The atomic-beam magnetic-resonance method has been used to measure the magnetic dipole and electric quadrupole hyperfine-structure interaction constants $A$ and $B$ for the ${}_{}{}^3{}_{}{}^{}D_3^{}{}_{}{}^{}$, ${}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}$, and ${}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}$ states of the... $\mathrm{}\left(6s\right)\mathrm{}\left(5d\right)\mathrm{}$ configuration for ${}_{}{}^{135}{}_{}{}^{}\mathrm{Ba}$ and ${}_{}{}^{137}{}_{}{}^{}\mathrm{Ba}$. The experimental results for ${}_{}{}^{135}{}_{}{}^{}\mathrm{Ba}$ are $A\left({}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}\right)=-73.429\mathrm{}\left(4\right)\mathrm{}$ MHz, $B\left({}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}\right)=+38.710\mathrm{}\left(15\right)\mathrm{}$ MHz, $A\left({}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}\right)=+370.6\mathrm{}\left(7\right)\mathrm{}$ MHz, $B\left({}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}\right)=+18.3\mathrm{}\left(22\right)\mathrm{}$ MHz, $A\left({}_{}{}^3{}_{}{}^{}D_3^{}{}_{}{}^{}\right)=+408.1\mathrm{}\left(14\right)\mathrm{}$ MHz, $B\left({}_{}{}^3{}_{}{}^{}D_3^{}{}_{}{}^{}\right)=+20\mathrm{}\left(8\right)\mathrm{}$ MHz; and for ${}_{}{}^{137}{}_{}{}^{}\mathrm{Ba}$, $A\left({}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}\right)=-82.180\mathrm{}\left(3\right)\mathrm{}$ MHz, $B\left({}_{}{}^1{}_{}{}^{}D_2^{}{}_{}{}^{}\right)=+59.564\mathrm{}\left(14\right)\mathrm{}$ MHz, $A\left({}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}\right)=+413.9\mathrm{}\left(9\right)\mathrm{}$ MHz, $B\left({}_{}{}^3{}_{}{}^{}D_2^{}{}_{}{}^{}\right)=+26.8\mathrm{}\left(30\right)\mathrm{}$ MHz, $A\left({}_{}{}^3{}_{}{}^{}D_3^{}{}_{}{}^{}\right)=+455.4\mathrm{}\left(16\right)\mathrm{}$ MHz, $B\left({}_{}{}^3{}_{}{}^{}D_3^{}{}_{}{}^{}\right)=+36\mathrm{}\left(9\right)\mathrm{}$ MHz. A comparison is made between the value of the electric quadrupole moment of the ${}_{}{}^{137}{}_{}{}^{}\mathrm{Ba}$ nucleus obtained from these results, which depend on the interaction of the $5d$ electron, and the value obtained from earlier results, which depend on the interaction of the $6p$ electron. There is a substantial difference in the values of Q(${}_{}{}^{137}{}_{}{}^{}\mathrm{Ba}$) obtained from these two sets of measurements if one does not take the quadrupole shielding into account. Taking the quadrupole shielding into account markedly reduces this difference.

A term representing interference between classical paths is isolated from the previously derived equation for the semiclassical shape of satellite bands. A band shape quantitatively accurate in the adiabatic one-perturber approximation is given by addition of the classicalpath contributions to the interference term. Comparison is made to a quantum-mechanical calculation of a satellite band in the red wing of the self-broadened Lyman-$\alpha$ line of hydrogen and to an experimentally observed band in the blue wing of the cesium 8521-$\mathrm{A}$ resonance line perturbed by argon. The problem of inversion to determine a difference potential from the observed band shape is discussed.

An investigation of the influence of self-consistency effects on the nuclear quadrupole antishielding factor ${\gamma }_{\infty }$ in case of ${\mathrm{Rb}}^{+}$ and ${\mathrm{Cs}}^{+}$ has been made. A new procedure is adopted to calculate this effect where use has been made of the first-order perturbed functions which had been utilized earlier to calculate the conventional ${\gamma }_{\infty }$. It is found that the effect of consistency is within 1% in both cases.

The lifetime and binding energy of the lowest-lying metastable quartet state, $\mathrm{}\left(1s2s2p\right)\mathrm{}{}_{}{}^4{}_{}{}^{}p°_{\frac{5}{2}}^{}{}_{}{}^{}$ in ${\mathrm{S}}^{13+}$ have been measured by studying the forbidden (spin-spin-induced) autoionizing decay in flight of the state after foil excitation. The state is found to be bound by -4988 ± 26 eV and has a lifetime of 1.1 ± 0.1 nsec. A summary of our previous measurements of these same quantities for other members of the lithiumlike sequence is also given, along with an estimate of the branching ratio for the autoionization process.

