Knowledge Bank

The A $\to$ X transition in OH is studied in emission from a Tesla discharge through Ar containing a small amount of water vapor. This system is used as a test case for investigating the capabilities of a CCD array detector (Photometrics CCD9000, 1024x256 pixels 25 $\mu$m on a side) in application to problems of high-resolution spectroscopy. The high information content of these spectra enables the 0-0 band to be least-squares fitted to a nonlinear model containing some 40 parameters, including 9 line-shape parameters. The combination of high signal-to-noise and large dynamic range makes possible the obsergation of OD lines amongst the OH lines, even though the natural abundance of OD is only 1/7000 that of OH. For example, in the residuals plot of Fig. 1 the prominent negative peaks near 3071.6, 3072.6, 3073.2, 3073.6, 3074.7, 3075.0, and 3076.0 \AA are all readily assignable to OD. When this species is included in the fit model, it should be possible to assess its abundance to better than 10% relative reliability --- a remarkable testament to the potential for this technology.