在許多實驗中，碘穩頻系統常用來作為雷射光頻率的標準。然而，量測碘分子的躍遷頻率時，大多數未考慮地磁對其頻率的影響。到目前為止，磁場對碘分子光譜影響的研究數量不多，主要因為碘分子對於磁場的敏感度較低。而引起碘分子光譜變化的原因在於不同量子態的 g值會有些許不同，導致因磁場產生的基曼分裂能量差不同，因此不同量子態基曼能階間的躍遷頻率會有些許偏離。

　　在我們的實驗中，觀察127I2 R(136)27-0、R(86)25-0、P(28)24-0及R(32)24-0超精細譜線在磁場下的變化，並量測其a10譜線的線寬，藉由勞倫茲曲線函數的線性疊加擬合出激發態與基態間的 g_F值差，在不同磁場下的平均值分別為0.0935、0.0765、0.0617及0.0571。雖然磁場對碘分子光譜在548.5奈米的影響目前無人探討，但得到的數值可合理解釋並預測譜線的變化。在未來，我們將量測其他譜線在磁場下的變化，並外加更多不同的磁場大小，以得到更精確的數值。

　　In many experiments, the iodine-stabilized laser system serves as an optical frequency standard. However, the geomagnetic field is usually not taken into consideration when measuring frequencies of molecular iodine hyperfine transitions. Up to now, the research on how the magnetic field affects absorption spectra of I2 hyperfine transitions is very limited because I2 is not sensitive to the magnetic field. The reason why spectra changes under the magnetic field is the difference in g-factor for different energy levels. As a result, each transition between Zeeman sublevels will exhibit different Zeeman shift.

　　In our experiment, the absorption spectra of 127I2 R(136) 27-0, R(86) 25-0, P(28) 24-0 and R(32) 24-0 hyperfine transitions are observed, and the linewidth of a10-lines is measured. By the superposition of several Lorentzian profiles, the difference of g_F-factors between the excited and ground state is calculated, and the average values of that under different magnetic fields are 0.0935, 0.0765, 0.0617 and 0.0571 respectively. Although the effect of the magnetic field on 127I2 hyperfine transitions at 548.5 nm is not theoretically investigated, the observed spectra can be reasonably explained and expected by the difference of g_F-factor obtained. In the future, more 127I2 hyperfine transitions under different magnetic fields can be investigated and better–controlled magnetic fields can be applied to get more precise g_F-factors.

Contents
1. Introduction
1.1 Motivation
1.2 Molecular Iodine
2. Theory
2.1 Hyperfine Spectra of Molecular Iodine
2.2 Zeeman Effect
2.3 Saturation Spectroscopy
3. Experiment
3.1 Light Source and Frequency Stabilization
3.2 Hyperfine Transition of Molecular Iodine
3.3 Magnetic Field and Polarization Variation
4. Results and Discussion
4.1 Data Analysis
4.2 Zeeman Splitting of I2 Hyperfine Spectra
4.3 Polarization Dependence on Pump beam
5. Conclusion
6. Bibliography

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