COLLISION NARROWING EFFECT IN HYDROGEN DEUTERIDE
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Date
1981
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Ohio State University
Abstract
``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 $\nu=0\rightarrow 5$ overtone of the hydrogen deuteride molecule. For the $\nu=0\rightarrow 1$ 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 $LiNbO_{3}$. The system has a resolution of about $10^{-4}$ $cm^{-1}$ and can be scanned more than $1 cm^{-1}$. Using a long path white-cell and ratio recording technique, we have observed $R_{1}(0)$, $R_{1}(1)$, $R_{1}(3)$ and $P_{1}(2)$ 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 $\nu=0\rightarrow 5$ overtone is optoacoustically detected using an amplitude modulated tunable dye laser with a linewidth of $\leq 3\times 10^{-4}\mbox{cm}^{-1}$. The sample cell is placed inside the dye laser cavity and the audio signal is lock-in detected. We have observed $R_{5}(0)$, $R_{5}(1)$, $R_{5}(2)$, $R_{5}(3)$, $P_{5}(1)$ and $P_{5}(2)$. Comparison of the narrowing and Broadening parameters will be reported.
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