Knowledge Bank

``we would like to report here the first direct observation of motional narrowing effect in the P and R branches of the fundamental as well as $\nu =0\to 5$ overtone of the hydrogen deuteride molecule. For the $\nu =0\to 1$ transition we have used a cw difference-frequency laser tunable in the 2.2-4.2 micron infrared region. This laser operates by mixing of an argon-ion laser with a tunable dye laser in the non-linear optical crystal $LiNbO3$. The system has a resolution of about $10-4$ $cm-1$ and can be scanned more than $1cm-1$. Using a long path white-cell and ratio recording technique, we have observed $R1(0)$, $R1(1)$, $R1(3)$ and $P1(2)$ lines over the pressure range of 15 to 700 Torr. The motional narrowing effect, manifested in a slightly sub-Doppler width of the absorption profile, has been observed in all lines. Also introduction of buffer gases (Xe, Ar, Kr) to the HD sample enhances the narrowing effect. From a least-squares-fit of the data to a soft collision model we have found the narrowing parameter to be $\nu =1.7$ MHz/Torr and pressure broadening coefficient $\nu =6$ MHz/Torr. A hard collision model predicts the same value of $\gamma$(for the same J value) but gives v about 35% lower. The $\nu =0\to 5$ overtone is optoacoustically detected using an amplitude modulated tunable dye laser with a linewidth of $\le 3\times 10-4cm-1$. The sample cell is placed inside the dye laser cavity and the audio signal is lock-in detected. We have observed $R5(0)$, $R5(1)$, $R5(2)$, $R5(3)$, $P5(1)$ and $P5(2)$. Comparison of the narrowing and Broadening parameters will be reported.