H3^+為結構最簡單的三原子分子離子，其結構由三個質子與兩個電子組成。H3+的簡單結構使得理論計算上可以達到很高的精確度。具有指標性的H3^+三原子分子離子，其理論計算方法可以廣泛應用在其他種類的三原子分子離子。

在2011年，我們團隊第一個利用飽和吸收光譜方式量測到H3+的躍遷頻率。陳炫辰建立了一套單頻連續波中紅外光的光摻量震盪器(optical parametric oscillator)系統，用來觀測中紅外分子離子飽和吸收光譜。實驗中陳炫辰利用調製頻率的方式，調製OPO的pump雷射的輸出頻率，經由鎖相放大器取得三階解調微分訊號，最後利用光纖光頻梳系統進行量測。最後得到的躍遷頻率與其他團隊的躍遷頻率相比有著很大的差異。

現在我們改善了之前的中紅外分子離子飽和吸收光譜儀系統，包括了延伸負光輝放電管、OPO、碘穩頻系統和可調式的差頻鎖相系統。我們使用了一台Nd:YAG雷射做為OPO的pump雷射，這台pump雷射來自可調式的差頻鎖相系統。OPO產生的signal光與idler光其波長範圍分別為1350 nm到1640 nm與3.2 μm到4 μm。除此之外，我們利用離子濃度調製與pump強度調製的方式取代了先前pump光輸出頻率的調製。利用這個方式我們量測到7條R branch的躍遷頻率。有6條的不準度都在MHz以下；和其他團隊的R branch躍遷頻率比起來，比較值都在MHz以下。在Q branch躍遷頻率上，我們也量測到Q(1,0)，其躍遷頻率的不準度在MHz以下。

H3+, which consists of three protons and two electrons, is the simplest polyatomic molecule. Due to its simple structure, the theoretical calculations to a very high accuracy can be performed. H3+ plays a role as benchmark molecule in other calculations of triatomic species.

In 2011, our group first measured the transition frequencies of H3+ by use of saturation absorption spectroscopy. Chen-Hsuan Chen built a CW optical parametric oscillator system for the observation of mid-infrared saturation absorption spectroscopy of molecular ions. He used the frequency modulation method in the experiment. He applied frequency modulation on the output frequency of pump laser of OPO and derived the three order error signal from lock-in amplifier. He measured the transition frequencies with fiber OFC. The transition frequencies of H3+ were large discrepancy comparing with other groups.

We have improved a mid-infrared saturation absorption spectroscopy system of molecular ions. The system includes extended negative glow discharge system, mid-infrared optical parametric oscillator (OPO), I2 stabilized Nd:YAG laser system and tunable offset locking system. We use Nd:YAG laser from tunable offset locking system as pump laser of OPO. The wavelength range of signal wave and idler wave is 1350 nm ~ 1640 nm and 3.2 μm ~ 4 μm, respectively. Furthermore, we replace the method of modulation on the output frequency of pump laser with concentration modulation of ion and pump intensity modulation. We have observed seven of R branch, six of them have sub-MHz uncertainties. Our results are consistent with other sub-Doppler measurements of R branch with a discrepancy MHz. One of the Q branch, Q(1,0), is also measured and have sub-MHz uncertainties.

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