Abstract
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.