Sensitivity of the Different Techniques

Resonant two-step ionization with two laser photons from pulsed or cw lasers represents the most versatile and sensitive detection technique. If laser Li excites all atoms or molecules that fly through the laser beam, single atoms or molecules can be detected [93, 99, 100] if the condition (1.48) can be fulfilled. 

We have seen in Sect. 1.4.1 that resonant two-photon ionization can reach a detection efficiency of 100 %, which means that every absorbed photon can be detected. The question is now how few molecules can be still detected According to Eq. (1.4a) the minimum number density of detectable absorbers is Ni AP/(PoctikL), where L is the absorption length and AP the absorbed power. If single absorbed photons can be detected, AP = h-v and we obtain for the incident laser power Pq = n hv 

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NRA and ERD can be used to study elemental/mo-lecular diffusion into a wide range of materials and can be used in conjunction with other complementary techniques such as PIXE and RBS. The sensitivity of the different techniques described here depends on a large number of factors including the experimental setup, detection efficiency, etc. but levels of about parts per thousand to parts per million have been reported. Excluding the position scanned microbeam NRA technique, depth resolutions ranging from several nanometers to several 100 nm for depths of up to several micrometers have been reported, with the resolution often degrading with depth. 

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