Abstract: Sixteen short bursts of photons in the energy range 0.2–1.5 MeV have been observed between 1969 July and 1972 July using widely separated spacecraft. Burst durations ranged from less than 0.1 s to ∼30 s, and time‐integrated flux densities from ∼10−5 ergs cm−2 to ∼2 × 10−4 ergs cm−2 in the energy range given. Significant time structure within bursts was observed. Directional information eliminates the Earth and Sun as sources.

Abstract: Angular correlations were measured by delayed coincidences between electrons of incident energy about 80 eV scattered inelastically from helium and photons from excited $3^{1}P$ and $2^{1}P$ states. From the angular correlation in each case we deduce the ratio of the differential cross sections for exciting the magnetic sublevels of the excited state and the phase between the corresponding excitation amplitudes. We compare the atomic radiation patterns with those predicted by the Born approximation.

Abstract: The 6th ESLAB Symposium, organised by the Space Science Department (formerly ESLAB) of the European Space Research and Technology Center, was held in Noord wijk from 26-29 September 1972. This year the theme was "Photon and Particle Interactions with Surfaces in Space." More than 60 scientists attended mainly from ESRO Member States and from America. The first part of the Symposium was devoted to introductory lectures and to papers on interactions with spacecraft. The second half dealt with the photon and particle interactions with celestial objects, and ended with a general discussion and presenta tions of areas where new developments are required. The purpose of this Symposium was to throw light on the importance of the prob lems which are evoked by E. A. Trendelenburg in his introductory remarks, and to sum up our present understanding of these phenomena. It is hoped that this book will prove useful to physicists and engineers who are actually involved in space ex periments and are concerned with interactions of these types. R. J. L. GRARD OPENING ADDRESS Gentlemen, I should like to welcome you to the 6th ESLAB Symposium. In the past we have always organised this Symposium jointly with our sister in stitute, ESRIN, in Frascati, but unfortunately reductions in the scientific budget have forced ESRO to terminate the activities of that laboratory. Nevertheless, we have decided to carryon the tradition, and we shall continue on our own organising this series of symposia on specialised subjects."

Abstract: Correlation effects in parametric photon-pair production are studied within the framework of a physically realistic model. The analysis, which is fully quantum mechanical, takes into account the finite sizes of the target and the beam cross section, and allows for dispersion and anisotropy in the linear susceptibility. The correlations in position and time at which the two members of a parametrically generated pair may be detected are carefully evaluated. These correlations, which have been measured experimentally, are intrinsically quantum mechanical; i.e., they can be explained by no theory in which the subharmonic fields are described purely by $c$-number functions. A complete solution, from which field correlation functions of arbitrarily high order may be evaluated, is obtained by a method which at the same time allows for an arbitrary degree of parametric gain. The solution is expressed entirely in terms of a particular two-point field-correlation-function, as evaluated in lowest order in the incident field strength, at points distant from the target. The function in question is found by directly examining the fluctuating currents in the material medium, rather than by eliminating the matter variables at the outset through the introduction of a nonlinear electomagnetic susceptibility.

Abstract: Fluorescence lifetimes of low pressure gaseous systems are necessary for the understanding of electronic relaxation in an isolated molecule. A detailed experimental and theoretical analysis of the technique of single photon decay spectroscopy is presented. This includes the construction of nanosecond light sources, the detection and timing of single photons, and a statistical analysis of the results. Standards for time calibration and detection sensitivity are suggested. This paper provides the basis for subsequent studies of the fluorescence decay of aromatic vapours.

Abstract: A method is considered for consistent analysis of a quantum-mechanical system situated in a potential that depends periodically on the time, for example, in the field of a strong classical electromagnetic wave The emission of such a system is considered, and particularly the shift of the fundamental frequency ω' and the appearance of satellites ω' ± hω

Abstract: The photoelectric yield, secondary electron yield and energy distribution of low energy electron emission from Apollo 14 and 15 dust samples are reported for photon energies, 4 to 21 eV, and incident electron energies, 50 to 2500 eV. The differential photoelectron flux and energy distribution spectrum due to solar irradiation are derived, and the results are used to estimate the electrical characteristics of the lunar photoelectron sheath. Auger electron spectroscopic analysis of the elemental composition of the lunar dust samples is described, and a comparison is made with the results obtained by the S-161 X-ray fluorescence experiment on-board the Apollo 15 Command Service Module, while in lunar orbit. The results are consistent with those expected for a finegrain, predominantly insulating, powder.

