Experimental Setup For Precision Spectroscopy Of Helium

The precision spectroscopy of atoms is a primary method to obtain fundamental physical constants and to test elementary laws in physics. Because of the relatively simple structure of the4He atom, the accuracy of the fine structure intervals can be theoretically calculated to the ppb level. On the experimental side, the accuracy of the measured23P0,1,2fine structure interval also reaches comparable level. It makes such measurement become an important method to verify the quantum electrodynamics the-ory. Shwartz has proposed that the comparison of the experimental and theoretical results can also be applied to determine the fine structure constant a. After about50years studies in this direction, the accuracy of the fine structure constant measured in this way has reached dozens of ppb, which approaches the accuracy of the CODATA (International Data Committee) recommended value. A variety of completely differ-ent determinations of the fine structure constant also provide a test of the elementary physical principles including the quantum electrodynamics theory.We developed a precision spectroscopy apparatus for measurements of the fine structure intervals of the23P0,1,2levels of4He. In the experimental setup, transverse cooling is applied to obtain an intense metastable helium atom beam, and to bend the atoms at the2351metastable state from the original atomic beam to decrease the back-ground due to21S0atoms and high-energy photons. In the spectral measurements, an external cavity diode laser is locked to a temperature controlled ultra stable optical cavity. The spectral scanning is accomplished by tuning the sideband of the frequency-locked diode laser produced by an electro-optic modulator to maintain sufficient fre-quency stability during the scan. The experimental methodology has been tested on the recently built apparatus, and the analysis shows that a sub-kHz precision is feasible.Varies technologies developed in this study have been applied in some other pre-cision spectroscopy studies. Laser locking on the thermal-stabilized super stable cavity has been used in the development of the laser-locked cavity ring down spectroscopy. We also proposed that such cavity ring down spectroscopy method can be applied to determine the Boltzmann constant. Numerical simulations show that the method has the potential to determine the Boltzmann constant to ppm accuracy, approaching to the optimal level among all the proposed methods.This thesis will focus on the setup that we have built for the determination of the23S1-23P0,1,2fine structure intervals of4He, as well as the precision spectroscopy studies using the developed techniques. The thesis consists six chapters. The first chapter describes the history of the theoretical and experimental studies of the preci-sion spectroscopy of the helium atom. The idea and configuration of our experimental setup is also presented. The second chapter describes the principle and structure of the helium atomic beam and the transverse laser cooling. The third chapter is devot-ed to the optical parts of the apparatus, including laser frequency locking system and the spectral scanning system. The forth chapter of this thesis introduces the finite el-ement method used in the design of the precision magnetic field. Chapter Five gives the preliminary experimental results and the analysis. The last chapter describes the laser locking and scanning technique developed in this study, and its application in the laser-locked cavity ring down precision spectroscopy. The proposal to determine the Boltzmann constant by measuring the molecular Doppler line width with the cavity ring-down spectroscopy method is also presented.