Aston 1919
F.W. Aston Proc. Cambridge Philos. Soc., 19 (1919), p. 317
The distribution of intensity along the positive ray parabolas of atoms and molecules of hydrogen and its possible explanation.
By
F. W. ASTON, M.A., Trinity College (D.Sc., Birmingham). Clerk Maxwell Student of the University of Cambridge.
[Read 19 May 1919.]
No one working with positive rays analysed by Sir J. J. Thomson's method can fail to notice the very remarkable intensity variation along the molecular and atomic parabolas described by him under the term 'beading.' It will be sufficient for the reader to refer to Plate III of his monograph on the subject (Rays of positive electric, p. 52) to realise how striking these can be. Beadings at points corresponding to energy greater than the normal have been quite satisfactorily accounted for by multiple charges (l.c., p. 46[1]), but the ones with which this paper is concerned have a smaller energy than the normal, actually half, and fractional charges are presumably impossible. Nevertheless they seem capable of a simple explanation and an opportunity of putting this to the test occurred recently while making some experiments to determine the best form and position of the cathode preliminary to the design of an apparatus to carry the analysis to higher degrees of precision.
The observations were made with an apparatus essentially of the form now well known (l.c., p. 20[2]) the discharge tube being arranged to be removable with the minimum trouble to change or move the cathode. As no camera suitable for photographic recording was immediately available or necessary a willemite screen and visual observation was employed. This form has many obvious disadvantages and in addition, owing to the enormous difference in sensitivity between the parabolas of hydrogen and those due to heavier elements the latter can only be seen with difficulty. It has however one notable advantage, namely that sudden and even momentary changes in intensity can be observed and correlated in time with changes in the discharge or in the intensity of other lines. As no accurate measurements were intended a large canal ray tube was employed so that the H1 and H2 parabolas could be easily seen even with the less effective types of cathode.
It was soon realised that the appearance on the screen was in general the sum of two superposed effects which could be only unravelled like the writings on a palimpsest by eliminating one of them. This by good fortune it was found possible to do under certain conditions. For the sake of clearness it is proposed to consider these two extreme types and their explanation before going on to describe the conditions under which they may be attained or approached. In the diagrams the fields of electric and magnetic forces are horizontal and such that positive ions will be deflected to the right and up, negative ones to the left and down. Brightness is roughly indicated by the width of the parabolic patch drawn.
Atomic type of discharge.
Fig. 1 illustrates the first or 'Atomic' type in which apparently the whole of the discharge is carried up to the face of the cathode by ions of atomic mass. Those which pass through the fields without collision produce the true primary streak on parabola m = 1, the head of which corresponds in energy to that obtained by the charge e falling through the full potential of the discharge. Now the pressure in the canal ray tube is never negligible being on the average at least half that in the discharge tube, and the ionisation along its length very intense so that in passing through it a large number will collide with electrons, atoms or molecules. The collision and capture of a single negative electron will result in a neutral atom striking the screen at the central undeflected spot O while the capture of two will cause the faint negative parabolic streak a1 as has already been described (l.c., p. 39[3]).
But besides these forms of collision by which the velocity of the atom is practically unaffected there is distinct evidence that it may collide with and capture another hydrogen atom. If the atom struck is negatively charged the resulting molecule will strike the central spot but if it is neutral and the collision is inelastic the resulting positive ray will have the same momentum (the atom struck being relatively at rest) but double the mass so that it will strike the molecular parabola at a point the same height above the X-axis as would the atom which generated it. Molecular rays formed in this manner will therefore form the streak b2 which, allowing for the geometrical difference in the curves will show a similar distribution of intensity to a1. Collision with a positively charged atom will obviously be unlikely to result in capture and those with heavier atoms will be referred to later. It is to be noted in connection with the brightness of these secondary streaks a1 and b2, which may conveniently be called 'satellites' to distinguish them from the 'secondary lines' already fully described (l.c., p. 32[https://archive.org/details/RaysOfPositiveElectricity/page/n42/mode/1up), that a1 is always very much fainter than its primary but b2 can be equally bright.
This atomic type of discharge with its pendant bright arc on the molecular parabola corresponding to similar momentum and half normal energy is most beautifully illustrated in Fig. 29 of Plate III already referred to. It was this photograph which suggested the above theory of its explanation.
Molecular type of discharge.
The extreme form in which the whole discharge is carried up to the cathode by ions of molecular mass is unattainable so far in practice and is probably impossible but its share in the illumination of the screen can be deduced by eliminating the superimposed atomic type and is indicated in Fig. 2.
The principal feature is a short and very bright spot of light b1 on the molecular parabola at the point corresponding in energy to a fall through the full potential of the discharge. It will be shown that all the ions causing this are probably generated in the negative glow. Besides this there are two symmetrical and equally bright positive and negative satellite patches a2 and a2 on the atomic parabola but of half the normal energy. The proposed explanation of these is somewhat similar to that considered by Sir J. J. Thomson (l.c., p. 94p[4]) and is as follows. The collision with and capture of a single negative electron by a positively charged molecule will not necessarily merely neutralise it and cause it to
hit the central spot O but may result in it splitting into two atoms one with a positive one with a negative charge. The energy of impact may be itself capable of causing this, if not some other cause, e.g. radiation, may effect the dissociation. In any case it would give exactly the observed result, i.e. two bright patches lying symmetrically on the extension of the line joining the primary spot to the origin at twice its distance from the latter, corresponding to half the mass but the same velocity.
