Fine structure of the neutral tantalum atom

During many decades several groups have performed optical spectroscopy in order to gain information on the excited states of the electron shells of the atoms. Nevertheless, for nearly all atoms one can find gaps and mistakes in commonly used wavelengths and level tables. Even for very intense spectral lines in some cases it is not known, between which energy levels the corresponding transition takes place. Especially for atoms with complicated spectra these circumstances are given, caused by not closed outer electron shells like an only partly filled d-shell. The investigations planned within this project are concerned with the tantalum atom having three electrons within the 5d shell. Our group is investigating the tantalum spectrum since 1990, supported by several collaborations. In our experiments we excite transitions within the Ta atom in order to observe the hyperfine pattern, using cw high-resolution laser spectroscopy, detecting either optogalvanic or laser-induced fluorescence signals. Due to the large amount of data already determined within the long-term cooperation between the groups from Hamburg and Graz, it is possible to identify fine-structure levels by their hyperfine constants, and to calculate the energy of new levels if such are involved in the transition. Together with the colleagues in Hamburg, up to now we could find approximately 70 previously unknown energy levels and to eliminate other levels listed wrongly in commonly used tables, and to classify a large manifold of spectral lines as transitions between certain energy levels. Our investigations have been extended to the UV and IR region by means of high-resolution Fourier-Transform spectra, from which we can extract information on lines which can not be classifed using known energy levels. Within this project we plan to continue systematically our investigations and to end with publishing new wavelengths and energy level tables of tantalum.

As generally known, at the beginning of the 20th century the analysis of the discrete spectral emission of free atoms led to the development of atom models. The emission of spectral lines could be interpreted as caused by transitions between stationary states of the electron shell, where the difference of the state energies is given to the emitted photon. The energetic structure of the states is known as fine structure. This picture finally led to the ideas of quantum mechanics (around 1925). In further spectroscopic investigations, the stationary state energies and the angular momenta of the electronic shell of nearly all atoms were determined. Of course also information on the first, second and higher ions are available. Most information of this kind was collected by the National Bureau of Standards (NBS, now National Institute of Standards, NIST) and published between 1949 and 1958. Since that time a huge number of single publications appeared, nevertheless these tables are used as standard reference. Nevertheless, these commonly used tables are by far not complete and contain in some cases errors, like energy levels which are not really existent, wrong angular momenta, and wrong designations. The present project was devoted to the investigation of one atom for which the information available on its electron shell was rather poor: tantalum. Besides the traditional analysis of the wavelengths of spectral emission lines, supported by investigations in magnetic fields for the determination of the angular momenta of the involved energy states (levels), we have used high resolution Fourier transform spectra combined with laser spectroscopy. Further we use the fact that the angular momentum of the tantalum nucleus is I = 7/2. The magnetic interaction between electron shell and nucleus as well as the fact that the nucleus is not spherical and possesses an electric quadrupole moment causes a so-called hyperfine splitting of the energy levels, described by the factors A and B. This interaction is large enough to split the observed spectral lines into up to 21 hyperfine components. Thus, additional to the commonly used physical quantities (level energy, parity, angular momentum J, magnetic g-factor) a level is characterized additionally by a pair of hyperfine constants A and B. We could show that this pair is unique for each level with certain value of J; if A and B are known. Based on this fact we tried to identify spectral lines which could not be explained (classified) as transition between already known energy levels. In most cases such lines were excited by laser light, and the combination of all experimental results led to the discovery of a large number of previously unknown energy levels. Today, our tantalum data base contains nearly 6000 classified Ta lines, and we have enlarged the number of known levels from ca. 270 to ca. 550. Ta now is the chemical element for which the hyperfine constants are known best. Besides all new information gained, still a huge number of lines is not interpreted, most of them lines of singly ionized tantalum in the ultraviolet region.