Several lines, observed between 9 and 30 eV in the energy spectra of electrons ejected in 100-eV to 10-keV collisions of ${\mathrm{H}}^{-}$ with He, Ar, and ${\mathrm{H}}_2$, are attributed to autodetaching states of ${\mathrm{H}}^{-}$. The energy of the lowest line associated with the $\left(2\mathrm{}{s}^2\right){}_{}{}^1{}_{}{}^{}S$ state of ${\mathrm{H}}^{-}$ was found to be 9.59±0.03 eV. Two lines due to ${\mathrm{Ar}}^{-}$ autodetaching states were used to calibrate the energy scale. The separations in energy between the $\left(2\mathrm{}{s}^2\right){}_{}{}^1{}_{}{}^{}S$ line of ${\mathrm{H}}^{-}$ and several other lines lying below the $n=2\mathrm{}$ and $n=3\mathrm{}$ levels of hydrogen were measured. The present results agree well with theoretical predictions and with the experimentally observed resonances in the elastic scattering of electrons on atomic hydrogen.

A formulation for the elastic scattering of electrons from molecules is presented based on an independent-particle model (IPM) and a fixed-nuclei approximation. Numerical computations are carried out for $e^{-}+{\mathrm{N}}_2$ scattering. A phenomenological, energy-independent IPM potential is found to be capable of qualitatively reproducing the total and differential cross sections over a wide range of energy from 10 to 500 eV.

A multichannel semiclassical collision theory, based on Feynman's path-integral formulation of quantum mechanics and discussed previously by us, is applied to the problem of vibrational excitation during collinear atom-diatom collisions. The diatom is treated as a harmonic oscillator and the atom-diatom interaction is an exponential repulsion. The semiclassical results are compared with the exact quantum-mechanical calculation of Secrest and Johnson and with the classical $S$-matrix theory of Miller and Marcus. It is found that the present theory is not as generally useful as the theory of Miller and Marcus.

The photoelectric cross section of molecular hydrogen has been measured by counting photoelectrons released by monochromatized x rays traversing highly purified ${\mathrm{H}}_2$ gas. Results are 54.0 ± 2.9 mb/atom at 5.41 keV and 12.0 ± 0.6 mb/atom at 8.39-keV incident-photon energy. The effect of molecular bonding is pronounced: The measured cross section, per atom, exceeds the theoretical atomic cross section by (44 ± 8)% at 5.41 keV and by (45 ± 7)% at 8.39 keV. A comparable effect has been predicted by Kaplan and Markin.

Collisions of 0.3-5-keV (4-20×${10}^6$ cm/sec) ${\mathrm{N}}_2^{+}$ ions with He and Ne targets have been studied by spectroscopic observation of the ${\mathrm{N}}_2^{+}$ first-negative-emission spectrum. Vibrational excitation within the collisionally excited $X{}_{}{}^2{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}\to B{}_{}{}^2{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ transition in ${\mathrm{N}}_2^{+}$ was studied by a measurement of the relative band intensities of the $\Delta v=-1\mathrm{}$ sequence of the first-negative system. Rotational excitation was examined by observation of the rotational line intensities of the (0,0) first-negative band ($\lambda =3914\mathrm{}$ ÅA). Although the ${\mathrm{N}}_2^{+}$ ions used for these experiments were highly vibrationally and rotationally excited by the ion-source discharge, the initial-energy distribution could be inferred from the ${\mathrm{N}}_2^{+}$ emission spectra produced by high-velocity ($v>\mathrm{}1.2\mathrm{}\times {10}^7$ cm/sec) collisions. At these velocities, populations of vibrational levels in the upper state were independent of projectile-ion velocity and the rotational-energy distribution corresponded to a Boltzmann distribution at 3700 °K. At lower velocities, the population of high vibrational states increased with decreasing velocities and deviations from a Boltzmann distribution among rotational states were observed. Excitation to high rotational states increased monotonically with decreasing projectile-ion velocity. No differences in the rotational and vibrational excitation produced by He and Ne were observed. These results show that for some ion-molecule collisions there is vibrational and rotational excitation in the direct electronic excitation (non-charge-transfer) channel.

A model to describe the slowing down of fast-moving heavy particles in matter owing to inelastic collisions is devised using the binary-encounter approximation, and stopping cross sections are calculated specifically for helium in hydrogen. In performing the calculations, each incident-beam charge-state component is treated separately, and the results are combined using the best available experimental information on the equilibrium charge-state fractions. The results, both for the partial stopping cross sections of the individual charge-state components and the total stopping cross section of the equilibrium mixture, are compared as a function of incident energy with the results of various experiments and with other theories. The calculated total stopping cross section is in reasonable agreement with experiment over a rather wide energy range and is superior at energies below 1 MeV to the results of the Bethe theory, but discrepancies exist between calculated and experimentally determined partial stopping cross sections. These discrepancies are discussed and possible explanations for them are suggested.

A previously developed model which describes the slowing down of energetic heavy charged particles in matter due to electronic collisions is generalized to multielectron target atoms. The model utilizes the binary-encounter approximation to describe the slowing-down collisions and explicitly takes into account the equilibrium charge-state composition of the incident ion beam. Extensive calculations of the electronic stopping cross sections of atomic targets from hydrogen to krypton for incident helium are made using the model, and the results are presented as a function of incident energy between 0.1 and 10 MeV for selected targets and as a function of target atomic number for various incident energies. Agreement with experiment is fairly good for light target elements, but the calculated results seriously overestimate measured cross sections for the heavy transition elements at large energies. Possible explanations of this discrepancy are discussed and comparisons are made with various other theories.

