litres, the pressure 1/300 of a mm. of mercury :
there was thus in the discharge-tube about 1/100 c.c. of argon at atmospheric pressure. This is about the amount in 1 c.c. of air at this pressure, and as the helium
line was visible along with the argon, we see that this method can detect the amount
of helium in 1 c.c. of air, which, according to Sir William Ramsay, is about 4 x 10 - 6 c.c,
even though this is mixed with an enormous excess of argon. The tube used for this
photograph was not at all well adapted for detecting small quantities of an impurity,
as the cathode had been in use for several weeks and the tube through it was almost
silted up by the sand-blast action of the positive rays, and was just about to be replaced by a new tube.
An interesting result, which is now being investigated, is that when very pure
nitrogen is in the discharge-tube the mercury line corresponding to the atom with
five charges becomes abnormally bright, brighter than those for the atom with four
or even three charges, though in other gases the greater the charge on the atom the
fainter the line.
I have much pleasure in thanking Mr F. W. Aston, B.A., Trinity College, for
the great assistance he has given me in these investigations.
Fig. l.
Fig. 2.
Download
Related flashcards
Particle physics

51 Cards

Create flashcards