The energy levels of lanthanide ions have been studied in great detail in the energy range up to $40\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$
(250nm). Recently an increased interest in the high energy levels between 40 000 and $70\phantom{\rule{0.2em}{0ex}}000\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ has emerged, partly triggered by the need for new luminescent materials for vacuum ultraviolet (VUV) excitation. Using synchrotron radiation many new energy levels have been discovered for various lanthanide ions. However, the spectral resolution of a synchrotron is limited and to resolve the complete energy level structure higher resolution tunable lasers are required. Unfortunately no high-resolution tunable lasers are available in the VUV. To overcome this problem two-photon spectroscopy may be applied. In this paper resonant two-photon spectroscopy is applied to measure the energy level structure of
Gd3+ in fluorides. Excited state excitation (ESE) from the
6P7∕2 level is shown to provide high-resolution spectra of the high energy levels of
Gd3+. The extension of the energy level structure is used to improve energy level calculations, which is especially beneficial for
Gd3+ where only a limited number of energy levels is available from conventional laser spectroscopy.
High-resolution three-photon laser spectroscopy has been performed for the first time to gain new spectroscopic information. Starting from the $6{s}^{2}^{1}S_{0}$ ground state of Yb,
6s15p1,3P1 Rydberg states were populated by two-photon-resonant, three-photon absorption via the
6s7s1S0 intermediate level. As a result of two-photon line narrowing Doppler-free spectra, showing well resolved isotope shifts and hyperfine structures, were obtained. From the hyperfine coupling constants
171,173A(1P1) the singlet-triplet mixing was derived.