Electron transfer in argon

When a singly-charged ion A collides with a normal atom B an electron may be transferred from 15 to A with the result that A becomes a neutral atom and B becomes a singly-charged ion. If the ionization potential of A is greater than that of B this process results in an evolution of energy equal to the difference between the ionization energies of A and B . If a doubly-charged ion A collides with a normal atom B , an electron being transferred from B to A during the collision, the process results in two singly-charged ions. The energy liberated in this process is equivalent to the difference between the second ionization potential of A and the first ionization potential of B . This may be partially or wholly employed in exciting one of the resulting ions or in increasing the kinetic energy of the separating particles.

Pressure broadening of spectral lines and van der waals forces II—Continuous broadening and discrete bands in pure mercury vapour

The spectrum of single transits, forming part of the pressure broadening, has been quantitatively investigated for Na—A and for Hg—A transits. It was found to be very nearly identical with the occurrence distribution of the perturbed eigenfrequencies, the intensity distribution being in agreement with the theoretical prediction I( v ) ~ Δ v -3/2 within the limits of its validity. As this agreement is a direct experimental test on the sixth power potential law of the van der Waals forces, given by London’s wave mechanical theory, it seemed to be of interest to investigate a case in which the velocity of the atoms is considerably less, so that any possible influence of the motion is smaller still. The interaction between two mercury atoms, i. e ., the broadening of the mercury resonance line by the mercury pressure itself, was therefore chosen for quantitative investigation. An excited atom interacts with a normal atom of the same kind at very large distances with a potential of force proportional to 1/ r 3 is the internuclear distance. The influence of this “resonance force” on pressure broadening has been treated theoretically by Weisskopf. Though it is considerable at very low pressures, resulting in large “optical impact diameters” for the Lorentz effect, it is very small at higher densities. This is connected with the dipole property of the forces: the interactions of several neighbouring atoms mainly cancel each other, leaving a very small residue only, the so-called coupling broadening. It was thus expected! that in the wing effect the resonance forces can be completely neglected.

The corpuscular X-ray spectra of the raio-elements

An interesting way in which an excited atom can emit its excess energy has been brought to light by the experiments of Robinson and of Auger. If, for example, an atom is ionised in the K state, then it may emit a quantum of radiation of some line of its K X-ray spectrum by means of a transition of an electron to the K level, but as an alternative method it may emit an electron instead, thus leaving the atom doubly ionised. One such process might be represented as [L I → K, L II → ∝] and the energy E of the ejected electron would be given by E = K abs — L Iabs — L IIabs — δ, where δ is a small correcting term to take into account that the work required to remove an electron from an ionised atom is slightly greater than that necessary in the case of a normal atom. Processes of this kind are essentially different from those giving rise to radiation since two electrons instead of one are concerned in the transition. The entire process must be considered as occurring simultaneously, and, to take as an example the case already mentioned, it has no meaning to attempt to state whether it is an L I electron which goes to the K state, and an L II electron which is ejected or vice versa . Two points of interest in this phenomenon are the investigation of the magnitude of the correction term δ, and of the relative probabilities of the different types of transition. It will be seen later that the possible transitions are considerably more numerous than with single electron transitions which give rise to radiation. This phenomenon has been studied by Robinson by analysing the ejected electrons with a magnetic field. A thin layer of the element under investigation is placed in the position of the source in the well-known semi-circular focussing apparatus, and is irradiated with X-rays of sufficiently high frequency to be able to eject electrons from the K state. There then follows a further electronic emission from these ionised atoms in the manner already described. Both sets of electrons are recorded photographically, and the various groups show up as lines or narrow bands on the photographic plate. A difficulty inherent in the nature of the experiment is that the groups of homogeneous electrons become slightly diffuse in emerging from the target which must have a certain thickness in order to yield groups of reasonable intensity.

The collision of slow electrons with atoms. II.—General theory and inelastic collisions

Although a satisfactory theory of the collisions of fast electrons with atoms is provided by the method of Born-Dirac, a complete theory of slow collisions has not yet been developed. This is due to the large number of complicating factors which cannot be neglected when the time of interaction between atoms and the incident wave is considerable. These complications include the distortion of the incident and scattered waves by the atomic field, the exchange of electrons between atom and colliding beam, and the disturbance of the atomic wave functions by the incident wave. The effect of the second of these phenomena was considered in detail to a first approximation by us and found to be of considerable importance. In this paper, the accuracy of the calculation could not be regarded as high inasmuch as the other disturbing effects were neglected. It was then shown in a later paper how the effect of the disturbance of the incident wave could be included in the calculation, and it was found that for the elastic scattering the agreement with experiment was considerably improved. On applying the method to the calculation of inelastic collision probabilities, the effect of the distortion of the incident wave was found to be small in comparison with, say, the effect of exchange. However, when one considers physically the excitation of an atom by electron impact, it is clear that the distortion of the outgoing wave by the field of the excited atom will be of greater importance than that of the incident wave by the normal atom. This is due to the greater spread of the field of the excited atom and also to the increased wave-length of the outgoing wave. In the same way, the distortion of the outgoing wave may be important in the case of elastic exchange.

The total ionisation due to the absorption in air of slow cathode rays

The object of the experiments here described is to measure the average ionisation produced by the absorption in air of an electron with definite initial energy. From this the average expenditure of energy associated with the formation of a pair of ions can be estimated. The initial energies considered ranged between 200 and 1000 electron-volts. Experiments on ionisation by electronic impact have generally been concerned, either with a determination of the ionisation potential of the gas, or with the ionisation per unit path due to an electron having a definite energy. The ionisation potential has been measured by determining the minimum energy a stream of electrons must have in order to ionise, even occasionally, a normal atom. It represents the energy expended by the ionising electron if no kinetic energy be transferred to either of the ions formed. If at the impact an atomic electron were ejected with appreciable kinetic energy, the energy expended by the ionising electron would be correspondingly increased above the ionisation potential. Also electrons may dissipate their energy by processes other than ionisation, notably by excitation and by dissociation of diatomic molecules. For these reasons the average expenditure of electronic energy per pair of ions should exceed the ionisation potential. The excess of this average energy would indicate the extent to which processes other than ionisation contribute to the dissipation of the initial kinetic energy of the electrons. The purpose of the present experiments is to obtain further information on this phase of the ionisation problem.