New even and odd parity fine structure levels of La I discovered by means of laser-induced fluorescence spectroscopy

In highly resolved Fourier-transform spectra as well as in laser-generated spectra of an Ar-La-plasma we found a huge number of spectral lines which could not be interpreted as transitions between known energy levels of Ar I, Ar II, La I and La II. If these lines show a hyperfine structure splitting larger than the Doppler width of a single component, the line must belong to La. Some of these lines were investigated by laser excitation, and we could find eight even and three odd before unknown energy levels of La I. Among them was the level 25558.774 cm^-1, even parity, J=3/2, A=248(2) MHz, which has an exceptionally low energy for a previously unknown level.