The general appearance on the screen when both types of discharge are present is indicated in Fig. 3.
Effect of different forms of cathode.
Experiments were performed with plane, concave and convex cathodes. Convex cathodes are the least efficient in producing bright effects but give the molecular type with the least atomic blurring. Concave ones are most efficient and throw the maximum energy into the atomic type which can be obtained practically pure with them under a moderate range of conditions. The original shape of cathode (l.c., p. 20[5]) may be said in a sense to combine both forms and was designed to give long aml bright parabolas at the same time allowing the discharge to pass easily at very low pressures. The present results however lead one to recommend a concave cathode similar to those used in X-ray focus tubes but pushed further forward into the neck of the bulb, for though this form requires a rather higher pressure this objection is more than counterbalanced by the great increase in efficiency. Plane cathodes, as was expected, give effects midway between the other forms.
Under very exact conditions of pressure, etc. it is possible to obtain the pure atomic type with plane cathodes but no conditions have yet been found under which convex ones will give it.
These results seem to indicate that atomic ions are formed by the passage of the stream of cathode rays through the Crooke,; dark space molecular ones tending rather to be formed in the negative glow. The axial intensity of the cathode stream i,; enormously increased by the concavity of the cathode while that of the negative glow does not appear t,o be affected to anything like the same extent.
Behaviour during change of pressure
The pressure in a freshly set up bulb always increases with running owing to the liberation of gas by heat etc. so that the changes due to gradual alteration of pressure can be observed most conveniently by exhausting highly, starting the coil and watching the events on the screen. Thus using a concave cathode of about 8 ems. radius of curvature set just in the neck of the discharge bulb the following sequence of events was observed. At very low pressures with a potential of about 50,000 volts the parabolas are very faint but correspond to the general type, the primary streak a1 and spot b1 being much brighter than their satellites (doubtless due to few collisions). As the pressure rises the discharge becomes curiously unsteady the spots on the screen become much fainter and change with flickering into the pure atomic type (Fig. 1), b1 having practically disappeared. This form of discharge which is evidently abnormal lasts for a certain time depending on the rate of increase of pressure. Then with absolute suddenness b1 flashes out intensely bright and with it appear at the same instant its satellites a2 and a2. At the same time the current through the bulb increases, the discharge settles down and the negative glow makes its appearance. As far as it was possible to judge the satellites a2 and a2 are of equal brightness and generally much brighter than the negative atomic satellite a1.
The appearance of the discharge bulb while the pure atomic type is shown on the screen is difficult to describe but quite characteristic and different from the general. Near its critical upper limit of pressure it was found possible to effect the change to the general type by bringing a magnet near the cathode and so disturbing the discharge. On removing the magnet the discharge at once reverted to the atomic type. This form of controlled change from the one to the other gave an excellent opportunity of testing the invariable association between the primary spots and their appropriate satellites.
Possible cause of disappearance of primary molecular rays
It is unlikely that change of pressure is itself the determining factor in the disappearance of the molecular type. This seems to be due to some disturbance in the discharge by the cathode stream (not caused by the diffuse one given by a convex cathode) which makes the formation of the negative glow impossible.
The facts so far may be brought into line fairly well by the somewhat speculative assumption that molecular rays can only originate freely in parts of the discharge where the electric force is very small, e.g. the negative glow, ionisation by more violent means in strong fields tending to cause simultaneous disruption of the molecule into its atomic constituents. This agrees with the observed fact that in general molecular arcs, or at least true primary molecular arcs, are shorter than atomic ones. It would also mean that a very short arc infers as origin a molecule capable of disruption. If this is so it offers interesting confirmatory evidence, if such were needed, that the substance X3 is molecular as this body often makes its appearance on the photographic plate as a short arc.
Effects with heavier elements
The inelastic collision of a hydrogen atomic positive ray with the atom of a heavy element would clearly result in the formation of a molecular ray of such low velocity that it might not be detected by a screen or plate and would in any case be deflected completely off the ordinary photograph.
The visual evidence on the screen although faint leaves little doubt that the formation of satellite arcs also takes place by atoms of heavier elements colliding to form molecules. There is also some evidence of this in many of the photographs, thus in Fig. 26 (l.c., p. 46[6]) taken with oxygen all four maxima are suggested. In Fig. 17 (p. 26) the satellite on the molecular parabola caused by the, capture of oxygen atoms by carbon atomic rays (or vice versa, but this is less likely) is unmistakable, in fact attention is called in the text to this remarkable increase in brightness.
Should the above theory of collision with capture prove correct the formation of compound molecules by this means opens an extremely interesting field of chemical research. Another important question raised is in what form the energy of the collision is radiated off by the rapidly rotating doublet formed.
In conclusion the author wishes to express his indebtedness to the Government Grant Committee for defraying the cost of some of the apparatus used in these experiments.
