No abstract

'"o.r e 'e to ire q tme tll $r e-w clon o earc q e.e r 'k mq treemlfton ot 0for-aton Send comments regarw nq tt,% bu rden " t-ale or an 3t -[,

By using intense pulsed coherent x-ray sources that are currently under development, it will be possible to obtain magnified three-dimensional images of elementary biological structures in the living state at precisely defined instants. For optimum contrast, sensitivity, and resolution, the hologram should be made with x-rays tuned to a resonance of nitrogen near 0.3 nanometer. Resolution will then be limited mainly by the hydrodynamic expansion that occurs while the necessary number of photons is being registered. Problems of technique are also briefly discussed.

This review is addressed to the development of lasers that might generate coherent radiation at ultrashort wavelengths by stimulating recoilless nuclear transitions in solids. First, the authors review the basic physics of stimulated emission, superradiance and the kinetics of lasing, with particular attention to those aspects that characterize recoilless nuclear transitions in solid hosts. Then they classify the various approaches to pumping that have been proposed for resolving the ''graser dilemma''-that the pump can destroy the conditions essential to gain-and discuss the general requirements for specification of an active nuclide and its solid host. The authors then classify and review those graser systems proposed since 1980 and prior to July 1996 in the published literature of the field, namely, (1) those that would pump directly, either with radiation or with intense bursts of neutrons; (2) those that would pump indirectly by first generating a nuclear isomer; (3) those that would eliminate the need for population inversion; and (4) several miscellaneous concepts. The significance of recent relevant experiments is described and discussed, and, finally, recommendations for future research are made. [S0034-6861(97) CONTENTS

A study of the kinetic energy distributions of ionic fragments produced by subpicosecond irradiation of N2 with 248-nm radiation at an intensity of -10' W/cm is reported. These measurements, in comparison to other findings involving molecular excitation with charged particles and soft x rays, reveal several important features of the nonlinear coupling. Four ionic dissociative channels are identified from the data on the multiphoton process. They are N2~N +N N2 +~N + N +, N2'+~N+ + N'+, and N& +~N +N', three of which are charge asymmetric.The data for the energy distributions are found to be in approximate conformance with a simple picture involving ionizing transitions occurring within a time of a few cycles of the ultraviolet wave at an internuclear separation close to that of the ground-state (X 'Xg ) molecule. The implication follows that a strong nonlinear mode of coupling is present which causes a high rate of energy transfer.A simple hypothesis is presented which unites the ability for rapid energy transfer with the observed tendency to produce charge-asymmetric dissociation.

We describe a holographic microscope with a spatial resolution approaching the diffraction limit. The instrument uses a tiny drop of glycerol as a lens to create the spherically diverging reference illumination necessary for Fourier-transform holography. Measurement of the point-spread function, which is obtained by imaging a knife edge in dark-field illumination, indicates a transverse resolution of 1.4 microm with wavelength lambda = 514.5 nm. Longitudinal resolution is obtained from the holograms by the numerical equivalent of optical sectioning. We describe the method of reconstruction and demonstrate the microscope's capability with selected biological specimens. The instrument offers two unique capabilities: (1) it can collect three-dimensional information in a single pulse of light, avoiding specimen damage and bleaching; and (2) it can record three-dimensional motion pictures from a series of light pulses. The conceptual design is applicable to a broad range of wavelengths and we discuss extension to the x-ray regime.

No abstract

No abstract