Discrete energy levels of ${\mathrm{H}}_3$ are observed by measuring the translational-energy spectrum of H-${\mathrm{H}}_2$ fragment pairs arising from predissociation of ${\mathrm{H}}_3$. The translational-energy spectrum exhibits structure in the range from 3.5 to about 6 eV. The excited ${\mathrm{H}}_3$ molecules are obtained in electron-capture collisions of 10-keV ${\mathrm{H}}_3^{+}$ ions on ${\mathrm{H}}_2$. The ${\mathrm{H}}_3^{+}$ ions are produced in an electron-impact ion source at high hydrogen pressure. The translational-energy spectrum of H-${\mathrm{H}}_2$ fragment pairs is measured with a flight-time-difference method. The energies and intensities of the observed peaks are compared with calculations by Porter. The observed levels can be attributed to vibrational levels in the upper Jahn-Teller sheet of ${\mathrm{H}}_3$. The spectrum can be described very well using the following parameters: The ground vibrational level in the upper Jahn-Teller sheet lies at 3.50 eV above H+${\mathrm{H}}_2$ in its ground vibrational level. The vibrational quanta are 0.32 and 0.49 eV for the breathing and degenerate vibrations, respectively.

Matrix equations for electron-molecule scattering including vibrational and rotational coupling are derived in the framework of the $T$-matrix expansion in discrete basis sets. The basis functions employed are products of scattered-electron orbitals with vibrational or rotational functions. The matrix elements are only marginally more difficult to calculate than in the adiabatic-nuclei version. It is expected that the implementation of the formalism for vibrational or rotational coupling (though possibly not both) is quite feasible.

The energy straggling of partially stripped ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ ions incident at energies from 3 to 36 MeV on a thin Al foil has been measured. The results are interpreted as due to a combination of the statistical processes of collisional energy loss and charge exchange. Analysis of the observed straggling in terms of these processes in the energy region 20-36 MeV leads to values of the electron-capture and -loss cross sections which are consistent with Bohr's simple theoretical description.

A general theoretical approach is proposed for the calculation of elastic, virbational, and rotational transitions for electron-molecule scattering at intermediate and high-electron-impact energies. In this formulation, contributions to the scattering process come from the incoherent sum of two dominant potentials: a short-range shielded nuclear Coulomb potential from individual atomic centers, and a permanent/induced long-range potential. Application to $e$-${\mathrm{N}}_2$ scattering from 50-500 eV incident electron energies has yielded good agreement with absolutely calibrated experiments. Comparisons with other theoretical approaches are made. The physical picture as well as the general features of electron-molecule scattering process are discussed within the framework of the two-potential approach.

An analysis of the differential cross section in the presence of orbiting is given. It is shown that the Regge representation of the scattering amplitude is particularly useful for such a case. The orbiting oscillations in the cross section of H-Hg are very pronounced and they can be described by only a few Regge poles. The velocity averaging can be done with ease and in the presence of orbiting can substantially change the differential cross section.

The problem of scattering of a charged particle (an electron) by a potential in the presence of single-mode classical electromagnetic field (a laser) in the dipole approximation is considered. No expansion in the strength of the field is made, but an expansion in the frequency of the field is carried out. This problem has been considered previously by Kroll and Watson, who showed that, if only the first two orders in $\omega$ are retained, then the scattering is describable by an on-shell $T$ matrix obtained in the absence of the field. They show this to be an essentially classical result. A different method is used to obtain their result and the next order in $\omega$ which yields the first off-shell correction is obtained. Far off-shell contributions are found in this order.

The behavior of the photodetachment cross section of the $S^{-}$ ion near thresholds in a magnetic field is derived. This cross section is shown to have an oscillatory dependence on photon energy. The calculation is based on first-order time-dependent perturbation theory, and final-state interactions are ignored. The weights of individual transitions between the various Zeeman sublevels for different light polarizations are given in terms of the appropriate angular momentum coupling coefficients. Broadening of the features of the cross section due to the motion of the ions is included in the calculation. The Wigner-law behavior known to be valid for zero magnetic field is shown to result from the magnetic field dependent cross section in the limit of low magnetic field. The theory is shown to be in good agreement with experimental results. The extension of the theory to other atomic negative ions is discussed.

