Knowledge Bank

In a sense, $H3+$ is the simplest polyatomic molecule. Its spectrum was first detected by $Oka1$ who observed the $\nu 2$ fundamental band ($\approx 2500cm-1$) using an infrared difference frequency laser. At the same time, Shy, Farley, Lamb, and $Wing2$ observed $\nu 2$ for $D3+(\approx 1800cm-1)$ using an ion beam apparatus with Doppler tuning and CO laser excitation. Subsequently, Wing's group also detected $H2D+$ and $HD2+$ in the $1700-2000cm-1$ region (the lower frequency ends of the Coriolis-coupled $\nu 2$ and $\nu 3$ bands), but detailed assignments of the spectra have not yet proven possible. Very recently, Amano and Watson have observed and analyzed $\nu 1$ of $H2D+(\approx 3000cm-1)$ with the difference frequency $source.3$ We are currently studying isotopic forms of $H3+$ using a diode laser spectrometer and a liquid-nitrogen-cooled multiple-traversal hollow-cathode $cell.4$ The limited and unpredictable tuning ranges of diodes make them rather unsuitable for measuring the widespread and sparse bands of these light molecules. However, diodes still provide the best present means to cover the region from about 2000 to $2400cm-1$ between the practical limits of the CO laser and the difference frequency source, respectively. This region contains most of the $\nu 2$ and $\nu 3$ band structure for both $H2D+$ and $HD2+$. We have already observed all 4 isotopic forms: $H3+,H2D+,HD2+$, and $D3+$, but the total number of lines is limited (total $\approx 10$ as of February, 1984). In collaboration with J.K.G. Watson, we are attempting to assign the observed lines, and the search for further transitions continues. The measurements to date, and their dependence on current, temperature, and $H2:D2$ mixing ratio will be discussed.