Absolute optical-emission cross sections have been measured for transitions in the wavelength range 3690–11 300 Å originating from 32 terms of excited oxygen atoms produced by electron impact on oxygen molecules with incident electron energy up to 500 eV. The excitation functions show a broad peak at about 90 eV with a shoulderlike structure of varying degree near 35 eV. The observed threshold energy is very close to the energy defect of the dissociation process. Near the threshold, dissociative excitation through partly bound Rydberg states of the oxygen molecule is believed to be a major mechanism for producing the observed atomic-oxygen emission, whereas simultaneous ionization and excitation followed by dissociation become important at energies above 50 eV. Combination of the measured cross sections with theoretical transition probabilities allows us to determine the absolute optical-emission cross sections for a series of atomic-oxygen transitions in the long-wavelength infrared region of the spectrum.

The electron excitation cross section both for the ${}_{}{}^3{}_{}{}^{}\mathrm{P}_2^{}{}_{}{}^{}$ and ${}_{}{}^3{}_{}{}^{}\mathrm{P}_0^{}{}_{}{}^{}$ metastable levels (1$s_5$ and 1$s_3$ in Paschen’s notation) and for the mixed ${}_{}{}^3{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ and ${}_{}{}^1{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$ resonant levels (1$s_4$ and 1$s_2$ in Paschen’s notation) of the 2$p^5$3s configuration of Ne have been measured. Apparent and direct excitation cross sections are reported for incident electron energies from threshold to 300 eV and are compared with results of different methods of measurements. Ground-state Ne atoms are excited by a monoenergetic electron beam to the 1s levels, and atoms in the 1s levels are pumped to the 2p levels of the 2$p^5$3p configuration by means of a cw dye laser. The laser-induced fluorescence from the 2p level is utilized to determine the electron excitation cross section for the 1s level. Methods for absolute calibration of the cross sections are given.

Cross sections for production of positive and negative charge for 10–300-keV/u ${\mathrm{He}}^{2+}$ ions on He, Ne, Ar, Kr, ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, CO, ${\mathrm{O}}_2$, ${\mathrm{CH}}_4$, ${\mathrm{N}}_2$O, and ${\mathrm{CO}}_2$ were measured by the transverse-field method. Single- and double-electron-capture cross sections at 5–150 keV/u for the same targets were measured by the method of deflection of different charge-state components of the beam after passing through a known length of target gas. A secondary-emission detector was used to detect the neutral component of the beam. A small least-squares adjustment of the cross sections was made to satisfy the equation ${\sigma }_{+}$=${\sigma }_{\mathrm{-}}$+${\sigma }_{21}$+2${\sigma }_{20}$ which follows from conservation of charge.

The photoionization cross section can be expressed as a particular matrix element of the resolvent of the molecular electronic Hamiltonian. By using a mixture of real and complex Gaussian basis functions to form a matrix representation of the Hamiltonian it is possible to use this expression to compute molecular photoionization cross sections from the results of a matrix diagonalization. This procedure is based on a variational principle for the resolvent matrix element from which the photoionization cross section is calculated, and can be applied in the presence of coupling between ionization channels. The interference between resonance features and the electron-ion scattering background is naturally incorporated by this method, and it is easily implemented for polyatomic systems. Results are reported for valence- and K-shell ionization of ${\mathrm{N}}_2$ at the static exchange level which demonstrate the utility of this method for molecular problems. Agreement with most other calculations is adequate and is especially good with those which compute the photoionization cross section directly from the electron-ion scattering wave function. The complex-basis-function technique is particularly successful in reproducing resonance features in these cross sections. A discussion of the variational behavior of the amplitude is given, and it is shown that the variational nature of these calculations provides a useful computational diagonostic.

K-shell ionization probabilities $P_K$ were measured for 2.2-MeV/u ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ ions with impact parameter b=16–51 fm on 24 targets ranging from Sc to Bi. For Bi, the distribution $P_K$(b) was also measured. Total K-shell ionization cross sections ${\sigma }_K$ for ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ and ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$ projectiles are reported for the same target range. Semiclassical calculations using the velocity-dependent electron binding energies from the relativistic two-center approximation of Andersen, Laegsgaard, and Lund reproduce both $P_K$ and ${\sigma }_K$ very well, except where polarization effects become important.

