Detector, atomic spectrometer density

Figure 6.12 Experimental two-color setup featuring an IR beamline, to generate intense shaped IR pump pulses, and a VIS probe beamline, to provide time-delayed probe pulses of a different color. Both beams are focused collinearly into a supersonic beam to interact with isolated K atoms and molecules. Photoelectrons released during the interaction are measured by an energy-calibrated TOE spectrometer. The following abbreviations are used SLM, spatial light modulator DL, delay line ND, continuous neutral density filter L, lens S, stretcher T, telescope DM, dichroic mirror MCP, multichannel plate detector.

A primary source is used which emits the element-specific radiation. Originally continuous sources were used and the primary radiation required was isolated with a high-resolution spectrometer. However, owing to the low radiant densities of these sources, detector noise limitations were encounterd or the spectral bandwidth was too large to obtain a sufficiently high sensitivity. Indeed, as the width of atomic spectral lines at atmospheric pressure is of the order of 2 pm, one would need for a spectral line with 7. = 400 nm a practical resolving power of 200 000 in order to obtain primary radiation that was as narrow as the absorption profile. This is absolutely necessary to realize the full sensitivity and power of detection of AAS. Therefore, it is generally more attractive to use a source which emits possibly only a few and usually narrow atomic spectral lines. Then low-cost monochromators can be used to isolate the radiation. 

Here, n is the number of atoms of this type per volume, is the inelastic mean free path and represents the depth of the region from which the elastic line can be observed", A is the surface area covered by both the radiation and the photoelectron detector,/hv is the incoming flux density of photons of energy hv, and the subscripts i and f refer to the initial and final states of excited electrons, respectively. This equation neither accounts for the size of the angular acceptance cone of the electron analyzer nor for the angular intensity variations due to the emission process or photoelectron diffraction effects (Section 3.2.23.4). Since electron spectrometers usually accept only a small emission cone of sohd angle it is often more... 

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