Multistage photoionisation of ytterbium

A theory is developed of multistage isotopically selective ionisation of ytterbium ions. This theory is based on the rate equations for the level populations and on the transfer equation for the laser line intensities. An analysis of the time characteristics of multistage photoionisation is used to find the optimal laser radiation intensities for different ionisation stages. A multilevel model of multistage photoionisation developed for a natural mixture of isotopes is used to identify the ranges of wavelengths where the photoionisation selectivity is the highest. A mechanism of degradation of a short but powerful laser pulse is considered and the length of an efficiently excited medium is found.