A search for the production of the 2$p^2$ ${\mathrm{}}^3$$P^e$ state of ${\mathrm{H}}^{\mathrm{-}}$ has been made by crossing a ${\mathrm{H}}^{\mathrm{-}}$(1${\mathrm{s}}^2$) ion beam of energies E<140 keV with a thermal Hg beam inside a static electric field. The formation of ${\mathrm{H}}^{\mathrm{-}}$(2${\mathrm{p}}^2$) ions and their subsequent decay to neutral H(1s) results in a feature on the angular distribution of the neutral particles produced in electron loss of the ground-state ${\mathrm{H}}^{\mathrm{-}}$(1${\mathrm{s}}^2$) ions. The absence of such a feature sets an upper limit of ${10}^{\mathrm{-}20}$ ${\mathrm{cm}}^2$ to the cross section for formation of the 2$p^2$ ${\mathrm{}}^3$$P^e$ state. Differential (with respect to angle) charge-transfer cross sections for scattering in the angular range 3°<θ<15° as well as the small-angle forward cross sections ${\sigma }_{10}$ and ${\sigma }_{\mathrm{-}10}$ for hydrogen ions passing through Hg vapor are presented.

An illustration is reported on the use of the Euclidean norm as a criterion of the quality of translation factors in the molecular model of atomic collisions. The relation between our norm and the deviation vector of Chang and Rapp [J. Chem. Phys. 59, 572 (1973)], and the computational simplicity of the calculation and minimization of the former quantity, are very appealing features of our approach. To show how the norm method can be applied, the ${\mathrm{He}}^{2+}$+H(1s)→${\mathrm{He}}^{+}$(2s,2p )+${\mathrm{H}}^{+}$ reaction is treated.

Using Demkov’s variational principle, the probability for electronic excitation is evaluated with the help of optimized basis states, which allow for a continuous transition from an atomic to a molecular description. The inclusion of cosliding translational factors provides an extension to transfer processes. Calculations are performed for radiative electron capture (REC) of a target K electron in the collision systems ${}_{}{}^{16}{}_{}{}^{}$O→${\mathrm{}}^{12}$C and ${}_{}{}^{32}{}_{}{}^{}$S→${\mathrm{}}^{20}$Ne. It is shown that even for fast collisions, an atomic description is only applicable in the REC peak region, while molecular effects have to be included in the tails.

A unitarized impact-parameter method is applied to calculate the electron-loss and excitation cross sections for ${\mathrm{He}}^{+}$ ions colliding with atoms. The projectile ionization and excitation are dominantly caused by the average potential field of the target atom (atomic number $Z_2$). The inelastic process of exciting the target atom contributes negligibly except for light target elements. We adopt the Molière potential to describe this average potential field. The energy dependences of the electron-loss cross sections in He, ${\mathrm{N}}_2$, and Ar targets are in good agreement with the reported data. In the case of the Kr target, the present theory yields larger cross sections than the data, especially below 1 MeV impact energy of a ${\mathrm{He}}^{+}$ projectile. The calculated loss cross sections at impact velocity ranging from 2$v_0$ to 6$v_0$ ($v_0$=2.18×${10}^8$ cm/s) show a weaker $Z_2$ dependence in the large $Z_2$ region than that given by the Bohr formula. As for the cross section for exciting the ground state of a projectile to the first excited state, a similar weak $Z_2$ dependence can be found. The recent experimental results using 40-MeV ${\mathrm{F}}^{8+}$ ions colliding with He, Ne, Ar, and Kr targets have supported this tendency.

We study laser-intensity effects in two-photon collisional redistribution of radiation. Thereby we concentrate on a special case of recent experimental interest, where the transition from the excited to the final state is saturated by a strong laser field. Both exciting lasers are, however, assumed weak in the sense that the dynamics of a collision between the radiator and a perturber is not significantly influenced by the laser fields. In particular, our treatment explains the intensity independence of the polarization dependence of total fluorescence from the final state which has been measured previously even if the transition from the excited to the final state is saturated.

