Knowledge Bank

The frequency tunable infrared source obtained by using the microwave modulation sidebands on $CO2$ laser radiation is sufficiently powerful ($5\sim 10mWatts$) and monochromatic $(\Delta \nu \le 20kHz)$ to conduct systematic saturation-dip spectroscopy in the $10\mu m$ region. Recently this technique was combined with the multiple reflection mirror system to increase the $sensitivity1$ so that it can be applied to molecular ion spectroscopy. We report on the first successful observation of this technique applied to the $\nu 2$ vibration-inversion-rotation transition of $H3O+$ ion. The magnetic field enhanced D.C. negative glow $discharge2$ of $\sim 30$ mTorr and low current produced the best result. So far the $\ast Q(6.6)$ transition has been observed with the signal to noise better than 10 with linewidth of $\sim 500kHz$ (FWHM). Using the frequency stabilized $CO2$ laser allows us to measure the frequency of the $H3O+$ transition with an absolute accuracy of $\le 300kHz$. The measurement of more transitions and attempts at observing rotational Zeeman effect and proton hyperfine structure are in progress.