The connection between the high-frequency end of the bremsstrahlung spectrum (tip bremsstrahlung) and the photoeffect, which was first noted by Fano, is shown to be valid also when the lowest-order radiative corrections are included. By exploiting this relation and by using results for the photoeffect given in the preceding paper, we obtain an explicit expression for the corrected tip-bremsstrahlung cross section. Other expressions derived previously for the radiative corrections to bremsstrahlung are not valid at the spectrum tip. Limiting cases of low and high incident electron energy are presented and discussed.

The absolute photodetachment cross section $\sigma$ and the solar detachment rate of ground electronic and vibrational energy state of C${\mathrm{O}}_3^{-}$ have been determined by using electron-detecting-drift-tube method. The C${\mathrm{O}}_3^{-}$ ion is identified by mass and mobility measurements. The cross section is measured with the use of 19 color filters. By fitting data with a least-squares program, it is found that $\sigma \left(\lambda \right)=[a_1+a_2{\lambda }^{\prime }+a_3{\lambda }^{\prime 2}+a_4{\lambda }^{\prime 3}]\theta ({\lambda }_0-\lambda )$, where ${\lambda }_0=459\mathrm{}$ nm (2.69 eV), ${\lambda }^{\prime }=(\lambda -385)$ nm, $a_1=(1.21\mathrm{}\pm 0.32)\times {10}^{-19\mathrm{}}$ ${\mathrm{cm}}^2$, $a_2=(-3.61\mathrm{}\pm 1.68)\times {10}^{-21\mathrm{}}$ ${\mathrm{cm}}^2$/nm, $a_3=(1.62\mathrm{}\pm 0.23)\times {10}^{-22\mathrm{}}$ ${\mathrm{cm}}^2$/${\mathrm{nm}}^2$, and $a_4=(2.00\mathrm{}\pm 0.37)\times {10}^{-24\mathrm{}}$ ${\mathrm{cm}}^2$/${\mathrm{nm}}^3$; $\theta$ is the Heaviside step function. The threshold energy of ${2.69}_{-0.14\mathrm{}}^{+0.07\mathrm{}}$ eV is identified as the electron affinity of C${\mathrm{O}}_3$. The solar photodetachment rate is 0.022 ± 0.003 ${\sec }^{-1\mathrm{}}$. The upper bound for the binding energy of C${\mathrm{O}}_3$ is determined as 0.6 ± 0.1 eV.

The presence of coherent radiation fields can modify various atomic, ionic, and molecular processes. We examine cases where the changes can be effected without actual absorption or emission of photons. The radiation fields cause optical Stark shifts of the energies and dress the nonadiabatic couplings between states. Rates of processes such as dissociation, predissociation, atomic and molecular collisions can be radiatively controlled. Formation of new avoided crossings due to radiative interaction gives rise to new phenomena. We give both exact numerical results from which the influences of the radiation field on arbitrary configurations can be calculated, and simple but accurate analytic results, from which these effects can be conveniently assessed. It is shown that the inelastic transition probability is decreased at a true crossing and is increased at an avoided crossing as a result of the presence of the nonresonant radiation field. Furthermore, for two parallel levels with constant nonadiabatic coupling, it is shown that the amplitudes of the nonadiabatic transition are invariant, but its flopping frequency is reduced by the field. Depending on final measurements with the field on or off, general behaviors of the inelastic transition probability as a function of the field parameters and the charge-system parameters are predicted and the possibility of inversion (from a field-free value of less than ½ to a value greater than ½) is demonstrated. Experimental investigation with iodine molecules is suggested. Comparison of our nonperturbative results with those of stationary perturbation theory shows that the latter is inadequate in an important parameter region where the field modification of processes is greater than 1%.

The theory of resonance Raman effect, resonantly enhanced two-photon absorption, and resonance fluorescence—valid also for strong fields—is treated in a unified and simplified way. Dressed states, Bloch equations, and perturbation theory are used to calculate line positions, and integrated intensities both for steady state and transient excitation. The case of adiabatic following is solved explicitly; it occurs when an off-resonance incident pulse is turned on and off slowly. It leads to the identification of Raman scattering, two-photon absorption, and Rayleigh scattering with an adiabatic process while fluorescence and consecutive two-photon absorption with nonadiabaticity. Time-dependent spectra are defined in a rigorous way. Our formulas agree in the various limits with those that appear in the literature.

