Aims. Following the recent detection of C 6 H − in the laboratory and in space we have succeeded in studying the microwave spectrum of C 4 H − . We report here the first detection in space of this negative ion. Methods. We have observed in the envelope of the carbon star IRC +10216 five lines corresponding to the J = 9−8, 11−10, 12−11, 14−13 and 15−14 rotational transitions of C 4 H − . The C 4 H − lines have a cusped shape, denoting that this ion is formed in the outer part of the envelope, like its neutral counterpart C 4 H. Results. The abundance of C 4 H − in IRC+10216 is 1/6 of the abundance of C 6 H − and 1/4200 of that of C 4 H. Conclusions. The detection of C 4 H− , after that of C 6 H − , confirms the theoretical prediction that C-chain anions are abundant in interstellar clouds and yields a first measurement of the electron radiative attachment rates.

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The v2 = 1←0, 2←1, and 3←2 bands of the methyl radical were observed in the gas phase by infrared tunable diode laser spectroscopy at 606.4531, 681.6369, and 731.0757 cm−1. The observed vibration–rotation spectra were analyzed to derive precise values for the rotational constants, centrifugal distortion constants, and spin–rotation interaction constants. The absence of N = odd, K = 0 levels in the v2 = even states and of N = even, K = 0 levels in the v2 = odd states was confirmed, indicating that each vibrational state is nondegenerate. The observed band origins, when analyzed, led to a potential function for the out-of-plane bending vibration that has no potential hump at the planar configuration. The large negative anharmonicity of this potential was ascribed to a vibronic interaction with excited electronic states. The methyl radical was generated by a 60 Hz discharge in di-tert-butyl peroxide and was found to have a lifetime of about 1.4 msec; its concentration in the absorption cell was estimated to be about 1013 molecules/cm3, provided that a recombination reaction is the only route for eliminating methyl radicals.