High-resolution observations of the infrared spectrum of neutral neon

Journal of Research of the National Institute of Standards and Technology, May-June, 2004 by Craig J. Sansonetti, Marion M. Blackwell, E.B. Saloman

For each spectrum the lines were initially measured with respect to the uncorrected wave number scale. A correction factor was then determined by taking the unweighted average of individual correction factors calculated from each of the standard lines. Not all standard lines appeared in all spectra. For spectra covering the range 7000 [Angstrom] to 11 000 [Angstrom] about 40 standards were used and for the longer wavelength regions about 27 standards were used. In all cases the average correction was in the range 4 to 6 parts in [10.sup.7] and the standard deviation of the individual values was about 1 part in [10.sup.7].

The final wave number for each line was calculated from the corrected values as the unweighted average of the individual measurements. For most lines there were two to four observations. For a few lines there was only a single observation; for others there were as many as six. The uncertainty for each line was calculated as the quadrature sum of three terms: the calibration uncertainty as measured by the standard deviation of the individual line correction factors, the standard deviation of the multiple measurements of the line, and the estimated precision with which the line position could be measured in the spectra. For most lines of moderate or greater intensity the uncertainty is dominated by the first two terms. The third term is calculated as the line width divided by twice the signal-to-noise ratio at the line center [15]. It is included to insure that weak or broad lines are not assigned an unreasonably low uncertainty because of an accidentally high degree of agreement between a small number of measurements.

For each Ne spectrum the radiometric response of the combination of FTS, filters, and detectors was determined by recording the spectrum of the standard tungsten strip lamp with the same filters, detectors, and FTS observing parameters. This spectrum was compared to the previously calibrated output of the strip lamp to generate an instrumental response curve that was used to adjust the integrated intensities of the spectral lines to a uniform linear dependence on the number of photons detected. With this choice for the calibration, the ratios of intensities of lines with a common upper level give directly the branching ratios of the various decay paths.

After calibrated intensities had been obtained for each spectrum, lines of moderate intensity in the overlapping spectral regions were used to determine scaling factors that were applied to place all of the spectra on a common intensity scale. The intensities from the multiple observations of each line were then averaged. For the two spectra in the 13 000 [Angstrom] to 50 000 [Angstrom] region, intensities of the 11 lines between 9218 [cm.sup.-1] and 9465 [cm.sup.-1] were omitted from the intensity average because they differed systematically from the intensities of the same lines measured in the two shorter wavelength regions. These lines were at the extreme end of the long wavelength region where the instrumental response was very low. Finally, the entire set of average intensities was scaled to obtain values on a linear scale from 1 to 100 000.