Table of contents

La méthode optique d'orientation atomique est située parmi l'ensemble des méthodes proposées et mises en oeuvre. L'excitation optique sélective des sous-niveaux Zeeman d'un niveau atomique excité se manifeste dans l'état de polarisation de la lumière de résonance optique. En excitant des atomes avec de la lumière de résonance polarisée circulairement, on peut également produire une population sélective des sous-niveaux Zeeman de l'état fondamental. Les applications des atomes ainsi orientés sont leur utilisation pour des expériences de résonance magnétique sur l'état excité et sur l'état fondamental, la possibilité d'orienter des noyaux atomiques et enfin la production d'électrons polarisés par effet photoélectrique des atomes orientés.

By use of a King furnace the absorption spectra of In vapour in the near and vacuum ultra-violet have been observed. Fourteen new absorption lines of the In I^b spectrum have been found in the Schumann region, and the normal sharp and diffuse series extended. Classification of the strongest of the vacuum ultra-violet lines leads to an amendment of the sp² term assignments of McCormick and Sawyer in the isoelectronic spectrum of Sn II.

The analysis of Davydov and of Tonks and Allis has been repeated to obtain the electrical and energy transport equations for a plasma subject to a magnetic field in which the isotropic part of the distribution is assumed Maxwellian but in which the momentum transfer due to electron self-interactions can be neglected. The validity of the various simplifying assumptions is considered. The exact coefficients have been evaluated and because of their unwieldy nature simpler approximate expressions are suggested and compared with the correct coefficients. The galvomagnetic and thermomagnetic effects are considered and it is shown that in addition to the Hall effect the Ettingshausen effect may also be important in laboratory discharges. The latter is suggested as a possible cause of the retrograde motion of an arc spot in a transverse magnetic field.

The β-spectrum of ²⁰⁴T1 has been measured using thin evaporated sources in a spheroidal field β-spectrometer. The end-point was found to be at 766±2 keV. Small deviations were found from the unique shape of spectrum predicted for a first-forbidden transition with a spin change of two units. The deviations extend up to about 400 keV.

The possibility that the decay is second forbidden and that the ground state of ²⁰⁴T1 has even parity is noted.

It is shown that the deviations of nuclear magnetic moments from the Schmidt lines can be accounted for in terms of simple interconfigurational mixing. In particular it has been found possible to explain the large deviation of ²⁰⁹Bi.

The paper gives the results of X-ray measurements on the variation of the lattice parameter of lithium with temperature. The mean thermal coefficient of expansion of lithium varies from 3.5 × 10^-5 at -914°C to 4.7 × 10^-5 at 20°C. The expansion of the material over this range of temperature is closely represented by the equation a_t = a₀(1 + αt + βt² + γt³) where a₀ = 3.4992kx; α = 4.6640 × 10^-5; β = 3.8791 × 10^-8; γ = -11.445 × 10^-11. After deformation at liquid air temperature lithium is found to contain a face-centred cubic structure as well as a body centred cubic structure, which confirms the findings of C. S. Barrett. The lattice parameters of these structures at -194°C were found to be 3.4760kx (body-centred cubic) and 4.370kx (face-centred cubic). After annealing the material at room temperature and afterwards obtaining its x-ray pattern at liquid air temperature the body-centred cubic structure remained alone.

It is shown that Feynman's proof of the existence of a generalized Bose-Einstein condensation in liquid helium can be carried through on the basis of fewer and less restrictive assumptions than he actually makes. The underlying physical idea proves to be very similar to those put forward by a number of other workers. His treatment using a Bijl wave function is also examined, and it is concluded that the resulting energy-spectrum may very well be nearly correct, in spite of the fact that it does not agree numerically with that assumed in the present form of the Landau-Khalatnikov theory. Some serious difficulties in the latter theory seem to call for an adjustment of the parameter Δ to a larger value. This is in just the direction needed to improve the agreement with Feynman's theory.

The Born approximation is applied to the collision of electrons with normal hydrogen molecules. Cross sections are calculated for collisions accompanied by the vibration-rotation transitions 0, 0 → n,l ([n = 0, 1], [l = 0, 2, 4]) for incident electron energies between 0 and 250 eV. Special attention is given to the choice of the interaction potential, the form of which has a marked effect upon the pure vibrational excitation function in the low-energy region where a sharp peak appears. The rotational excitation does not show the same sensitivity. Elastic scattering is also treated and the results are in satisfactory agreement with those of previous workers.

The photo-ionization cross section of lithium is calculated using Hartree-Fock wave functions. Good agreement is obtained between the dipole length and dipole velocity formulae for the cross section over the wavelength range considered, 2300 Å-1800 Å. The mean value at the spectral head is found to be 1.16 × 10^-18 cm², considerably smaller than either of the previous theoretical estimates by Hargreaves (1929) and by Trumpy (1929). The cross section curve rises on the short wavelength side of the series limit, as was found by Hargreaves.

Experiments are described in which quanta emitted by water under alpha-particle bombardment have been detected. Differentiation between light pulses arising from liquid water and any gas present above the water surface has been accomplished by utilizing the difference in their times of duration. From measurements made of the absorption of the quanta from water by various gases an upper limit of the wavelength of 1900 Å is suggested. The importance of quantum emission in radiation chemical experiments is discussed but, owing to the apparently very high absorption coefficient of the water, it is difficult to assess the contribution from this phenomenon.

Previous calculations of the photo-ionization cross section of neon have been extended to wavelengths just beyond the L₁ edge at 256Å. The absolute value of the calculated cross section is found to be in reasonable agreement with the experimental results of Lee and Weissler and of Ditchburn and Marr, but compared with the calculated results the experimental curve rises more steeply just beyond the threshold and falls more rapidly at higher energies.

In two recent papers by the author in cooperation with Devons and with Lindsay, investigations of internally produced pairs in various nuclear reactions in solid targets have been reported. In this paper an apparatus is described for the investigation of internal pairs produced in a ¹⁵N gas target by proton bombardment. A search was made for a cascade transition from the 13.05 MeV level in ¹⁶O through the 6.06 MeV level to the ground state, but no indication for such a reaction could be found.

Several new band systems have been observed in the absorption spectra of BiSe and BiTe molecules in the vapour state at high temperatures (1000° to 1300°C). All the band systems lie in the quartz ultra-violet region. The band-heads have been analysed and the following constants obtained:

Molecule System	Å	νe	ωedouble prime	ωedouble primexedouble prime	ωe'	ωe'xe'
BiSe I	2900-2700	35617.5	264.7	0.40	304.00	2.00
BiSe II	2350-2200	44425	264.7	0.40	316.00	2.00
BiTe I	2942-2814	-	208.5	0.52	-	-
BiTe II	2454-2382	-	208.5	0.52	-	-
BiTe III	2390-2315	42870	208.5	0.52	164.40	0.40
BiTe IV	2374-2279	43116	208.5	0.52	263.00	0.96
BiTe V	2276-2200	-	208.5	0.52	-	-