The authors have recorded beam-foil spectra of Fe v and Fe vi between 1100 and 1900 Å at ion energies ranging from 0.4 to 1.5 MeV. For the first time, lifetimes have been measured for thirteen $3d^{3}4p$ levels in Fe v and six $3d^{2}4p$ levels in Fe vi. These lifetime results are not in agreement with the available theoretical values deduced from the transition probabilities calculated by Abbott.

A previously derived low-frequency approximation for scattering in a laser field represents the amplitude for scattering with the absorption or emission of a specified number of photons in terms of the physical field-free scattering amplitude, a result which holds even with the inclusion of a correction term of first order in the frequency of the field. In the general case, where the polarization of the field is arbitrary and the dipole approximation is not assumed, the cross section depends on the phase as well as the magnitude of the field-free scattering amplitude. It is shown here that this phase dependence disappears when the cross section is summed over all possible final states of the field. The result of this summation is identical (within the limits set by the domain of validity of the low-frequency approximation itself) to that which would be predicted from a classical description of the motion of the electron in the field, the collision taking place instantaneously and without influence from the field. This sum rule and its classical interpretation, obtained here in a nonrelativistic potential scattering model, is of the form derived some time ago by Brown and Goble on the basis of general field-theoretic considerations.

The specific primary excitation of helium and of neon has been measured. Momentum-separated particles from an accelerator, identified by Čerenkov counters, were directed through a series of 69 spark chambers. The specific primary excitation was derived from the measured spark-chamber efficiency as a function of particle momentum. The numbers of primary electrons at the minimum of ionization $N_{min}$ and at the Fermi plateau $N_{\mathrm{pl}}$, per centimeter of gas at normal pressure and temperature, are the following: in helium $N_{min}=3.5\mathrm{}\pm 0.2$; $N_{\mathrm{pl}}=5.1\mathrm{}\pm 0.4$; in neon $N_{min}=11.6\mathrm{}\pm 0.3$; $N_{\mathrm{pl}}=18.1\mathrm{}\pm 0.7$. For a particle with momentum $p$ and mass $m$ the Fermi plateau starts at ${\left(\frac{p}{m}\right)}_{\mathrm{pl}}\sim 80$ for helium and at ${\left(\frac{p}{m}\right)}_{\mathrm{pl}}\sim 70$ for neon. Significant discrepancies exist between the various experimental and theoretical results. The main cause of the experimental discrepancies can be explained by the different energy sensitivities of the detectors.

Configuration-interaction (CI) potential curves have been calculated for the ten lowest singlet molecular states of Mg${\mathrm{H}}^{+}$. The $X{}_{}{}^1{}_{}{}^{}\Sigma$, $A{}_{}{}^1{}_{}{}^{}\Sigma$, and $B{}_{}{}^1{}_{}{}^{}\Pi$ molecular states are calculated to be bound by 1.93, 1.93, and 0.18 eV at equilibrium separations 3.16, 3.84, and $4.41a_0$, respectively. These potential-well characteristics are in reasonable accord with analyses performed on spectroscopic data. The CI potential curves were employed in a three-channel scattering calculation to estimate ${\mathrm{H}}^{+}$ + Mg electron-capture cross sections at velocities (0.1-1.4) × ${10}^8$ cm/sec. Capture with excitation of the ion dominates with H + ${\mathrm{Mg}}^{+}$ ($3p$) being the major electron-capture channel at 1 × ${10}^8$ cm/sec. At higher velocities, a classical-trajectory Monte-Carlo method was used to calculate both the electron-capture and impact-ionization across sections for the ${\mathrm{H}}^{+}$ + Mg collision pair. The calculated electron-capture cross sections are in reasonable agreement with the recent measurements of Morgan and Eriksen.

The authors have observed the alignment of the $5p{}_{}{}^2{}_{}{}^{}P_{\frac{3}{2}}^{}{}_{}{}^{}$ state of strontium ions produced in Penning collisions between an unpolarized beam of helium metastable atoms and a strontium vapor target. The alignment is shown by a linear polarization of the optical emission from the excited ion. For a 66-meV beam of helium metastable atoms a 3.5% linear polarization of the emission relative to the beam axis was measured. It is shown how the alignment may be used to determine the probabilities for populating the various final quasimolecular states of the ion-atom pair. The alignment of the Penning ions is an important new parameter in the description of these reactions.

