| Precision spectroscopy in physics plays a crucial role not only in testing physical theory but also in determining the fundamental physical constants. It improves our understanding of the physical world. As the simplest multi-electron atom, helium is an ideal system from various aspects of physics. The helium atom has two electrons, therefore there is no exact solution but numerical results in a series of approximations, even in the non-relativistic limit. The energy structure of helium is very important in developing quantum electrodynamics of multi-electron atomic system.The helium fine structure splitting is more difficult to be calculated accurately. However, helium offers considerable advantages for precise measurements than hy-drogen:the life time of the23P level of Helium is more than sixty times longer than that of Hydrogen, and the fine structure interval of He is three times larger than that of H (30GHz versus10GHz). The4He atom has a zero nuclear spin, therefore no hyperfine structure. It makes the energy levels of He are more simple than those of hydrogen. In addition, helium atoms can be decelerated and deflected using the laser cooling technique. These factors make helium a more attractive in the determination of the fine structure constant α in an atomic system. Combined with the latest theoretical calculations, a measurement with300Hz accuracy corresponds to determining α with a relative uncertainty of5ppb.This thesis develops a new apparatus for precision spectroscopy of the fine struc-ture intervals of helium. An intense metastable helium atomic beam is produced, trans-verse cooled and collimated by a resonanting laser. Atoms in the triplet metastable state are deflected by laser and separated from the beam background. Combining with pump laser and deflect magnet, atoms at a single quantum state are selected and de-tected. The fine structure interval between J=1and J=2in the helium23PJ state is measured with an experimental uncertainty of1.0kHz. The measured frequency is v12=2291177.4(1.0) kHz.This dissertation is organized as following:Chapter1summarizes the reported ex-perimental and theoretical studies on the helium fine structure. chapter2describes the details of the experimental apparatus built in Hefei, including preparation of metastable atomic beam, laser transverse cooling, two dimensions optical molasses, laser deflect-ing and state selecting, and spectral measurement. Chapter3presents the data analysis and the discussions on the error budget, as well as the preliminary results. Finally, chapter4presents the prospects on future improvements of the experiment. |
