Table of contents

It is shown that plasma component separation, 'demixing', occurs in an alcohol vortex arc, in contrast to the mixing present in some other liquid vortex arcs. New transition probability measurements are reported for five CI lines and one SII line. An electron density determination based on the H_beta half-width is only 85% of the result from optically thin radiation measurements.

Absolute cross sections for single detachment from H^- by electron impact have been measured by an inclined beam technique. At interaction energies close to 14.5 eV the detachment function shows structure which is attributed to the formation of H^2- with a lifetime of the order of 10^-15s.

A correction for the quadrupole-quadrupole interaction energy is applied to the quadrupole moment of CO₂ measured from the far infrared collision-induced spectrum. The resultant values for the quadrupole moment of CO₂, obtained using two different theoretical equations, are (4.6+or-0.3)*10^-20 esu cm² and (4.3+or-0.4)*10^-20 esu cm² respectively ((4.6+or-0.3)*10^-31 C m² and (4.3+or-0.4)*10^-31 C m² in SI units).

The close-coupling method for atom-electron collisions is modified to take account of the full polarizability of the atomic states. This effect is tested on the calculations of the polarization of lithium atom resonance lines and the results are in better agreement with the measurements of Hafner and Kleinpoppen (1967).

The relativistic effects on bound levels of atomic systems with particular application to systems of astrophysical importance are investigated by including the most important contributions to the low Z Pauli approximation for the Hamiltonian. This work represents an extension of a general purpose computer program for calculating non-relativistic energy levels of complex atoms, described by Eissner and Nussbaumer (abstr. A10729 of 1970). Their algebraic approach has been adapted to the problems of evaluating matrix elements of the relativistic operators. In particular the program should give fairly good intermediate coupling wave functions. Thus it may be used as a basis for the calculation of intermediate coupling collision strengths and oscillator strengths. Numerical results are given for the magnesium I, sodium I and fluorine I isoelectronic sequences. In each case the calculations reproduce the observed Z dependence of the term differences in a much more satisfactory way than the non-relativistic calculations. Also calculations of spin-orbit parameters are found to compare well both with observed and the calculated parameters of other workers.

The authors have measured the apparent cross section for excitation of 5016 AA radiation (3¹P to 2¹S) by impact of 100 eV electrons on helium as a function of pressure in cylindrical excitation chambers of various proportions. The analysis of Phelps (1958) appears to describe the data provided that his upper bound to the fraction of resonance photons which reach the walls is taken and that the distance from the observed axial region to the closest wall is taken as the effective radius.

For Pt. II see abstr. A44297 of 1969. By using a basis set of symmetrized atomic orbitals the influence of electron exchange and nuclear symmetry is examined for the 2s and 2p collisional excitations of hydrogen by a hydrogen atom with impact energy E in the range 0.25<or=E<or=225 keV. These effects reduce all the cross sections and affect the 2s excitation more than the 2p excitation. At high energies, the results tend to the Born values, indicating that the neglect, in this treatment, of the phase factors which account for the translational motion of the atomic electrons, does not cause much error for excitation.

A classical treatment of inelastic collisions between neutral atoms and charged particles is presented. Detailed balance between the rates for the forward and reverse transitions is satisfied. The theory is used to calculate the cross sections for ionization of atomic hydrogen by electron and proton impact, and for the excitation by electrons of various levels of hydrogen. Comparison of the results with the Born cross sections and experimental data shows that the theoretical description is fairly successful for inelastic transitions to the continuum or to a discrete level sufficiently close to the continuum.

The authors describe an atomic beam resonance experiment to search for an electric dipole movement in the ³P₂ metastable state in xenon. The experiment avoids the ' nu *E' difficulty of earlier work by using an atomic state which has a high differential polarizability. Small residual effects are cancelled by comparison between xenon and krypton. The result is interpreted to give a limit on the electric dipole moment of the electron mod d_e mod =(0.7+or-2.2)*10^-24 e cm at 90% confidence level.

It is predicted theoretically that mercury-cadmium sensitized fluorescence should be modulated in general, when a rotating magnetic field is applied, and a steady magnetic field perpendicular to the plane of the rotating field. The time dependence of the density matrix, expressed in terms of irreducible tensors, is calculated. The final expression for the intensity of the sensitized fluorescence, as a function of time, is shown to be similar to the corresponding one for resonance fluorescence (both cases referring to 'broad-band' excitation).

It has recently been found that when a molecular gas is excited by electronic bombardment the subsequently emitted light is polarized. In this paper the authors investigate the possibility of studying molecular states excited by this technique in the presence of magnetic and/or electric fields. The measurements are made on the polarization of the light emitted from a level for which the total angular momentum is known. They extend to diatomic molecules the theory of irreducible tensor operators, previously applied to such experiments in atoms. All resonant effects observed in atoms may be expected in molecules, for instance in the presence of the magnetic field the effects of level crossing and magnetic resonance, including their transmission in a cascade decay. When an electric field is added, new effects may arise in polar molecules and make it possible to measure the permanent electric dipole moment and the Lambda doubling in some cases.

Using a local effective potential to account for electron exchange, the authors have analysed the zero-energy scattering of ortho-positronium by helium atoms. The effective potential was chosen so that the results obtained in the static approximation agreed with the static-exchange results of Fraser and Kraidy (1966). Correlation was introduced in two different approximations which agree well. A decrease is found in the scattering length a from 1.72 a₀ to 1.39 a₀, and an increase of more than a factor of 2 in the quenching rate of the positronium (from Z_eff=0.042 to 0.10), as compared with the static-exchange approximation. This gives improved agreement with the experimental value Z_eff approximately=0.18.

