On fitting deuterated methylidyne spectra in fusion plasmas

We discuss how the spectra of deuterated methylidyne, CD, must be analysed and fitted to experimental measurements in fusion plasmas, especially plasmas generated by the JET tokamak. The A 2Δ → X 2Π spectra can be reasonably well fitted by attributing a single rotational temperature to all three vibrational levels v=0,1,2, but the vibrational levels themselves are not thermally populated (v=2 is depleted). Off-diagonal vibrational transitions are negligible. We conclude that the data are sufficiently well fitted for the ratio of total light emitted from this CD transition to that observed in the wavelength region 429.4 nm to 430.9 nm to be calculated to about 6% accuracy. We find that there is probably a contamination, perhaps from the 3D 1Π → 2P 1Σ transition in D2 in the 429.4 nm to 430.9 nm observation region. (orig.)