Knowledge Bank

Although the extremely slow conversion between different nuclear spin modifications of molecules in radiative and collisional processes has been well-established, no observation of non-thermal ratios of spin modifications has been reported except for $H2$ and its isotopic species. We have studied the $\nu 2$ fundamental band of $H3+$ using normal and para $H2$ as the source gases to see if we could detect a different ortho-to-para ratio of $H3+$. The reaction$\backslash [H2+H2+\to H3++H\backslash ]$ generates only para $H3+$ if para-$H2$ is $used1$, and the subsequent proton hop reaction $\backslash [H3++H¯2\to H¯2H++H2\backslash ]$ will retain para-$H3+$ while the hydrogen exchange reaction $\backslash [H3++H¯2\to H2H¯++HH¯\backslash ]$ is expected to convert $para-H3+$ to $ortho-H3+$. We have used two pairs of transitions, $R(4,3)-$ and $R(4,4)-$ separated by $0.13cm-1$ and $R(1,0)+$ and $R(1,1)+$ separated by $0.32cm-1$, for measurements of relative intensities. A clear difference in the intensity ratios of the ortho and para lines was detected depending on whether normal or $para-H2$ was used. The ion-neutral chemical kinetics will be discussed to explain the observed results.