The energy levels of lanthanide ions have been studied in great detail in the energy range up to 40 000 cm −1 ͑250 nm͒. Recently an increased interest in the high energy levels between 40 000 and 70 000 cm −1 has emerged, partly triggered by the need for new luminescent materials for vacuum ultraviolet ͑VUV͒ 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 twophoton spectroscopy is applied to measure the energy level structure of Gd 3+ in fluorides. Excited state excitation ͑ESE͒ from the 6 P 7/2 level is shown to provide high-resolution spectra of the high energy levels of Gd 3+ . The extension of the energy level structure is used to improve energy level calculations, which is especially beneficial for Gd 3+ where only a limited number of energy levels is available from conventional laser spectroscopy.

The increasing knowledge of the energy level structure of the lanthanides has led to new applications for these ions. The Gd 3þ /Eu 3þ quantum cutting couple was discovered a few years ago and may be applied in phosphors for fluorescent tubes based on a VUV xenon discharge. The VUV absorption of the phosphor LiGdF 4 :Eu 3þ is poor and therefore a sensitizer is needed. This contribution describes an investigation of the use of the strong 4f 12 !4f 11 5d VUV absorption bands of thulium ions as a sensitizer for the Gd 3þ /Eu 3þ quantum cutting couple. The incorporation of thulium in the quantum cutting phosphor LiGdF 4 does result in sensitization of Gd 3þ but not in downconversion in the Gd 3þ /Eu 3þ couple. A competing downconversion process between gadolinium and thulium occurs resulting in infrared emission of thulium, which is not suitable for lighting applications.

The energy levels of lanthanide ions have been studied in great detail in the energy range up to 40 000 cm À1 (250 nm). Recently, an increased interest in the high-energy levels between 40 000 and 70 000 cm À1 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 many 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 tunable lasers are available in the VUV. To overcome this problem two-photon spectroscopy may be applied. In this contribution the use of resonant and non-resonant two-photon spectroscopy is applied to measure the energy level structure of Gd 3+ in fluorides. Non-resonant two-photon excitation and resonant excited state absorption from the 6 P 7/2 level is shown to provide high-resolution spectra of the high-energy levels of Gd 3+ . The extension of the energy level structure may be beneficial for energy level calculations, especially for Gd 3+ where only a limited number of energy levels is available from conventional laser spectroscopy. r