Determination Of Boltzmann Constant Based On Cavity Ring-down Spectroscopy

Molecular spectroscopy is one of the most important method to study the molec-ular structure, internal energy distribution and the interaction between molecules. In last century, with the technology development in lasers and spectroscopy, the inven-tion of the optical frequency comb in particular, molecular spectroscopy has been more and more applied in the field of precision measurement, including testing basic laws of physics and symmetry, and the determination of fundamental physical constants, such as the fine structure constant ? and Boltzmann constant k_B. This thesis is focused on the development of a cavity ring down spectroscopy instrument for precision mea-surement, using various advanced technologies such as precise temperature control and laser frequency lock. The performance of the instrument is demonstrated by retrieving a few molecular line frequencies with very high precision, and the determination of the Boltzmann constant.The main content of this thesis is as follows:The first chapter introduces the background of the measurement of Boltzmann constant, different methods used by groups all over the world, such as Acoustic Gas Thermometry (AGT), Dielectric-constant Gas Thermometry (DCGT), Johnson Noise Thermometry (JNT), and Doppler Broadening Thermometry (DBT). In particular, the idea is given for the DBT measurement based on cavity ring-down spectroscopy. The principle of cavity ring down spectroscopy is briefly introduced, and also some broaden-ing effects that may be encountered in this study, such as collisional broadening, transit time broadening and saturated broadening etc.The second chapter introduces the design and build-up of the experiment appara-tus, including the mechanical design and temperature control of the sample chamber,laser frequency locking and scanning. In order to measure the Boltzmann constant pre-cisely, some major improvements have been carried out during last few years. The latest experimental scheme can be not only used for the determination of Boltzmann constant,but also in some other precision measurements.The third chapter introduces the analysis on the results using three different experi-mental schemes, and also the preliminary results on the measurement of the Boltzmann constant. At the same time, a method was developed to measure the Lamp dip of a molecular transition. Sub-kHz accuracy has been realized in the determination of a few CO lines. At last, perspectives are presented on further improvements in the de-termination of the Boltzmann constant as well as some other precision spectroscopy of molecules.