本實驗主要的目的是利用速度調制的方式來測量HeH+基頻帶(fundamental band)P(7)的吸收譜線頻率，未來與飽和吸收光譜方式所測量到的譜線頻率做比較，得到的實驗數據能夠增進分子常數的準確度。
我們使用一套窄線寬、可調、高功率的中紅外差頻雷射，將鈦藍寶石雷射(Ti-Sapphire laser)與釹釔鋁石榴石雷射(Nd-YAG laser)，通過有周期性極化反轉的鈮酸鋰晶體(PPLN)，產生中紅外波段的差頻雷射(DFG, Difference Frequency Generation )，本實驗主要是使用4.3 μm左右的波長，測量HeH+基頻帶P(7)的譜線躍遷頻率。
將差頻雷射光源，通過兩端具有電極的放電管(discharge cell)，其放電管長度約為1.1 m，外徑約為1.5 cm，並將氫氣與氦氣以1：99的比例充入放電管中，總氣壓約為2 torr，利用交流電高電壓放電的方式合成HeH+的分子。
速度調制的實驗裝置主要是由訊號產生器(function generator)、功率放大器(power amplifier)、高頻變壓器(high frequency transformer)所組成，訊號產生器頻率約為15 kHz，放電管電壓約為1 kV，電流最大為200 mA的交流電。
由於放電管的螢光雜訊，對偵測器的影響太大，以至於我們並沒有掃到HeH+基頻帶P(7)的吸收光譜，為了能測量出吸收光譜，首要工作在於降低放電管螢光雜訊對偵測器的影響。

We report the absorption spectroscopy of 4He1H+ at 4.3 μm by velocity modulation. The light source is CW mid-infrared difference frequency generation (DFG) generated in periodically poled lithium niobate (PPLN) nonlinear crystal by mixing a Nd-YAG laser and a Ti-Sapphire laser. The Nd-YAG laser is stabilized onto 127I2. The Ti-Sapphire laser is scanned near 852 nm. The discharge cell is full of HeH+ generated in 99 % helium and 1 % hydride, total pressure is about 2 torr . The experimental setup of velocity modulation consists of function generator, power amplifier, and high frequency transformer. The two electrodes of discharge cell are connected to AC sine wave source about 15 kHz, and the voltage is about 1 kV. We find the background arising from emission of the discharge too large to search the signal of HeH+ spectroscopy. We still try to find out the solutions to diminish the background of the emission.

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