Oxford University Press, Publications of Astronomical Society of Japan, 4(57), p. 605-609, 2005
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A search has been carried out for the C2- ion in diffuse clouds toward HD 23180, HD 24912, HD 24398, HD 46711, and HD 50064 using the HIDES spectrometer on the Okayama 188-cm telescope. An upper limit of 8.3 × 1010, 1.8 × 1012 cm-2 was obtained for the C2- column density. The upper limit value (3.8 × 1011 cm-2) toward HD 23180 is two orders of magnitude smaller than that of the C2 radical. Possible production mechanisms for C2- are discussed.