Multiphoton single and double ionisation of strontium in the range 532-541 nm

Abstract

By focusing an excimer pumped dye laser on an atomic beam, the authors have observed multiphoton multiple ionisation of the neutral strontium atom in the tuned wavelength range 532-41 nm. Sr+ and Sr++ yields were measured with a time of flight spectrometer. At least three photons are required to produce Sr+ and eight photons to produce Sr++. As a function of the wavelength, the Sr+ yield exhibits resonances which correspond to two-photon excitation of bound states (5s2-5p2 1D2 and 5s2-5p2 1S0) and three-photon excitation of autoionising states 4d5f J=1 and J=3 of the neutral atom. The Sr++ yield exhibits four broad smooth resonances that they interpret, from known spectroscopic data, as two-photon (5p1/2-4f), three-photon (5s-6p1/2, 3/2) and four-photon (5s-7d) resonances in the singly charged ion. The latter transitions start from a singly charged ion in the 5s or 5p1/2 state reached after three- or four-photon ionisation of the neutral ground state. The variation of the Sr++ pattern as a function of the dye laser beam intensity shows that resonances are shifted and broadened when the intensity is increased. These features have been interpreted by a perturbative treatment of Sr+ ionisation based on the use of an effective Hamiltonian which takes into account higher-order terms involved in the calculation of resonance effects. Calculated multiphoton ionisation probabilities are in good agreement with experimental data.