Probing vacuum ultraviolet energy levels of trivalent gadolinium by two-photon spectroscopy

The energy levels of lanthanide ions have been studied in great detail in the energy range up to 40 000 cm−1 (250 nm). 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 (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 Gd3+ in fluorides. Non-resonant two-photon excitation and resonant excited state absorption 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 may be beneficial for energy level calculations, especially for Gd3+ where only a limited number of energy levels is available from conventional laser spectroscopy.