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Several moderately strong emission bands were discovered in the optical spectrum of comet Halley at 4930\AA\, 5300\AA\ and 6000\AA\ . These bands have been observed in three previous comets, and here we present evidence for the strongest band in another comet. The similar structure and simultaneous appearance of all three bands indicate that they arise from a single molecular species. The strongest unidentified band at 5300\AA\ was observed in the spectrum of comet Nakamura-Nishamura-Macholtz (1994m) obtained in July 1994 with the Multiple Mirror Telescope using the red channel spectrograph. We present low resolution spectra together with evidence that the unidentified band is due to a molecular ion. We also demonstrate that no known laboratory spectra of likely diatomic or triatomic molecular ions match the unidentified molecular emission features observed in comets.
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