Multiphoton ionization of xenon, krypton, and argon atoms has been investigated by using a tunable-wavelength bandwidth-limited 30-psec laser pulse of ${10}^{13}$ W/${\mathrm{cm}}^2$ at 1.06 μm. The laser linewidth is 0.8 ${\mathrm{cm}}^{-1\mathrm{}}$, and the laser wavelength can be tuned over a range of 80 Å. No resonant effects have been observed within this wavelength range, although resonance conditions are satisfied. The absence of resonance effects could be explained in terms of the short laser pulse duration. Laser light polarization effects were still measured. Moreover, the ionization energy of krypton has been observed to be lowered by 86±6 ${\mathrm{cm}}^{-1\mathrm{}}$ under the influence of a laser electric field of 1.2×${10}^8$ ${\mathrm{Vcm}}^{-1\mathrm{}}$.

We suggest that the statistical properties of the scattered radiation in resonance-fluorescence experiments may be affected significantly by the existence of atomic correlations. The scattered light spectrum from two- and three-atom collective systems has been calculated and compared with the one-atom spectrum. The differences are quite significant for weak fields, but become less pronounced as the intensity of the driving field is increased. In addition, we have calculated the scattered intensity correlation function for collectively interacting systems, and found that its behavior is very different from that of the single-atom intensity correlation function, both for weak and strong incident fields. The implications of our findings for the observation of photon antibunching are also discussed.

The interest in precise knowledge of the intensity factors for investigating molecular structure is pointed out and illustrated by some typical recent examples. Diatomic intensity factors are derived. Starting from pure coupling-case wave functions, the intensity factors related to transitions between states belonging to $a_{\alpha }$, $a_{\beta }$, $b_{\beta J}$, $b_{\beta S}$, and $b_{\beta N}$ coupling cases, in the spin-forbidden or spin-allowed hypothesis, are derived. Cases are considered for which hyperfine structure is fully observed, partially observed, or absent.

Numerical calculations of level shifts and power-broadening linewidths for double-photon absorption in truncated rigid-rotor models are presented. This work demonstrates the feasibility of a previously suggested calculational method. It is shown that level shifts in double-photon absorption deduced from three-state models are misleading since other states dipole-coupled to states directly involved in the transition make important contributions to the shifts.

We display the time dependence of populations in coherently excited three-level atoms. In particular we examine the relationship between ion production and the dynamical parameters: Rabi frequencies, detunings, and loss rate.

The comparison between the classical and quantum theories of optical coherence is presented by using an idea used in the problems of modulation of light beams of classical fields. In such problems it is assumed that it is possible to vary the mean light intensity of a beam without changing its statistical properties defined by a set of coherence functions. We generalize this idea and introduce precisely the class of optical fields which are consistent for modulation. It is shown that all the quantum fields of this class have a positive $P$ representation and are strictly equivalent to classical fields. Moreover, when the field is assumed to be stationary, an interpretation of this condition is given which in particular makes precise the relations between photon-counting and light-intensity measurements. Finally it is shown that all the quantum fields without $P$ representation cannot be consistent for modulation, and the condition of consistency for modulation appears as a characteristic property of fields strictly equivalent to classical ones.

A model is developed to describe the scattering of fast test particles in an infinite background plasma in which conditions are such that the probability of elastic scattering by plasma ions much exceeds the probability of elastic scattering by plasma electrons. Encounters leading to large angles of scattering are excluded in calculating the scattering kernel and the slowing-down density. These functions are evaluated using the quantum-mechanical elastic scattering cross sections. The encounter density is obtained by solving the space-and time-independent Boltzmann equation in the presence and in the absence of fusion reactions between the test particles and the plasma ions. Asymptotic energy distributions in the slowing-down range below the source energy are obtained for both cases. The average density and mean energy of the test particles in the slowing-down range are also evaluated. The results are found for arbitrary sources of test particles, taking into account spread in the initial energy of the particles. Binary and multicomponent plasmas are considered.

The time-energy spectrum of fast particles released in background plasmas is evaluated for the cases of monoenergetic impulses and sources dispersed in time and energy, as well as for general source spectra. Distant encounters are assumed to contribute mainly to the energy degradation of the test particles in the slowing-down range in which ions are treated as fixed centers, and in which the probability of elastic scattering by electrons is insignificant compared to the probability of scattering by plasma ions. The results give a slowing-down time close to that evaluated by approaches that account for close encounters. Asymptotic solutions of the Boltzmann equation are also obtained, and the effects of the confinement time of the test particles on the time-energy spectrum are taken into account. Confinement times characteristic of open-ended devices, tokamaks, and Bohm-like plasmas are considered. Pitch-angle scattering is not considered here, and hence only a rough estimate of the parameters relevant to neoclassical processes is given. The average density and mean energy of the test particles are obtained in the slowing-down range for these three types of plasmas. Discrepancies resulting from the classical treatment of scattering as opposed to the quantum-mechanical treatment are pointed out. A multienergy approach is introduced for numerical calculations; such an approach is necessary in particular for model-dependent studies. The approach is rather flexible and can accommodate different types of spatial dependence and scattering processes.

