This paper reviews the experimental studies of displacive phase transitions in solids. Primary emphasis is upon inelastic light scattering and neutron scattering; related infrared reflectivity measurements, as well as x-ray and EPR analyses are also summarized. Several prototype structures are considered in detail: (1) the rocksalt IV-VI semiconductors PbTe, SnTe, and GeTe; (2) the ferroelectric perovskites exemplified by PbTi${\mathrm{O}}_3$ and BaTi${\mathrm{O}}_3$; (3) perovskites which exhibit cell-doubling transitions, such as LaAl${\mathrm{O}}_3$, SrTi${\mathrm{O}}_3$, and KMn${\mathrm{F}}_3$; (4) crystals having the $\alpha$-quartz structure, including Ge${\mathrm{O}}_2$, Si${\mathrm{O}}_2$, and AlP${\mathrm{O}}_4$; (5) the "improper ferroelectrics" ${\mathrm{Gd}}_2$${\left(\mathrm{M}\mathrm{o}{\mathrm{O}}_4\right)}_3$ and ${\mathrm{Tb}}_2$${\left(\mathrm{M}\mathrm{o}{\mathrm{O}}_4\right)}_3$; (6) the V-VI-VII semiconductors typified by SbSI; (7) the hydrogen-bonded ferroelectrics of the K${\mathrm{H}}_2$P${\mathrm{O}}_4$ family; (8) Jahn-Teller systems such as DyV${\mathrm{O}}_4$ and RbCo${\mathrm{F}}_3$, in which structural distortions occur as secondary effects; (9) order-disorder systems such as NaN${\mathrm{O}}_2$ and the ammonium halides (N${\mathrm{H}}_4$Cl, N${\mathrm{H}}_4$Br), in which no "soft mode" occurs in the spectral region ($\omega >{10}^{11}$ Hz) probed by ir, Raman, and neutron spectroscopy; (10) $\beta$-tungsten ($A-15\mathrm{}$) structures such as ${\mathrm{V}}_3$Si and ${\mathrm{Nb}}_3$Sn, which exhibit high-temperature superconductivity. These crystal categories are used to illustrate several phenomena of current physical interest: Specifically, we discuss harmonic and anharmonic mode coupling; "critical exponents" $\beta$ differing from one-half in the temperature dependences of the order parameter $\varphi \mathrm{}\left(T\right)={\varphi }_0{(T-T_0)}^{\beta }$ and of the soft-mode frequency $\omega \mathrm{}\left(T\right)={\omega }_0{(T-T_0)}^{\beta }$; and the recently discovered "central" modes centered at zero frequency, which grow in intensity as the transition temperature $T_0$ is approached from above or below. The review covers the period 1940-1972. A few 1973 works are mentioned for which the author had preprints in 1972 or very early 1973. This review is in no sense a comprehensive survey of ferroelectricity. Readers are referred to the following earlier reviews on that subject: Silverman (1966, 1969), Cochran and Cowley (1967), Blinc (1968), Murzin et al. (1968), Nettleton (1970), and Blinc and Zaks (1972).

A more or less self-contained introductory review is presented of the so-called Higgs phenomenon. This is the mechanism by which, in a certain class of gauge theories, the "photon" and would-be Goldstone scalar mesons conspire together to produce massive vector mesons via a "spontaneous" breaking of gauge invariance. It is conceivable that this is the way in which nature has chosen to unify weak and electromagnetic interactions. It is hoped that a reader of this review will come to understand the meaning of the first three sentences in this abstract and will then be able to proceed to confront a rapidly growing literature in the subject of gauge theories.