The authors study the phenomenon of coherent Raman beats in ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$${\mathrm{H}}_3$ using a C${\mathrm{O}}_2$ laser and Stark switching. In particular, a three-level system in which the two upper levels always remain split by a few MHz is prepared by switching the transitions into resonance with a short Stark pulse whose bandwidth is large enough to coherently excite both transitions. When one of the coherently excited levels remains in resonance with the laser after the Stark pulse, this system exhibits a qualitatively new effect which has not been seen heretofore. For Stark shifts on the order of the upper-state splitting or smaller, the Raman beat is amplitude modulated at a frequency related to the optical nutation frequency. This is interpreted as being due to an interaction between the two-photon coherent Raman beat process and a single-photon optical nutation process which occurs simultaneously. Numerical calculations as well as a simple analytic model are presented to support this interpretation. By reducing the laser power, one can make the modulation of the Raman beat disappear. From the Raman signal in this regime the permanent electric dipole moment of ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$${\mathrm{H}}_3$ in an excited vibrational state is determined and the Raman beat decay rate measured. By comparing the latter result with a delayed optical nutation measurement we show that phase-changing collisions are negligible for the transition studied.

A relativistic amplitude for elastic scattering of electrons and positrons by hydrogen atoms or hydrogenlike ions in the ground state is derived in the straight-line Glauber approximation without exchange terms. A small-angle approximation valid to first order in the fine-structure constant is used for the relativistic corrections to the input amplitudes describing the scattering of the projectile by the proton and the electron of the target, and spin-flip terms are included. The results are given in closed form in terms of hypergeometric functions.

Electron-impact excitation of the Li $3S$, $4S$, $3D$, and $4D$ states have been measured from threshold to 1200-eV impact energy. The intensities of emission lines from these states have been measured relative to the $2P-2S$ resonance line, and cross sections are obtained by normalizing to the resonance-line cross section and correcting for branching ratios. Cascade corrections to these total cross sections have been made at the higher energies, using available Born cross sections. The resulting direct cross sections show a uniform convergence at high energy to available Born cross sections and to $E^{-1\mathrm{}}$ behavior. At lower energies the $S$- and $D$-state cross sections have very different forms: the $S$ states rise abruptly at threshold and drop rapidly past a narrow peak, while the $D$ states rise gradually to a very broad maximum. Comparisons are made to available cross section calculations.

A semiempirical analysis is made of the contributions of the two lowest resonant states to dissociative attachment and vibrational excitation in low-energy collisions of electrons with ${\mathrm{H}}_2$ and ${\mathrm{D}}_2$. The resonance models are based both on ab initio calculations and on fits to experimental data. The dissociative-attachment cross section near threshold is enhanced significantly by vibrational or rotational excitation of the initial molecule. Near 10 eV, contributions from both resonances are required to explain the observed cross sections for vibrational excitation.

Cross sections have been computed for single and double electron transfer from the ground state of a helium atom to the ground state of an $\alpha$ particle at high velocities by means of the independent-electron approximation with the Bates-Born version of first-order perturbation theory. Results are in reasonable agreement with experiment for both single and double electron capture.

Five-state close-coupling equations for electron-heliumlike-ion scattering are solved in the energy region from near threshold to five times threshold energy. Collision strengths for excitation of the ground state to the $n=2\mathrm{}$ states ($2{}_{}{}^3{}_{}{}^{}S$, $2{}_{}{}^1{}_{}{}^{}S$, $2{}_{}{}^3{}_{}{}^{}P_{}^0{}_{}{}^{}$, and $2{}_{}{}^1{}_{}{}^{}P_{}^0{}_{}{}^{}$) are presented for Liii, Cv, Ovii, and Sixiii. Fair quantitative agreement is obtained with the measurements of Rogers et al. for $\Omega (1\mathrm{}{}_{}{}^1{}_{}{}^{}S, 2{}_{}{}^3{}_{}{}^{}P_{}^0{}_{}{}^{})$ for Liii. Rate coefficients are given in parametrized form.

Absolute optical electron-impact excitation functions for 24 transitions of the sharp, principal, diffuse, and fundamental spectral series of potassium have been measured. The determination of the density of the potassium vapor in the collision chamber was made by measuring the degree of transmission, by the vapor, of potassium resonance radiation generated externally in a fluorescence cell. From the measured optical excitation functions direct excitation functions have been determined for 14 states ($5S$, $6S$, $7S$, $8S$, $4P$, $5P$, $6P$, $7P$, $3D$, $5D$, $6D$, $5F$, $6F$, and $7F$) with the aid of known radiative-transition probabilities compiled in the literature. Theoretical calculations of these same 14 excitation functions, as well as $4D$ and $4F$, were carried out by means of the Born approximation. The $4P$, $5P$, $5S$, $3D$, and $4D$ direct excitation functions at intermediate energies (10-25 eV) were also calculated by the method of multistate close coupling, neglecting projectile-target-electron exchange. The high-energy (above 100 eV) Born-approximation cross sections agree with the experimental results for $4P$ and for all $S$ states, but are lower than experimental results, by 30-40%, for the $D$ and $F$ states. At intermediate energies the close-coupling excitation calculations agree well with the experimental excitation functions for $4P$ and $5P$, but are significantly higher than experimental values for $5S$ and $3D$. The discrepancies between the experimental and theoretical results are probably due to a combination of systematic experimental errors, errors in the available transition-probability values, and errors in the theoretical excitation functions introduced by the use of approximate excited-state wave functions (Hartree-Fock-Slater), or, in the case of intermediate or low energies, by the neglect of projectile-target-electron exchange. The polarization of the $4P-4S$ and $3D-4P$ radiation produced by electron impact was measured, and the results were used to determine the direct excitation functions of the separate magnetic sublevels of the $4P$ state.