The zero-field mobilities of positive helium ions in helium vapor have been measured as functions of both pressure and temperature between 2.0 and 5.2°K. These mobilities are much lower than would be predicted from known ion mobilities at 76°K. These data can be explained in terms of kinetic theory with a clusteredion hypothesis.

The validity of analysis of parallel-plate drift-tube experiments by use of diffusion theory is examined. Monte Carlo simulations demonstrate the inadequacy of such analyses for electron motion near an absorbing cathode. However, diffusion theory results are verified for situations in which cathodic absorption is negligible, provided that a parameter $\beta$, the ratio of energy relaxation distance to drift-tube length, is much smaller than unity. For experimental circumstances in which cathode effects may be distinguished from those of anodic absorption, circuit time constant, and ionization, the Monte Carlo results can be used to augment diffusion theory for interpretation of the observed transients in terms of transport coefficients.

By a self-consistent procedure, the velocity autocorrelation functions of both liquid and gaseous argon have been calculated without introducing any arbitrary parameters. The results are in satisfactory agreement with computer experiments. The correlation functions are primarily determined by the nearest-neighbor coordination. Because of the strong hard-core repulsion, the behaviors are more vibratory and damp out quickly at high densities and are more diffusive at lower densities.

In this investigation the basic causes of glow collapse and/or glow-to-arc transition have been examined for convection-cooled, externally sustained molecular-discharge conditions. The results of this analysis show that externally sustained molecular discharges of the type used in C${\mathrm{O}}_2$ lasers are inherently unstable. In particular, it is shown that there are two distinct mechanisms which can provide the driving force for thermal instability: (i) electron vibrational excitation leading to a surge in vibrational temperature; and (ii) vibrational reservoir collapse accompanied by a decrease in vibrational temperature. The plasma conditions and basic collisional processes contributing to the occurrence of these thermal instabilities are discussed in detail, with particular emphasis directed toward charged-particle production and loss mechanisms.

Density fluctuations in a semi-infinite collisional electron plasma are studied using "two-stream" distribution functions. Microscopic boundary conditions, imposed separately upon each of the velocity streams, are formulated with the inclusion of both specular reflection and diffuse scattering contributions. The strong damping of surface modes noted by previous investigators is shown to be, in part, a consequence of the use of pure specular reflection boundary conditions. With increasing diffuse scattering contributions the surface modes are seen to become an enhanced and identifiable spectral feature.

Qualitative arguments based on the physics of vortex stretching indicate that Kolmogorov's mean-field theory of strong turbulence must be unstable and tend to a state in which fluctuations become increasingly singular at short distances, as proposed by Kolmogorov and Oboukhov in 1962. The number of coupled degrees of freedom in the Navier-Stokes equations is then greatly reduced by a wave-packet analysis, and the essence of the vortex stretching picture is recovered. Numerical integration of the reduced equations confirms the qualitative arguments for intermittency and gives exponents and probability distributions in crude agreement with experiment and the log-normality hypothesis.

The unified theory of line broadening by electron perturbers is generalized to include the case of a weakly turbulent plasma. The collision operator in the line-shape expression is shown to be the sum of two terms, both containing effects arising from the nonequilibrium nature of the plasma. One of the two terms represents the influence of individual atom-particle interactions occurring via the nonequilibrium dielectric plasma medium. The other term is due to the interaction of the atom with the turbulent waves. Both terms contain damping and diffusion effects arising from the plasma turbulence.

It is shown that a nematic liquid crystal in the presence of a magnetic field vector rotating on the surface of a right circular cone can have an equilibrium state in which the director rotates synchronously with the applied field. For field cone angles of less than 54.7°, this state is stable at all angular frequencies. At larger angles a stability limit exists at a finite rotation speed. Two types of instability modes are predicted, depending upon the field cone angle. The implications of these results for torque-magnetometer and electron-resonance-spectroscopy studies are discussed.