A method of measuring refractive indices of gases in the vacuum ultraviolet using the electron beam from a 4.5-MeV microtron to produce Čerenkov radiation in the gases is described. In this method the threshold pressures for Čerenkov radiation in the ultraviolet and visible regions are compared for electrons of the same velocity. The ultraviolet detectors used in the present experiment are photomultipliers with gold photocathodes and windows of: (a) fused silica, and (b) “Spectrosil” to give results at about 1900 Å and a nitric oxide ionization chamber with a lithium fluoride window for results at about 1200 Å.
Preliminary results are quoted for the refractive indices of argon and nitrogen at the above wavelengths. The calculation of dispersion formulas has been deferred until further measurements have been made.
© 1963 Optical Society of America
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