Total differential scattering cross sections for ion-atom collisions are measured and compared with predictions from various interatomic potentials. Ions with atomic numbers $6\le Z_1\le 54$ and energies 2.5-400 keV are scattered by xenon through 1-6°. For closest separations, $\le 1.5a_0$, the scattering behaves nearly elastically with universal scaling properties, and individual statistical potentials give successful predictions including peak structures. For larger separations a strong $Z_1$ dependence appears and statistical potentials fail.

In a slow collision between a multiply charged projectile ion and a neutral target atom, self-consistent molecular orbitals in the entrance channel can be defined by introducing a variational constraint which enforces the initial charge sharing and reflects the charge-equilibration process during the collision. In the present work, the interaction of an auxiliary electric field with the electronic dipole moment along the internuclear line is added to an effective single-electron Hamiltonian in order to simulate a constrained Hartree-Fock procedure. In this way, correlation diagrams for the systems ${\mathrm{N}}^{2+}$ + O and ${\mathrm{Ne}}^{4+}$ + Ne are computed. The diagrams exhibit crossings between energy curves of like symmetry. The correlations are found to be the same as in the corresponding charge-balanced (or neutral) systems and are hence consistent with existing experimental data.

A modified Glauber method proposed recently for electron-atom scattering is extended to the analysis of the 1-2 excitation of atomic hydrogen by electron impact. Exchange effects are not neglected, but included in the calculation through the exact eikonal exchange formulas. The results are compared with experimental data acquired by absolute measurements. Good agreement with experimental data is found. The differential cross sections calculated with the conventional Glauber amplitude, but without neglecting the exact eikonal exchange effects, are also obtained and shown for comparison.

Born differential and total cross sections for electron-impact ionization of atomic oxygen are presented. A configuration-interaction wave function was used in the representation of the initial state, and a close-coupling wave function, in which the three ground-state terms of the residual ion are retained in the close-coupling expansion, was used in the description of the final state.

The authors discuss two discrete-basis-function approaches to the soluion of the $T$-matrix equations for the scattering of electrons by atoms or molecules. Both methods, one of which is based on the Schwinger variational principle, have major advantages over the previously proposed $T$-matrix methods and do not require large-basis-set expansions. Results are reported for $s$- and $p$-wave scattering for helium.

$K$-shell ionization of thick targets of Ti, Fe, Ni, and Zn by 60-150-keV protons has been investigated. Experimental x-ray production cross sections have been determined as a function of proton energy. Absolute ionization cross sections are compared to predictions of the perturbed-stationary-state theory, including Coulomb deflection and approximate relativistic corrections (CPSSR). It is found that the CPSSR systematically overpredicts the values for the experimental data at the lower proton energies.

The authors report studies of collision-induced features of the $3S_{\frac{1}{2}}\to 3P_{\frac{1}{2}}\to 4D_{\frac{3}{2}}$ two-photon absorption in atomic-sodium vapor undergoing collisions with several rare-gas perturbers. The results yield the collisional redistribution function which describes the nonresonant excitation of the $3P_{\frac{1}{2}}$ intermediate state. Good agreement with theory is obtained only if effects of $3P_{\frac{1}{2}}\leftrightarrow 3P_{\frac{3}{2}}$ state-changing collisions are included in the calculations. Results are used to obtain a value for a pressure-broadening coefficient in the Na-Kr system.

Using a continuous-wave, single-frequency, tunable dye laser, the authors have made accurate absorption measurements of the band oscillator strength for various rotational-electronic lines in the ${}_{}{}^2{}_{}{}^{}\Sigma _{}^{+}{}_{}{}^{}(v^{\prime }=0)\leftarrow {}_{}{}^2{}_{}{}^{}\Pi (v^{\prime \prime }=0)$ transitions of OH. The band oscillator strength is seen to decrease linearly with the energy of rotational excitation. It is established that this decrease results solely from lengthening of the internuclear distance due to rotation.