Photoionization detection of rare atoms and radioactive isotopes

3 Photoionization detection of rare atoms and radioactive isotopes 

Resonance three-step ionization has been used successfully, for example, for detecting trace amounts of platinum-group elements in natural samples by the scheme shown in 

The efficiency of this method was again demonstrated by Bekov et al. (1988) when detecting Rh traces in natural samples. Extraordinarily high concentrations of iridium and other siderophiles were discovered in the Cretaceous/Tertiary (K/T) boundary deposits and interpreted (Alvarez et al. 1980) as a result of a large extraterrestrial body falling upon the Earth, causing mass extinction of the biota dominant in the 

The photoionization detection of atoms in a buffer gas is advantageous over such detection in a thermal atomic beam because the atoms being detected reside for a long time in the region irradiated by the laser pulses. For example, the time it takes for atoms to diffuse out of a spherical region with a diameter of a is in this case given by 

Photoionization detection in a buffer gas has also been used to study the properties of superheavy (transuranium) elements with charge numbers Z 92. Isotopes of such elements can only be produced by fission reactions in heavy-ion collisions or by transfer reactions using radioactive targets. The elements produced can be placed in an optical buffer-gas cell for the purpose of laser resonance photoionization spectroscopy. This was successfully demonstrated with atoms of such radioactive elements as americium (Z = 95) (Backe et al. 2000), einsteinium (Z = 99) (Kohler et al. 1997), and fermium (Z = 100) (Sewtz et al. 2003). 

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