Abstract
This dissertation present a versatile mid-IR molecular ion saturated absorption spectrometer capable of measuring rovibrational transition frequencies with sub-MHz accuracy. An extended negative glow discharge tube was used to produce molecular ions. It has the advantages of higher concentration of positive ions and near fi eld-free. The molecular ion transition is probed with sub-Doppler spectra enabled by an optical parametric oscillator (OPO). To determine the transition frequency, the OPO signal frequency was locked to an optical frequency comb. A tunable offset locking system was used to lock the OPO pump frequency to an iodine-stabilized Nd:YAG laser. With this offset locking system, the OPO idler frequency could scan precisely and obtain the saturated absorption pro file. The accuracy of the OPO idler frequency is 7 kHz, demonstrated by measuring the absolute frequency of the F2(2) component of the P(7) transition in the ν3 band of methane, which is recommended by CIPM as a frequency standard. Furthermore, intensity modulation and ion concentration modulation were employed to increase the signal-to-noise ratio (SNR) of the saturated absorption signal. Using this spectrometer, we measured 16 ν2 fundamental band transitions of H3+ and 9 fundamental band transitions of HeH+. The transition frequencies with acceptable SNR were able to determine to sub-MHz accuracy, which is better than the current theoretical calculations by two orders of accuracy (~300 MHz) for these two molecular ions. In addition, the homogeneous linewidth broadening parameters of H3+ R(1,0) transition influenced by discharge conditions were studied. In addition, absolute frequencies of 23 carbon dioxide transitions ranging from J = 2 to 70 for both the P and R branches in the [1001,0201]II ← 0000 band near 2.7 μm had been measured to the uncertainties varying from 7 to 17 kHz by using this spectrometer. A re fined set of molecular constants were obtained which gave the differences between measured and calculated values of less than 7 kHz.