It is shown that vibration-rotation interaction terms in the Hamiltonian of tetrahedral molecules like ${\mathrm{CH}}_4$ permit transitions, in the ground vibrational and electronic states, in which only the rotational quantum numbers change. Selection rules, spectral line positions, and absolute intensities are calculated. Effects of tetrahedral fine-structure splittings and nuclear-spin statistical weights are considered. Possible competing transitions are discussed.

The finite coherence time of the driving field plays an important role in the rates of energy exchange taking place in nonlinear processes. As a typical example, the problems of the quantum parametric amplifier and frequency converter driven by a fluctuating pump are considered. A closed hierarchy of equations for the moments of the fields is given, which in turn is used for evaluating the average number of photons per mode and the relative variance as functions of time. Significant deviations from the case where the pump field is coherent are found.

The thermal-lens effect is used to study the dynamics of heat diffusion in transparent media. Values of the thermal diffusivity and thermal conductivity are obtained for several liquids.

This paper presents a single mathematical expression of the form $\tau ={\tau }_0\exp (-\epsilon T^{\lambda })$ governing the temperature dependence of acoustic and dielectric relaxation times simultaneously for water, methyl, and ethyl alcohol; and on this basis a similar qualitative expression has been suggested for their pressure variations too.

It is argued that the "local" electric polarizability (or, in the band picture, its "interband" component) has major effects on the top of the phonon spectrum of certain superconductors. This causes interesting pressure and isotopic dependences of $T_c$ and might furnish and explanation for the variation of $T_c$ with electron number in these superconductors.

It is shown that measurement of the derivative of the second-order Raman spectrum with respect to wavelength of the scattered light gives information on the Van Hove critical points of the combined two-phonon energy function. The results of such a measurement on SrTi${\mathrm{O}}_3$ at 10°K are presented and discussed.

The spectral response of photoconductivity of clean, cleaved silicon surfaces shows at 80°K a broad shoulder for photon energies lower than the band gap. This shoulder exhibits a sequence of minima, equally spaced with 59 meV. From the spectral dependence of photoresponse and the energetic position of the minima, it is concluded that the photoconductance of the clean, cleaved surface is caused by indirect transitions from the valence band to surface states 0.5 eV above the valence-band edge.

The energy loss of 5-eV electrons reflected from clean and from oxidized (111) silicon surfaces has been investigated. A surface phonon with $\hslash \omega \approx 55$ meV, $k\approx 0$ is found at the clean surface. After exposure to ≅${10}^{-5\mathrm{}}$ Torr min oxygen, three energy losses at 48, 90, and 125 meV occur. Models for the electron-phonon interaction, the nature of the surface phonon, and the structure of the oxidized surface are proposed.

It is shown that, in a semiconductor exhibiting negative differential mobility, the coupling between diffusion effects and the tendency for space-charge accumulation can lead to temporal growth rather than spatial amplification if the dielectric relaxation frequency exceeds a certain threshold value. This conclusion results from an analysis of the dispersion relation for longitudinal waves in the semiconductor. The criterion for temporal growth is a condition for absolute instability, as opposed to convective instability, which would indicate rather spatial amplification.

It is pointed out that the incorporation of band structure in the theory of elastic tunneling in metal-insulator-metal structures does not make an appreciable enough change in the tunnel current to warrant its deduction from measured voltage-current data. It is emphasized that if it is to be at all meaningful, the essential barrier parameters (effective work function, thickness, and effective mass) and the barrier shape must be consistently and precisely specified. This has so far not been achieved.

Intrinsic broadening of the exciton linewidth in a Mott-Hubbard insulator is predicted, within an idealized model, as a result of restrictions on the migration of $d$-state holes via electron exchange. This broadening should decrease above the Néel point.

We report that the 2-eV electroreflectance structures in germanium can be represented by a two-dimensional critical line which contains the $L$ point and extends over approximately 80% of the distance from $L$ to $\Gamma$.

Measurements of the spectrum of $\gamma$ rays from the reaction ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}(n,\gamma ){}_{}{}^{209}{}_{}{}^{}\mathrm{Pb}$ have been made at incident neutron energies of 9.2, 11.2, and 13.2 MeV. The observed spectral shapes disagree strongly with the predictions of compound-nuclear theory and agree quite well with those predicted by semidirect (collective) capture theory.

An experiment to determine the bremsstrahlung cross section in proton-$\alpha$-particle scattering was performed for incident proton energies between 7.0 and 12.0 MeV. It was found that in order to obtain agreement with the measured coplanar values, the theoretical calculation must include the contribution of the so-called "rescattering term."

The effects of final-state interactions on nonleptonic hyperon decay amplitudes are evaluated within the current-algebra framework of partially conserved axial-vector currents. Good agreement with experimental amplitudes is obtained for both $S$- and $P$-wave decays.

The multiple-scattering correction to the spectator momentum distribution is calculated using Glauber theory. It is shown to dominate when the transverse component of the spectator momentum is greater than about 400 MeV/c, yielding an overall enhancement of high-momentum events.

One of the precesses which may be observed in $e^{\pm }{e}^{-}$ colliding-beam experiments is the deep inelastic electron-photon scattering. We examine the feasibility of such an experiment assuming an appropriate scaling limit.

We have measured the cross section for the reaction $\overline{p}p\to K_S^{}{}_{}{}^{0}K_L^{}{}_{}{}^0$ from 100-800 MeV/c. A significant enhancement in the cross section is observed at a mean momentum of 600 MeV/c. A Legendre-polynomial analysis of the angular distributions also shows an enhancement in the $A_2$ coefficient in the same momentum range. A plausible explanation of the data indicates a new meson state $\rho \mathrm{}\left(1970\right)\mathrm{}$ with a mass $M=1968\mathrm{}$ MeV, width $\Gamma =35\mathrm{}$ MeV, and $J^{\mathrm{PC}}=1^{--}$.

$K$-shell x-ray yields were measured for Al and Ni bombarded by H, D, He, and Li of kinetic energies between 1 and 7.5 MeV/amu. Ratios of $K$-shell ionization cross sections for projectiles of different atomic numbers $Z_1$, at equal velocities, show pronounced deviations from the $Z_1^{}{}_{}{}^2$ dependence predicted in the plane-wave Born approximation. The effect can be understood in terms of distortions of the $K$ shell in the field of the ionizing particles.

The dissociative excitation ${\mathrm{H}}_2+e\to \mathrm{H}\mathrm{}\left(2s\right)+\mathrm{H}+e$ has been studied with a time-of-flight method. Measurements of the excitation function and the angular distribution of the metastable atoms show that the slow atoms arise from dissociation out of $e{}_{}{}^3{}_{}{}^{}\Sigma _{\mathrm{u}}^{}{}_{}{}^{+}$ and $B^{\prime }{}_{}{}^1{}_{}{}^{}\Sigma _{\mathrm{u}}^{}{}_{}{}^{+}$ states and from predissociation out of the $d{}_{}{}^3{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{+}$ and $D{}_{}{}^1{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{+}$ states. The fast atoms arise from a previously unreported ${}_{}{}^1{}_{}{}^{}\Pi _{\mathrm{u}}^{}{}_{}{}^{}$ state that corresponds to a separated-atom limit in which both atoms are in $n=2\mathrm{}$ states.