Abstract
ABSTRACT We report the application of a CW mid-infrared difference frequency generation (DFG) source with Optical Frequency Comb (OFC) to molecule and molecular ion spectroscopy. This DFG source has the characteristics of narrow linewidth (< 100 kHz), wide tuning range (2.66 − 4.77 μm) and good power level (> 3 mW). We have accomplished two high precision spectroscopic measurements: molecular spectroscopy of N2O and molecular ion spectroscopy of ?3+. First, 44 transition lines of sub-Doppler profile of the 1000 ← 0000 N2O fundamental band at 4.5 μm are observed with the frequency modulation spectroscopy. The Nd:YAG laser is stabilized onto the iodine frequency standard line. The Ti:sapphire laser is locked onto the center of N2O transition and then beat with OFC for frequency measurement. The absolute frequencies of those transition lines are derived by fDFG =fTis − fYAG. The measurements of absolute frequencies for transitions up to J = 100 of the 1000 ← 0000 band are accomplished with an accuracy better than 95 kHz. For the molecular constant fitting analysis, we collaborated with Dr. Brian Drouin of Jet Propulsion Laboratory (JPL). With the help of the refined constants, the accuracy has been improved by two orders of magnitude compared with current HITRAN database. The tri-atomic hydrogen molecular ion ?3+, consisting of three protons and two electrons, is the simplest polyatomic molecule. Due to its simple structure, it is the benchmark of accurate calculation of polyatomic molecule. So far, most of H3+ transitions are observed by the velocity modulation spectroscopy which eliminates the strong absorption line of neutral gas. The transition frequency accuracy is about 150~300 MHz. Recently, our lab and McCall’s group have measured the saturated absorption spectrum of H3+. These two works achieve frequency accuracy better than 1 MHz with the help of optic frequency comb (OFC). Although saturation spectroscopy provides high accuracy, but the spectroscopic systems are more complex and the signal are smaller. In this dissertation, we use the velocity modulation spectroscopy to detect the vibration-rotation absorption transition of H3+ molecular ion, and measure its absolute frequency. For R(1,0) line of H3+, the frequency accuracy of the absorption transition is improved to 4 MHz. In the future, we will improve the stability of the frequency locking of our Ti:sapphire frequency comb and PPLN DFG and test the measurement accuracy of a weak absorption line. Finally, we will extensively measure the weak absorption lines of H3+.