Improvements in spectral resolution and sensitivity of a cesium based resonance fluorescence imaging monochromator (RFIM) are described. Alignment of the two lasers used in the excitation scheme in a co-propagating geometry have resulted in a detector spectral bandpass (270 MHz) that is narrower than the Doppler broadened profile (380 MHz) of cesium vapor at room temperature. Improvements in the experimental design, namely the addition of an image intensifier and cooled charge-coupled device (CCD) camera, have resulted in increased sensitivity; we report a limit of detection of 10<sup>8</sup> photons per pixel limited by image intensifier noise. Images of the cesium D<sub>2</sub> line emission (852.12 nm) from a cesium hollow cathode lamp were compared to an argon emission line (4S<sup>o</sup><sub>1/2</sub> → 4P<sub>3/2</sub>) at 852.14 nm from a silver/argon hollow cathode lamp as a demonstration of the spectral resolution. The ability of the RFIM to discriminate between photons exciting the <i>F</i> = 4 vs. <i>F</i> = 3 ground state hyperfine level (a frequency difference of 9.2 GHz) of the cesium 6<sup>2</sup>S<sub>1/2</sub> state was also investigated to further demonstrate the spectral selectivity. The potential use of the described RFIM for chemical imaging applications is discussed.
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