The mean transverse emission energy (MTEE) of electrons from GaAs photocathodes has been derived from limiting resolution measurements on proximity-focused image intensifiers. The value of 107+ 18 meV obtained for the (111)B emitting surface of GaAs/GazAlpz)As/GaP photocathodes is approximately two orders higher than that calculated by other workers, and does not depend on the applied electric field in the range 1000-4000 V mm-l. In contrast to previous work a comparable value has also been measured at 2 V mm-1 using a LEED optic assembly with correction for the distortion of the trajectories of these very low-energy electrons. The rough emitting surface of practical photocathodes may be responsible for the high transverse emission energies and a model is considered in which electrons are emitted from a sinusoidal surface. It is shown that the observed field-independent MTEE is obtained with a period-to-amplitude ratio of about four, and that the period lies between 2.5 and 100 nm. An initial study of photocathode surfaces by scanning electron microscopy has shown that a surface structure is produced by the heat cleaning process on a scale which is within this range.
A study has been made of the relationship between the facetting and surface structure observed upon (111)B and (100)GaAs photocathode surfaces and the mean transverse emission energy of the photoelectrons emitted from these cathodes. It is concluded that the mean transverse emission is largely independent of the degree of facetting and the presence of surface structure; however, a relationship exists between the temperature at which the cathode surface is cleaned and the mean transverse emission energy.
For orbiting astronomical telescopes and rocket spectroscopy the great angular dispersion of the Fabry-Perot interferometer should permit easier guidance tolerance for a given spectral resolving power with the added profit of the physical compactness of an etalon spectrometer or spectrograph. Moreover the superiority in luminosity and illumination of the interferometer instruments would permit shorter exposures and greater time resolution. Work aimed at extending the spectral range of the interferometer for high resolution work on the solar Frauenhofer spectrum (3 000 A-1 800 A) from stabilized rockets is reviewed. The first
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