The spectroscopy of hadrons, both bosons and baryons, is critically examined with respect to the $\mathrm{SU}\left(3\right)\mathrm{}$ classification scheme. This is done with respect to the Gell-Mann/Okubo mass formula and the partial decay rates of member states of individual $\mathrm{SU}\left(3\right)\mathrm{}$ multiplet families. The agreement is exceedingly good. The baryons are catalogued into octets with $J^P=\frac{1}{2^{+}}, \frac{5}{2^{+}}, \mathrm{and} \frac{5}{2^{-}}$, nonets with $J^P=\frac{3}{2^{-}}, \frac{7}{2^{-}}, \mathrm{and} \frac{1}{2^{-}}$, and decimets with $J^P=\frac{3}{2^{+}} \mathrm{and} \frac{7}{2^{+}}$ Among the bosons, nonet structure is well established for $J^{\mathrm{PC}} \mathrm{multiplets}=0^{-+}, 1^{--}, \mathrm{and} 2^{++}$, with less firm evidence existing for a host of other multiplets, $J^{\mathrm{PC}}=0^{++}, 1^{+-}, 1^{-+}, 2^{--},2^{-+} \mathrm{and} 3^{--}$.

The motion of energetic charged particles inside a monocrystalline solid can be strongly influenced by channeling and blocking effects. The present article reviews the theory, the experimental studies, and some of the applications of these effects. The coverage of the published literature extends through June 1973.

A quantum foundation is developed for the methods employed in the calculation of the pre-equilibrium component of the energy spectrum of nuclear reaction products. It involves the application of the statistical theory of nuclear reactions to chained doorway states.

The literature of the past three years on the two-photon process for particle production $e^{\pm }+e^{-}\to e^{\pm }+{\gamma }^{*}+e^{-}+{\gamma }^{*}\to e^{\pm }+e^{-}+X$ (where $X$ is any $C=+\mathrm{state}$) or $p+p\to X_1+{\gamma }^{*}+X_2+{\gamma }^{*}\to X_1+X_2+{\mu }^{+}+{\mu }^{-}$ (where $X_1$,$X_2$ are hadron states) is reviewed in some detail. Both the theoretical aspects and the experimental feasibility of various processes are discussed especially for experimentalists' convenience.

The basic idea of the renormalization group is introduced and illustrative examples are presented. Emphasis is put on the application to the theory of critical phenomena. This article is prepared for pedagogical purposes. It is written at a level that a second-year graduate student in physical sciences can understand. No previous knowledge of critical phenomena or field theory is needed. We make no attempt to survey the field or cover a wide range of subjects. On the contrary, we limit the scope to the most basic aspects. We choose to elaborate at length to make the basic idea clear and the definitions precise, and to go through the examples very carefully. We feel that once these basic aspect are understood, there will be no difficulty in confronting the rapidly expanding literature on this subject.

Extensions of percolation theory to treat transport are described. Resistor networks, from which resistors are removed at random, provide the natural generalization of the lattice models for which percolation thresholds and percolation probabilities have previously been considered. The normalized conductance, $G$, of such networks proves to be a sharply defined quantity with a characteristic concentration dependence near threshold which appears sensitive only to dimensionality. Numerical results are presented for several families of $3D$ and $2D$ network models. Except close to threshold, the models based on bond percolation are accurately described by a simple effective medium theory, which can also treat continuous media or situations less drastic than the percolation models, for example, materials in which local conductivity has a continuous distribution of values. The present calculations provide the first quantitative test of this theory. A "Green's function" derivation of the effective medium theory, which makes contact with similar treatments of disordered alloys, is presented. Finally, a general expression for the conductance of a percolation model is obtained which relates $G$ to the spin-stiffness coefficient, $D$, of an appropriately defined model dilute ferromagnet. We use this relationship to argue that the "percolation channels" through which the current flows above threshold must be regarded as three dimensional.

A new formulation is presented for the angular distribution and the polarization of light excited by atomic and electronic collisions and modulated in time by the action of internal and external fields. The formulation disentangles geometrical and dynamical effects and stresses the extraction of data on the alignment and orientation of radiating atoms from observations of the emitted light. The treatment is set in the context of recent experimental and theoretical literature and points to new avenues of research.

The technique of measuring the energy distribution of electrons which have been field emitted from a cold cathode is considered. The general historical and introductory theory is presented. A survey of the experimental techniques and existing energy analyzers is given. Specific studies on clean metal surfaces in which work functions, band structure effects, surface states, thermal effects, and many-body effects have been studied are reviewed from both the experimental and theoretical points of view. Field emission energy distributions have been particularly valuable in studies of atoms chemisorbed on surfaces. Several theories of enhanced resonance tunneling due to chemisorbed atoms are discussed. Specific systems studied experimentally are reviewed. Inelastic adsorbate enhanced tunneling is also treated.

