Saturation technique for the measurement of photoionization cross-section of atomic excited states-A review

A review on the saturation technique is presented which has made utilized extensively for the absolute measurements of the photoionization cross section of the atomic excited states. This technique employed two step photoionization, in first step the state whose photoionization cross section to be measured is excited and in second step the excited state is photoionized. The energy of the exciting laser kept fixed and that of the ionizing laser is varied until the photo-ion signal gets saturated. Least square fitting of the photo-ion signals verses the ionizing laser energy density yields the photoionization cross section. Different experimental methodologies in combination with saturation technique have been employed for the measurement of photoionization cross section. These include heat cell with capacitive electrodes, atomic beam apparatus, thermionic diode ion detector, magneto optical traps (MOT), atomic beam-time of flight (TOF) mass spectrometer and DC/RF glow discharge cell. Thermionic diode ion detector and DC discharge cell offer an additional advantage of the measurement of photo absorption cross section (oscillator strength) of the atomic excited states. In addition, the availability of intense tunable dye laser made it possible to probe different region of continuum i.e. to search the regions of copper minima.