氦原子為最簡單的多電子原子，相較於其他多電子原子，更容易將理論計算與實驗結果相互驗證，在精密量測的領域中佔有相當重要的地位。
　　本論文中，我們架設了一套穩定的亞穩態氦原子束系統。首先，我們將報告它是如何被設計以及如何運作。我們以射頻放電將基態氦原子激發產生亞穩態氦原子源，透過多次量測確認系統的穩定性，並且分析混入氬原子對亞穩態氦原子源與螢光光譜的影響。雷射與原子束會以互相垂直的方向交互作用，我們透過偵測螢光訊號來觀察原子束的許多特性，包含氦氣與氬氣氣壓的影響、速度分佈、發散角、原子數目以及原子束通量。

　　Helium, as the simplest multi-electron atom, is a good testing ground to verify the difference between theoretical calculations and experimental measurements. As a consequence, helium plays an important role in the field of precision measurements.
　　In this thesis, we built a stable metastable helium atomic beam system. Firstly, we will report how we designed this system and how it works. The metastable helium source is produced by radio frequency (RF) discharge excitation. We checked the stability by numerous measurements, and analyzed how argon mixture affects helium beam. Then we applied the crossed-beam method and observed the characteristics of atomic beam by detecting the fluorescence signal. The properties of atomic beam will all be studied in this thesis, including the influence of helium and argon pressure, speed distribution, divergence angle, number of atoms and atomic beam flux.

摘要.....i
Abstract.....ii
致謝 (Acknowledgements).....iii
Contents.....v
List of Figures.....viii
List of Tables.....ix
1 Introduction.....1
1.1 Motivation.....1
1.2 Transitions at 1083 nm in atomic helium.....1
2 Theoretical background.....3
2.1 Atomic and molecular beam.....3
2.1.1 Mean free path.....3
2.1.2 Knudsen number.....4
2.2 Doppler-free laser spectroscopy.....5
2.2.1 Doppler broadening.....5
2.2.2 Saturated absorption spectroscopy.....6
2.2.3 Crossed-beam method.....6
2.3 Discharges.....7
2.3.1 Plasma production.....7
2.3.2 RF-driven discharge system.....9
3 Laser system.....10
3.1 Saturated absorption spectroscopy for helium at 1083 nm.....10
3.2 Frequency stabilization of DFB laser.....11
3.3 ECDL at 1083 nm for atomic beam measurement.....13
4 Vacuum system.....16
4.1 Atomic beam setup.....16
4.2 Our structure of metastable helium source.....18
4.3 Fluorescence detection chamber.....22
4.4 Vacuum clearance.....23
5 Characteristics of atomic beam.....25
5.1 Fluorescence spectroscopy of helium 2 3S1→2 3P1,2 transitions at 1083 nm.....25
5.2 Observation of mixing argon in helium beam.....30
5.3 Speed distribution and divergence angle.....32
5.4 Atomic beam flux.....35
6 Conclusion and future works.....37
6.1 Conclusion.....37
6.2 Future works.....38
References.....39
A Python program for simulation of beam flow.....41
B PI control circuit.....43

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