The rapid measurement of spectral intensity with an oscillating Fabry-Perot spectrometer: isotope abundance in mercury

A high-resolution photoelectric spectrometer employing a mechanically scanned Fabry - Perot interferometer is described. The spectrometer produces high-finesse spectral profiles continuously and rapidly at repetitive frequencies of up to 1000 c/s corresponding to a time resolving limit of 4 u s for a scan of 5 orders. The display is on an oscilloscope or a pen-recorder function-plotter. The instrument has been tested with the high-frequency discharge spectrum of mercury. Single intensity measurements are reproducible to better than 0.5%. The wavelength scale is linear to 0.5% over an order and can be corrected to four times this accuracy. Hyperfine structures agree well with recent determinations which use both pressure scanned and photographic Fabry-Perot systems. The mercury isotope abundances have been obtained from the spectral intensity measurements agreeing well with mass-spectrometer values. The r.m.s. deviation for a single determination is less than 0.5% of the percentage abundance. Possible applications to rapidly varying phenomena in gas discharges and shock waves, the determination of refractive indices and extension to the ultra-violet region for spectroscopy from an earth satellite are briefly considered.