An attempt to detect the C 2 intercombination transition lines in comet

In this paper we report the first attempt to detect directly the intercombination transition lines of the C 2 molecule in a comet. Indeed, these transitions play a key role in the C 2 fluorescence process but had never been observed directly so far. We conducted an observational program with the Cerro Tololo Interamerican Observatory 4 m telescope, located in Chile, on September 15 th and 16 th , 1997. The spectral range covered was 1.99-2.15 µm, in order to detect the emission lines of the X 1 Σ + g −a 3 Π u (3,0) band, that should be degraded to the blue with a bandhead located at 2.127 µm. This band was chosen because of its expected brightness and its wavelength, corresponding to a good atmospheric transmission and a relatively low sky background. In this paper we present the theoretical calculation allowing to model the expected spectrum and the results obtained with our observational data. These results, unfortunately , cannot allow the detection of the expected emission lines. This is due to a too strong noise, generated by the sky background emission lines, that cannot be totally removed from the spectrum.