Photoionisation laser spectroscopy of ytterbium atoms in the presence of a DC electric field

The authors report a study of the stepwise photoionisation of ytterbium atoms near the first ionisation threshold in the presence of a DC external electric field. Several spectra corresponding to ionisation from the levels 4f¹⁴6s7s ¹S₀ and ³S₁, 4f¹⁴6s6p ¹P₁⁰ and 4f ¹⁴6s5d ¹D₂ and to various polarisation schemes of the exciting light beams were recorded by detecting the ion signal. Modulations of the photoionisation cross sections above the field-free ionisation limit were observed and investigated: for the 6s7s ¹S ₀ and ³S₁ initial levels, the spectra exhibit modulations only for pi polarisation of the last excitation, spectra obtained via the 6s6p ¹P₁⁰ level exhibit modulations for the pi pi polarisation scheme only. An experimentally unprecedented result is obtained for the 6s5d ¹D₂ level: in that case the largest modulation rate is obtained for sigma polarisation of the last excitation. The behaviour of the rate and of the period of modulation as a function of energy were studied. It was observed that the period of modulation as a function of energy were studied. It was observed that the period of modulation at the ionisation limit varies with the electric field intensity F according to a F^3/4 law. The structure of the Stark spectra below the field-free ionisation limit was also investigated. Narrow resonances, all having a symmetrical shape, were recorded. They could be identified by calculating their energies using perturbation theory. A number of doubly excited levels were recorded, their influence on the spectra was analysed.