The frequency characteristics that determine the response rate of atomic narrow-band fluorescence filters are calculated. Atomic cesium and rubidium vapors are used as the working medium. The response rates of various filters are compared. The cesium filter at a wavelength of 456 nm turned out to be the best.
The dependence of the quantum efficiency of fluorescence cesium vapor filters, which allow one to select with a high contrast the desired narrow-band signal against the background of broadband (solar) radiation, is studied. The most important characteristic of such filters, apart from the spectral band, is the quantum efficiency; i.e., the ratio of the number of florescent photons to the number of absorbed photons of excitation. It is experimentally shown that addition of a buffer gas to working cesium atoms can substantially increase the quantum efficiency as compared with that of working atoms in vacuum. The dependence of the quantum efficiency on the characteristic of transitions that are involved in population of atomic levels from which fluorescence occurs, and the processes resulting in an increase in the quantum efficiency of atomic fluorescence filters are considered.
A direct experiment is performed to determine the quantum efficiency of a cesium fluorescence filter. The fluorescence spectra of cesium atoms are recorded under excitation of the upper states of the second resonance doublet with a Bell–Bloom cesium lamp. Introduction of different noble gases into the cell with cesium leads to the appearance of additional fluorescence photons. It is found that a fluorescence filter based on atomic cesium vapor with addition of helium in the working cell has the highest efficiency and response rate of all known fluorescence filters based on alkali-metal atomic vapors.
It is shown that the excitation spectrum of atomic Cs vapor around 455.5 nm (21947 cm−1), observed by Marling et al., contains 0.1 cm−1 structure which is not expected on the basis of hyperfine constants previously reported in the literature. If this structure is extraneous it follows that a Cs vapor resonance ‘‘filter’’ designed to fluoresce in the red when irradiated on its blue (455.5 nm) resonance can suppress background light better than has heretofore been thought. Additional experiments to determine the origin and relevance of this structure are therefore recommended.