We present a detailed description of new observations of parity nonconservation in the $6^2$$P_{1/2}$→$7^2$$P_{1/2}$ transition (293 nm) in atomic ${}_{{\mathrm{}}^2{\mathrm{}}_{81}^{05}}{}^{}{}_{}{}^{}\mathrm{Tl}$. Linearly polarized 293-nm photons, polarization ɛ^, are absorbed by $6^2$$P_{1/2}$ atoms in crossed E and B fields. The transition probability for each Zeeman component contains a pseudoscalar term, proportional to ɛ^⋅Bɛ^⋅E×B, arising from interference between the parity-nonconserving E1 amplitude ${\mathrm{scrE}}_P$ and the Stark E1 amplitude βE. This is detected by observation of fluorescence at 535 nm accompanying the decay ${(7\mathrm{}}^2$$P_{1/2}$→) $7^2$$S_{1/2}$→$6^2$$P_{3/2}$. Systematic corrections, due to imperfect linear polarization and misaligned E and B fields, are determined precisely by auxiliary experiments and found to be small. Our final result: Im[${\mathrm{scrE}}_{P/\beta }$]${\Vert }_{\mathrm{expt}}$=-(1.73±0 .26±0.07) mV/cm, where the first uncertainty is statistical, and the second is systematic, is compared with results of other experiments and with theoretical estimates based on the standard SU(2)×U(1) electroweak model.

The Faraday rotation and circular dichroism of dilute gases in longitudinal magnetic fields and subject to strong radiation fields are studied. The use of irreducible tensorial sets as bare-atom states which are then dressed by the laser field allows a simple incorporation of the various multipole relaxation rates into the theory. The treatment considers magnetic fields which produce Zeeman shifts much smaller than inverse collision times but which may be greater or smaller than the multipole relaxation rates. Doppler broadening is also considered. Explicit expressions are given for dipole transitions in a two-level system with arbitrary angular momentum for strong and weak magnetic fields.

The theoretical formulation of the nonlinear optical activity is utilized to describe the signal intensity in forward scattering. For cases in which the ratio of the Rabi frequency of an isolated transition to its relaxation rate is less than one, a perturbation procedure in this parameter is presented. The successive terms in the resulting series are interpreted as higher-order coherences or induced multipole moments. Application is made to the hyperfine components of the sodium D transitions where experimental data is described by a third-order calculation. Good agreement is obtained without invoking an induced hexadecapole moment as was proposed originally.

We have observed a doubly excited autoionization resonance to broaden with increasing electric field. This broadening is consistent with the quadratic field dependence predicted by a simple perturbative model, but the magnitude of the observed broadening is about four times larger than predicted. Possible reasons for this discrepancy are discussed.

The energies and widths of a 6$p_{3/2}$nd J=3 autoionizing series are reported for n=8–17. Above the 6$p_{1/2}$ ion limit the series is found to have approximately constant values for the quantum defect (μ≊2.85) and for the scaled widths (${\Gamma }_n$$n^{\mathrm{*}3}$≊0.07 a.u.). Comparisons are made with the previously reported energies and widths of the other two J=3 6pnd autoionizing series [F. Gounand et al., Phys. Rev. A 27, 1925 (1983)]. The interaction of all three series is analyzed using a six-channel multichannel-quantum-defect-theory model which successfully accounts for energies, widths, and complex excitation spectra for the three series. Conclusions concerning state wave functions are drawn from the data and implications regarding the strengths of different electron interactions are discussed. Additionally, information is obtained concerning the interaction of the bound perturber 5d7d ${\mathrm{}}^3$$F_2$ with the 6s14d ${\mathrm{}}^1$,3${\mathrm{D}}_2$ states.

