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

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Abstract
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 4f146s5d 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 1S0 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 F3/4 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.