With a fixed-frequency Arii laser (488 nm), the photoelectron spectrum of ${\mathrm{Fe}}^{-}$ has been obtained. The electron affinity of iron is 0.164 ± 0.035 eV. It has been found that the cross section for detachment of $s$ electrons is much larger than that for detachment of $d$ electrons at this wavelength. Also, the various fine-structure transitions do not occur in the ($2J^{\prime \prime }+1\mathrm{}$) ($2J^{\prime }+1\mathrm{}$) statistical weighting of the neutral ($J^{\prime }$) and ion ($J^{\prime \prime }$) spin-orbit states. An explanation for this result extends previous calculations, deriving explicit, usable expressions for fine-structure transition intensities, which are then found to agree with those observed experimentally.

We report a total collisional quenching rate of (6.4±0.3)×${10}^{-12\mathrm{}}$ ${\mathrm{cm}}^3$ ${\sec }^{-1\mathrm{}}$ at 292°K (or a thermally averaged cross section of 0.36×${10}^{-16\mathrm{}}$ ${\mathrm{cm}}^2$) for He $3{}_{}{}^3{}_{}{}^{}S$ atoms colliding with ground-state He atoms. This rate is determined by using a pulsed dye laser to populate the $3{}_{}{}^3{}_{}{}^{}S$ level in a He aftergow and then observing the decay of the $3{}_{}{}^3{}_{}{}^{}S\to 2{}_{}{}^3{}_{}{}^{}P$ fluorescence. We see no evidence for three-body collisional quenching of $3{}_{}{}^3{}_{}{}^{}S$ atoms at pressures up to 300 Torr.

High-resolution laser-rf double-resonance spectroscopy has been performed in the $4f^{6}6s^2{}_{}{}^7{}_{}{}^{}F_6^{}{}_{}{}^{}$ (4020.66 ${\mathrm{cm}}^{-1\mathrm{}}$), $4f^{6}5d6s{}_{}{}^9{}_{}{}^{}H_2^{}{}_{}{}^{}$ (11044.90 ${\mathrm{cm}}^{-1\mathrm{}}$), $4f^{6}5d6s{}_{}{}^9{}_{}{}^{}H_3^{}{}_{}{}^{}$ (11406.50 ${\mathrm{cm}}^{-1\mathrm{}}$), and $4f^{6}5d6s{}_{}{}^7{}_{}{}^{}H_3^{}{}_{}{}^{}$ (13542.80 ${\mathrm{cm}}^{-1\mathrm{}}$) metastable levels of Samarium i. Hyperfine parameters have been deduced with an absolute accuracy of ∼ 2 kHz. By means of laser-induced fluorescence, hyperfine constants have been determined in excited levels up to ∼ 30560 ${\mathrm{cm}}^{-1\mathrm{}}$, and isotope shifts measured for 11 lines.

The laser-rf double-resonance technique has been used to study the hyperfine structure (hfs) of the excited $3d^{4}4s{}_{}{}^4{}_{}{}^{}D$ levels of ${}_{}{}^{15}{}_{}{}^{}\mathrm{V}$ in a continuation of earlier atomic-beam magnetic-resonance studies of the lower levels. The new results make it possible for the first time to distinguish clearly between the contact hfs due to the $4s$ electron and that due to core polarization associated with the $3d$ electrons. The values found for the radial hfs integrals from a systematic analysis of both the new and earlier results are compared with the relativistic Hartree-Fock values of Lindgren and Rosén. The magnetic octopole hfs interaction is measured for one level. Hyperfine constants and lifetime information, deduced from laser-fluorescence studies, are presented for the levels $3d^{4}4p{}_{}{}^4{}_{}{}^{}P_{\frac{5}{2},\frac{3}{2},\frac{1}{2}}^o{}_{}{}^{}$ at ∼25000 ${\mathrm{cm}}^{-1\mathrm{}}$ exitation.

A theoretical framework is presented from which precise electronic $g$ values for molecules may be calculated. The theory is applied to the calculation of $g$ factors and related magnetic properties for ${\mathrm{H}}_2^{+}$, H${\mathrm{D}}^{+}$, and ${\mathrm{D}}_2^{+}$. The average $g$ value for the ground state of each of these molecular ions is predicted to be shifted by about 3 ppm from the $g$ factor for atomic hydrogen. The results are expected to be accurate to about 0.1 ppm, and when compared with future experimental measurements may provide the first precise test of the theory of molecular $g$ factors.