The paper treats the absorption of light by a helical polymer consisting of monomer molecules whose nuclei can vibrate. The exchange of electrons between monomers is neglected. With monomers having only a single excited electronic state, sum rules are derived for the first three moments of the energy of photons absorbed into the corresponding band in the polymer. There are no further sum rules like the three obtained. The sum rules remain valid in the time-dependent Hartree approximation.

The anomalies found in the spin splitting of a number of ² Sigma states of diatomic molecules are explained by taking the rotational stretching (centrifugal distortion) of the molecule into account by using an adequate quantum-mechanical treatment. This calculation also yields both the true rotational dependence of the parameters p and q describing the Lambda doubling of the ² Pi states, and the centrifugal distortion correction terms to the rotational energy. For the ² Pi states the dependence of the spin-orbit coupling constant A on the internuclear distance gives rise to an additional correction term to the energy of the form A_JJ(J+1). An expression for the parameter A_J in terms of spectroscopic quantities where values are usually known is derived, and tested for several examples.

An attempt is made to explain the variation in the scintillation efficiency of unactivated NaI crystals at low temperature as the specific ionization of the incident radiation varies between 1 and 25 meV cm² g^-1. The theory presented indicates that the variation of luminescent output from pure NaI with temperature is to be associated with the temperature dependent probability of radiative exciton decay, but the theory shows that the exciton formation probability, which the experimental results indicate is also temperature dependent, should be taken into account. Finally, the theory is applied with some success to data for an activated inorganic scintillator used at room temperature.

For pt. IX see abstr. A36447 of 1970. Expressions are derived for the host and guest fluorescence response functions and quantum yields of thin and thick, pure and mixed organic crystals. In addition to exciton migration and transfer, the effects of host exciton traps and of radiative migration and transfer are considered. Host exciton traps and radiative migration are shown to be important in thick anthracene and mixed anthracene-tetracene crystals.

Using flash photolysis and flash discharge techniques, 28 bands of the f¹ Delta _u-a¹ Delta _g transition of S₂ were photographed at high resolution in absorption. Vibrational and rotational analysis of these bands was made. The observation of this system in absorption allows the a ¹ Delta _g state to be identified as arising from the ground state electronic configuration pi _u⁴ pi _g². The predissociation occurring between the v=10 and v=11 levels of the f¹ Delta _u state is probably a c⁺ type predissociation caused by a repulsive ³ Delta _u state.

Using a double ionization chamber in one half of which negative ions are formed and in the other half of which they suffer detachment, experiments on the collisional detachment from O₃^- and O₂^- in oxygen at E/p at 20 degrees C equal to 45-50 V cm^-1torr^-1 give values for the detachment coefficient per unit pressure delta /p approximately 0.01. Pressure variations (20-60 torr) are discussed. For Townsend discharges in these circumstances in oxygen detachment is thus relatively unimportant, in that delta << eta (the attachment coefficient).

230 newly classified spectral lines emitted from higly ionized elements and belonging to the sodium, magnesium, aluminium, silicon, phosphorus, sulphur and chlorine isoelectronic sequences are reported as a result of observations of laser-produced plasmas formed from elements in the period between chlorine and iron. The transitions studied occur between low-lying energy levels and are of importance both to the study of forbidden lines of astrophysical interest and to the identification of iron emission lines in solar spectra. The observation of the Fe XIII 3s²3p²³P-3s3p³³S multiplet provides the most direct confirmation of the identity of the solar forbidden lines at 10 746.8 AA and 10 797.95 AA. Revised wavelengths of emission lines in the scandium spectrum between 320 AA and 800 AA are listed.

Half-widths of several chlorine II lines have been measured behind the reflected shock wave produced in an electromagnetically driven 'T' tube operated in dichlorodifluoromethane. The electron density (2.64-4.27*10¹⁷ cm^-3) was determined by laser interferometry at two different wavelengths while the electron temperature (13 300-17 800 K) was measured from relative intensities of Cl II lines. Measured Cl II linewidths compare fairly well with the theory of Griem. Some difficulties in computing linewidths are underlined.

The absorption spectra of MgH and MgD have been examined under high resolution and several new features observed. In the A² Pi -x² Sigma ⁺ transition the ⁰P₁₂ satellite branch has been identified and assigned for both molecules. Measurements on the C² pi -x² sigma ⁺ system of MgH indicate that the C state is inverted with A= -3.7 cm^-1. The MgD absorptions at 236.4 nm and 236.0 nm, previously considered to belong to independent electronic transitions, have been shown to be, respectively, the P and Q heads of the 1-0 band of the C² Pi -x ² Sigma ⁺ system. Rotational analysis yields the following constants for the C² Pi state of MgD; B_e=3.256 cm^-1, D_e=9.2*10^-5 cm^-1, beta _e=0.25*10^-5 cm^-1, r_e=0.1669 nm and Delta G(¹/₂)=1140.1 cm^-1.

About 90 lines of Ne VII and Ne VIII have been observed in the region 50-140 AA with a theta -pinch discharge as a light source. Use was made of a rotating shutter in front of the spectrograph to eliminate impurity lines.