Abstract
The aim of this dissertation is to accomplish high precision spectroscopic measurements on 12C16O2 spectra in the mid-infrared region. The research involves two spectroscopic methods and focuses on two different absorption bands. We report the application of a CW mid-infrared difference frequency generation (DFG) source generated in periodically poled lithium niobate (PPLN) nonlinear crystal by mixing a Nd:YAG laser and a Ti:sapphire laser to the saturation spectroscopy of 12C16O2 at 4.3 μm and 2.7 μm region. This DFG source has a characteristics of high power (> 6 mW) and narrow linewidth (< 100 kHz). Optical frequency comb (OFC) system is the second niche of this work. It plays a significant role in absolute frequency measurement of Ti:sapphire laser. The operating spectrum of our OFC system is from 500 nm to 1450 nm and the frequency accuracy is better than 10−12. First, 56 transition lines of sub-Doppler profile of the 0001-0000 band at 4.3 μm region are observed by means of a pump-probe scheme with frequency modulation spectroscopy. The Nd:YAG laser is stabilized onto the frequency standard line, 127I2 hyperfine transition R(56)32-0 a10 component. The Ti:sapphire laser is locked onto the center of CO2 transition and then beat with OFC for frequency measurement. The absolute frequencies of those transition lines are derived by the formula, fline = fTis − fYAG. The measurements of absolute frequencies for transitions up to J’ = 62 of the 0001-0000 band are accomplished for the first time with an given uncertainty of 30 kHz. The second work is to observe the sub-Doppler spectra of the [1001,0201]I-0000 band at 2.7 μm region by adopting saturated 4.3 μm fluorescence approach. Offset locking method is employed for stabilizing and precise tuning Nd:YAG laser. Ti:sapphire laser is stabilized onto a ultra-stable Fabry-Perot cavity and frequency counted by OFC. After fitting the saturated profile, the transition frequency can be determined. We successfully carry out absolute frequency measurements on transitions up to J’ = 30 of the [1001,0201]I-0000 band with an uncertainty of 40 kHz which is 15 times better than previous results. In addition, a new set of molecular constants of 0000 and [1001,0201]I states have been determined. The STD errors of our fitting are 29 kHz (9.7 × 10−7 cm−1) and 46 kHz (1.5 × 10−6 cm−1) respectively which are one order of magnitude better than previous results. With the help of our constants, one can provide more accurate linepositions to revise the current HITRAN database and construct more accurate molecular model of 12C16O2.