Molecular-dynamics studies in argon at 148 amagats are presented for gaining information on the dynamical properties responsible for the depolarized light scattering from simple fluids. The total and pair-correlation functions are computed within the simple dipole—induced-dipole model of polarizability anisotropy. The pair spectral shape is derived. These results are compared with a theoretical analysis based on a continued-fraction approach. The necessary frequency moments are calculated both in the low-density limit and taking into account first-order density corrections, and compared with the molecular-dynamics data. The agreement between the theoretical spectra and molecular-dynamics data shows the validity of the memory-function approach. The comparison with the real experimental results allows one to test the relevant physical contributions to the polarizability anisotropy.

Predictions of bremsstrahlung angular distributions from incident electrons of kinetic energy 1-500 keV are examined using numerical data obtained from a numerical calculation in partial waves, under the assumption that the process is described as a single-electron transition in an atomic central potential. For low-$Z$ elements the Born approximation or the Elwert-Haug approach, modified with form-factor screening, give good predictions for the shape of the angular distributions, except at forward and backward angles. That is, the results are good in the main region which contributes to the energy spectrum. For high-$Z$ elements such predictions are accurate within about 15%. Analytic properties of the shape of the angular distribution from simpler theories suggest a simple way to parametrize the results.

The $L\alpha$ spectra for elements between ${}_{40}{}^{}{}_{}{}^{}\mathrm{Zr}$ and ${}_{48}{}^{}{}_{}{}^{}\mathrm{Cd}$ have been measured at high instrumental resolution. The natural widths of the diagram $L{\alpha }_1$ line ($L_3-M_5$ transitions) of these elements are determined. The observed linewidths from Mo to Ag are systematically narrower than those predicted by theory. We suggest that a reason for this disagreement are too large theoretical Auger transition rates assumed for the $M_{4,5}$ shells of Mo to Ag. Owing to high resolution, characteristic features of satellites of $L{\alpha }_{1,2}$ diagram lines are revealed.

Results of Hartree-Fock and multiconfiguration Hartree-Fock calculations using only two configurations (degenerate in zero order within the context of the nuclear-charge-expansion perturbation method) for ${}_{}{}^4{}_{}{}^{}S_{}^e{}_{}{}^{}$ doubly excited states of lithium isoelectronic sequence are used to propose an orbital configuration classification of these states, extending the similar classification by Holoien and Geltman for ${}_{}{}^4{}_{}{}^{}S_{}^e{}_{}{}^{}$ states of lithium only.

The controversy over whether to use the length or the velocity form of the interaction of electric dipole radiation with atoms is resolved on the basis of gauge invariance. When the unperturbed Hamiltonian is chosen to be the atomic Hamiltonian, the condition that the probability amplitudes are gauge invariant implies that the interaction is of the length form, $-q\overrightarrow{\mathrm{E}}·\overrightarrow{\mathrm{r}}$, not the velocity form, $-\left(\frac{q}{\mathrm{mc}}\right)\overrightarrow{\mathrm{A}}·\overrightarrow{\mathrm{p}}$. Hartree-Fock theory is examined and shown to be form invariant under local gauge transformations, i.e., gauge invariant. A comparison is made between the length and velocity forms of the interaction and oscillator strengths. If the true Hamiltonian is nonlocal, only the length form of the dipole oscillator strength is valid. If the true Hamiltonian is local, the length form of the dipole oscillator strength may be transformed to the velocity form.

The wavelength of the $K{\alpha }_1$ line from an electron-bombarded natural W anode has been measured in terms of that of an ${\mathrm{I}}_2$-stabilized HeNe laser. This visible laser is a reference point for current Rydberg determinations and for the present-day definition of length. The resulting wavelength value, namely 0.20901 349 Å (0.90 ppm) (energy = 59.319233 keV) appears to resolve historical inconsistencies in the x-ray route to $\gamma$ wavelengths.

Hydrogen molecules were excited into the ${\mathrm{H}}_2\mathrm{}\left(a\right)\mathrm{}$ state by impact of ${\mathrm{Ar}}^{+}$ ions in a beam at 0.5-48 eV relative energy. Optical emission in the ${\mathrm{H}}_2\mathrm{}(a-b)\mathrm{}$ continuum as well as in several H, Ar, and ${\mathrm{Ar}}^{+}$ lines was measured. Intense ${\mathrm{H}}_2\mathrm{}\left(a\right)\mathrm{}$ emission occurring below the thermodynamic threshold for ${\mathrm{H}}_2\mathrm{}\left(a\right)\mathrm{}$ excitation by ground state ${\mathrm{Ar}}^{+}$ indicates that transfer of electronic energy from metastable ${\mathrm{Ar}}^{+}$ is the dominant excitation process. The reaction is promising as a pumping mechanism for a new tunable uv laser.