The two-photon excitation and three-photon ionization of atomic cesium is investigated over the 6550-6950-Å wavelength region with a tunable-dye-laser source having a 0.06-0.08-Å linewidth and a space-charge ionization detector sensitive to a few ions per second. An online data-acquisition computer provides significant signal-to-noise recovery. The resulting dispersion curve for photoionization is interpreted in terms of the two-photon transitions from the ground $6{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ level to resonant $n{}_{}{}^2{}_{}{}^{}D$ and $n{}_{}{}^2{}_{}{}^{}S$ intermediate states, and represents, to the authors' knowledge, the first such two-photon absorption spectrum of Cs. Transitions from $n=9\mathrm{}\mathrm{to}13$ have been recorded for the $6{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}-n{}_{}{}^2{}_{}{}^{}D_{\frac{5}{2},\frac{3}{2}}^{}{}_{}{}^{}$ series and from $n=11\mathrm{}\mathrm{to}14$ for the $6{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}-n{}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}$ series.

The practical problem of consistent use of a phase convention in calculating differential cross sections from scattering or reactance matrices is discussed, and in two instances (electron-hydrogen-atom and positron-hydrogen-atom scattering) where an inconsistency has been found, we correct some previously published differential-cross-section formulas. We present the first correct calculations of close-coupling-approximation differential cross sections for the $1s-2p$ electron-hydrogen-atom scattering process. We compare the differential cross sections from the 3-state and the 6-state close-coupling-approximation scattering matrices of Burke and co-workers.

The excitation of argon and helium metastables by electron impact was studied in the energy range from threshold to about 50 eV with a time-of-flight method. The metastables were detected by Auger ejection of secondary electrons from a CuBeO surface. The argon cross section was obtained in two different ways yielding peak cross sections of 3.4 × ${10}^{-17\mathrm{}}$ and 3.7 × ${10}^{-17\mathrm{}}$ ${\mathrm{cm}}^2$ at 22 eV, respectively. The estimated uncertainty for both values is a factor of 2. The helium cross section was measured with a trapped-electron method near threshold and found to have a peak value of (6.2 ± 2.0) × ${10}^{-18\mathrm{}}$ ${\mathrm{cm}}^2$ at the 25-eV maximum. The secondaryelectron yields of the metastable detector used in this work are discussed in detail. The effect of metastable recoil is also discussed. Argon metastables suffer only negligible recoil because of their large atomic weight, but this is not the case for the lighter helium metastables.

By exploiting the soluble infinite tridiagonal (Jacobi)-matrix problem generated by evaluating a zeroth-order scattering Hamiltonian $H_0$ in a certain $L^2$ basis set, we obtain phase shifts, wave functions, etc., which are exact for a full Hamiltonian $H$ in which only the potential $V$ is approximated. Only bound-bound ($L^2$) matrix elements of the Hamiltonian and finite matrix manipulations are needed. The method is worked out here for $s$-wave scattering using Laguerre basis functions. Kato improvement of the results and necessary generalizations to many channels are treated.

The method of the preceding paper is here applied to $s$-wave electron-hydrogen collisions. A pseudo-state model is developed and applied to elastic and inelastic scattering above and below the ionization threshold where we compare with the results of Burke and Mitchell. Additional calculations show the effect of Kato correction, the effect of the closed-channel asymptotic forms, and include a comparison with the results of Schwartz.

The joint energy and angular-distribution function of protons transmitted through a thin foil when incident as 2-MeV ${\mathrm{H}}_2^{+}$ has been measured. The experimental result is compared with simple calculations and satisfactory agreement is obtained.

The Glauber theory for electron-atom collisions is extended to include the polarization effects. The results for the differential and the total elastic-scattering cross sections are presented, and it is observed that the agreement with the available data is improved with the inclusion of such effects.

The total scattering cross section of molecular nitrogen for electrons in the energy range 0.3 to 1.6 eV has been measured absolutely by single-electron time-of-flight spectrometry, in which care has been taken to ensure positive removal of the scattered electrons and the gathering and reduction of data were in accordance with a thorough analysis of both statistical and systematic errors. The total cross section between 0.9 and 1.5 eV is found to be numerically equal to the momentum-transfer cross section of Englehardt, Phelps, and Risk; at lower energies it is appreciably lower than the momentum-transfer cross section. No evidence is found for the periodic structure reported in this cross section by Golden.

The impulse approximation of Chew is applied to the calculation of the cross sections for rotational and vibrational excitation of diatomic molecules by collisions with an atom. The assumptions underlying the impulse approximation are discussed and found to be justified for these processes. A closure approximation is used to perform the sums over rotational quantum numbers in the expression for the vibrational-excitation cross section and the resulting expression depends on two factors; an oscillator form factor and the scattering amplitudes for atom-atom scattering. Vibrational-transition probabilities, evaluated for oxygen-carbon monoxide collisions assuming that the oscillator is harmonic and using a rough estimate for the typical atom-atom scattering amplitudes, are in qualitative agreement with experiment. Further refinements and extensions of the model are discussed.