In this review we present the energies, configuration, and other properties of resonances (also called "compound states" and "temporary negative ions") in diatomic molecules. Much of the information is presented in the form of tables and energy level diagrams. Vibrational, rotational, and electronic excitation are discussed whenever these processes have given information on resonances; often these excitation processes proceed via resonances. The paper is divided according to molecular species (${\mathrm{H}}_2$, ${\mathrm{N}}_2$, CO, NO, ${\mathrm{O}}_2$), but the main conclusions are discussed by the nature of the processes involved.

Electrons colliding with atoms can form, at well-defined energies, compound states consisting of the target atom plus the incident electron. The compound states, which are also called "resonances" or "temporary negative ions," often dominate electron collision processes. In this review we discuss the experimental methods which are useful for studying these resonances, and review the results obtained by various investigators. We list the energies and the widths of resonances for H, He, Ne, Ar, Kr, Xe, Li, Na, Hg, and O. The configurations and other properties of resonances in atoms are discussed. Whenever applicable, results are presented in the form of tables and energy level diagrams.

The moment of inertial $\mathcal{I}$ of deformed nuclei increases with increasing angular momentum $I$ in the ground-state rotational band. Recent energy measurements of rare-earth nuclei show that while the increase is smooth at low angular momenta, at about spin 14-16 in a number of nuclei there is a sudden substantial rise in the moment of inertia. In some cases $\mathcal{I}$ rises faster than $I$ so that the rotational frequency $\omega =\frac{I}{\mathcal{I}}$ actually decreases with increasing $I$ producing a characteristic backbending $\mathcal{I}$ vs $\omega$ curve. A singular behavior at about these spin values was predicted over ten years ago by Mottelson and Valatin as a Coriolis antipairing effect. The present article will discuss the experimental data and describe calculations of the nuclear moment of inertia. Calculations made both before and after the recent experiments will be considered.

The complications of calculating the photoabsorption cross section of atoms for frequencies in the far ultraviolet and soft x-ray regions motivate an analysis of this problem on the basis of the Bloch semiclassical model of hydrodynamic charge oscillation in the neutral Thomas-Fermi atom. The hydrodynamic modes of oscillation of the neutral Thomas-Fermi atom have a continuous frequency spectrum. The resulting photoabsorption cross section is a continuous function of frequency which scales with atomic number $Z$ and in this sense is a universal cross section, approximately applicable to all heavy atoms. Numerical solutions of the normal mode functions of dipole charge oscillation are used to calculate the photoabsorption cross section in a range of photon energies $0.816 \mathrm{Z}\mathrm{eV}<ħ\omega <272\mathrm{} \mathrm{Z}\mathrm{eV}$, the range where the model is expected to be most realistic. In this range the semiclassical hydrodynamic cross section agrees with experimental data for the noble gases as well as could be expected for a cross section applicable to all atoms. The model cross section, extended to zero and infinite frequencies by analytical calculation, checks the sum rule to within 2%; but gives a value $I=4.95\mathrm{}$ $Z$ eV for the logarithmic mean excitation energy of stopping power formulas. The unrealistically low value of $\frac{I}{Z}$ results because the Thomas-Fermi atom exaggerates the number of electrons which absorb at low frequencies. Use of the hydrodynamic cross section in the approximate range of validity of this model $ħ\omega >0.816\mathrm{}$ Z eV gives $I=12.4\mathrm{}{e}^{\beta }$ Z eV, where $\beta$ depends upon the oscillator strength below $ħ\omega =0.816\mathrm{}$ Z eV which cannot be satisfactorily determined from the Bloch model. Used within its limitations the Bloch model of hydrodynamic oscillation in the statistical atom provides a useful method of estimating photoabsorption cross sections and could possibly be applied to other atomic processes.

This review of the properties of leptons, mesons, and baryons is an updating of Review of Particle Properties, Particle Data Group (Phys. Letters 39B, No. 1 (1972)). Data are evaluated, listed, averaged, and summarized in tables. A data booklet is also available.