Improved 3d photoabsorption spectra of the series of rare-earth fluorides have been obtained by recording total electron yield and using monochromatized synchrotron radiation. They are compared with predicted spectra resulting from an average-of-configuration calculation of wave functions employing radial integrals adjusted to the observed spectrum in each case; results are presented here for Ce, Pr, Ho, and Er spectra. Improved resolution of the Ce spectrum permitted for the first time a determination of the proper scaling for all of the electrostatic parameters.

We have measured the position of the magnetic field level crossing of the J=2, M=2, and the J=M=0 sublevels of the 3 ${}_{}{}^3{}_{}{}^{}P$ state of helium. Time-resolved level-crossing spectroscopy, coupled with photoionization detection, were used to achieve accuracy of 1.5 ppm. The result, quoted in NMR frequency in water, is 9695.023(15) kHz, in good agreement with earlier, less precise measurements.

The consequence of time-reversal symmetry in a general second-order optical process between degenerate initial and final states is elucidated. The principle is applied to the pseudoscalar natural-optical-rotation operator ($R_L$⋅$M_L$), which involves the interference of an electric-multipole ($R_L$) transition and a magnetic-multipole ($M_L$) transition. In addition to deriving point-group selection rules, time-reversal effects on matrix elements between degenerate states of a chiral versus an achiral molecule are compared and contrasted. Other non–time-reversal-degenerate states in chiral versus achiral molecules ($D_3$ versus $C_{3v}$ and $D_4$ versus $D_{2d}$) are also compared.

The analogy to the Jahn-Teller effect, which also involves (albeit first-order vibronic) matrix elements over degenerate states, is mentioned, especially with regard to the formal ‘‘splitting’’ of the degenerate levels by the pseudoscalar optical-rotation operator. A comprehensive search among the point groups for degenerate-state self-products that will give rise to the pseudoscalar function is conducted. Detailed results for the coupling of (degenerate) irreducible representations for important point groups (and their subgroups) are worked out for $T_d$ (T), O, $D_{6d}$ ($D_6$, $D_3$, $C_{6v}$, $C_{3v}$, $C_6$, $C_3$, $D_{2d}$), and $D_{4d}$. The couplings to give dipolar (L=1), quadrupolar (L=2), and octopolar (L=3) pseudoscalars are shown explicitly.

Attention is called to the differences in the transformation of a magnetic-multipole operator and an electric-multipole operator, even though they both belong to the same irreducible representation. The resulting differences in coupling coefficients for degenerate states are given to help determine nonvanishing optical-rotation matrix elements. A summary and a discussion of the conservation of parity and time symmetry in optical rotation are given.

The problem of soliton interaction in optical-fiber communication systems is investigated analytically with the assumption of incoherent soliton interaction, i.e., interaction through nonlinear intensity overlap only. This situation is found to lead to attraction forces between consecutive pulses that are weaker than those in previously investigated coherent cases. The subsequent maximum system bandwidth is correspondingly improved.

The nonlinear evolution of the free-electron-laser amplifier is investigated numerically for a configuration consisting of a helical wiggler and axial guide magnetic fields. A set of coupled nonlinear differential equations is derived in three dimensions which governs the self-consistent evolution of either the TE or TM modes in a loss-free cylindrical waveguide and the trajectories of an ensemble of electrons. The initial conditions are chosen to model the adiabatic injection of a cold, cylindrically symmetric electron beam into an interaction region in which the wiggler amplitude rises slowly from zero to a constant level in ten wiggler periods. Both self-field and space-charge effects have been neglected in the formulation, and the analysis is valid for the high-gain Compton regime of operation. Numerical simulations are conducted to model an amplifier operating in the neighborhood of 35 GHz, and for electron-beam energies of 250 keV and 1 MeV. (The free-electron-laser operating at electron-beam energies less than 500 keV is called the ubitron.) The growth rate in the linear regime prior to saturation is found to be in substantial agreement with the predictions based on a linear theory of the instability, and the saturation efficiency is consistent with that expected on the basis of simple, heuristic phase-trapping arguments. Substantial enhancements in the efficiency are found to occur due to the presence of the axial guide field.