Laser atomic fluorescence excitation and emission spectra of Na in an air acetylene flame are shown. in the excitation spectrum the laser was scanned through various wavelengths while the detector monochromator was held fixed near 589 nm. In the emission spectrum, the laser was fixed at 589.0 nm and the detector monochromator wavelength was varied. Explain why the emission spectrum gives one broad band, whereas the excitation spectrum gives two sharp lines. How can the excitation linewidths be much narrower than the detector monochromator bandwidth?