Abstract: The decay curve of a spectral line with unresolved structure produced by a foil-excited atomic beam is considered in terms of a spherical tensor analysis of the density operator. In the case of L-S coupling, no external field and no cascades, the general expression is displayed and the necessary conditions for the appearance of quantum beats are discussed. By considering a series of special cases, some observable implications of the commonly assumed properties of the beam–foil excitation process are derived.

Abstract: This contribution attempts to review certain resonance effects which occur in the dynamic behaviour of semiconductor lasers. To study these effects theoretically, a rate equation approach is used for single-mode operation in the region of lasing threshold. The two basic rate equations are given and their transient solutions discussed. The existence of two time constants in these equations, viz. the electron lifetimeτ e and the photon lifetimeτ e; gives rise to a characteristic resonance frequency in the GHz region. This resonance manifests itself in transient ‘spiking’ effects, in quantum noise phenomena, and in high-frequency modulation experiments. In a modified form the resonance frequency may also be studied in lasers with external cavities and in double-diode configurations (or, equivalently, conventional devices with non-uniform excitation along the cavity length). In the latter two examples mentioned above, the resonance is excited by optical feedback of the laser radiation into the active medium via a ‘lossy’ or insufficiently inverted region. In the ‘spiking’ oscillations commonly observed at the commencement of laser operation, the initial ‘population overshoot’ is the cause of the resonance. For the case of quantum noise it is the requirement that the photon and electron populations have integer values which supplies the driving force-a true quantum effect. High-frequency modulation experiments directly reveal the same resonance frequency where a strong maximum in modulation intensity occurs.

Abstract: We have measured various coincidence rates between four photomultiplier tubes viewing cascade photons on opposite sides of dielectric beam splitters. This experimental configuration, we show, is sensitive to differences between the classical and quantum field-theoretic predictions for the photoelectric effect. The results, to a high degree of statistical accuracy, contradict the predictions by any classical or semiclassical theory in which the probability of photoemission is proportional to the classical intensity.

Abstract: Atomic bremsstrahlung-polarization correlations between incident electron and emitted photon are discussed for incident-electron kinetic energies in the range from 5 keV to 1 MeV. Results are reported from an exact numerical calculation in partial-wave formulation using screened potentials. It is shown that bremsstrahlung can be used as an analyzer of polarized electrons, a transmitter of polarization from electrons to photons, or a source of polarized radiation. Comparisons are made with previous work where available.

Abstract: The density matrix method is used to investigate the passage of light (the frequency of which corresponds to the exciton excitation) through an optically isotropic crystal, The investigation is made in a wave packet representation which enables one to observe the spatial motion of photons and excitons. The radiationless energy transfer from an electronic excitation to a dissipative subsystem is taken into account. [Russian Text Ignored].

Abstract: The photon energy dependence of the circular polarization efficiency (i.e. the ratio of the two-photon ionization probability with circularly polarized photons to that with linearly polarized photons) and the angular distribution of the ejected electrons are calculated for Li, Na and K atoms.

Abstract: Optical transition radiation produced by 40-60 MeV electrons crossing thin metallic foils has been studied in order to provide a beam diagnostic device. The radiation yield is roughly one photon per 100 electrons and the intensity appears to be Z independent ; the influence of bremsstrahlung is then fairly weak. Measurements of angular and ? dependence, spectral density and polarisation have been carried out and results are in good agreement with theories. When multiple scattering angles are negligible (< ?-1), a strong eneray dependence, close to ?3 , of the light intensity radiated into small solid angles is found. Moreover, the radiation is peaked in a direction making an angle ?-1 with the beam axis. Suitable optical arrangement make it possible beam position and profile determination. Measurement of the polarisation plane rotation by means of a scanning mirror provides a method of determining the incident angle of the beam onto the target. This device looks promising for diagnostics on particle beams-electrons as well as protons-with normalized energies ? in the range 1-103.

Abstract: The total hadronic photoabsorption cross sections of a number of nuclei (C, Al, Cu, Nb, Sn, Ta, Pb) have been studied in detail using a tagged photon beam over the energy range 1.7-4 GeV. The results are described, and compared with models of photoabsorption. (auth)

Abstract: By measuring the photon intensity emitted by He excited by electron impact in the 60 eV region as a function of the electron energy, with an apparatus capable of excellent signal to noise ratio and...

Abstract: A new formula for induced transition amplitudes in intense photon beams is derived which relates the photon-number description to the coherent-field description According to this formula transition amplitudes describing processes in which $m$ photons are added to the intense one-mode beam or subtracted from that beam are equal to the Fourier coefficients of the corresponding amplitude in the external field, expanded with respect to the phase of the classical field The same formula is used to study the expectation values of dynamical variables in $n$-photon states leading to the conclusion that for large $n$ such states are very much like classical statistical states with a given amplitude and evenly distributed phase