New energy levels of La I deduced from the investigation of red spectral lines

By means of laser spectroscopy and optogalvanic detection, we searched for unknown spectral lines of atomic lanthanum (La I) in the red region of the visible spectrum (6900–6200 Å). Most of the discovered lines could be interpreted as transitions between known energy levels due to wavelength and observed hyperfine structure pattern. For lines which we could not classify we searched for laser-induced fluorescence in order to identify at least one level involved in the transition. In this way we discovered 25 energy levels of La I having even parity. For two levels we could improve the accuracy of energy and hyperfine structure constants. These 27 levels classify 213 spectral lines.