VIBRATIONALLY EXCITED C4H

Rotational spectra in four new excited vibrational levels of the linear carbon chain radical C4H were observed in the millimeter band between 69 and 364 GHz in a low pressure glow discharge, and two of these were observed in a supersonic molecular beam between 19 and 38 GHz. All have rotational constants within 0.4% of the X 2 &Sgr; + ?> ground vibrational state of C4H and were assigned to new bending vibrational levels, two each with 2 &Sgr; ?> and 2 &Pgr; ?> vibrational symmetry. The new levels are tentatively assigned to the 1 &ngr; 6 ?> and 1 &ngr; 5 ?> bending vibrational modes (both with 2 &Pgr; ?> symmetry), and the 1 &ngr; 6 + 1 &ngr; 7 ?> and 1 &ngr; 5 + 1 &ngr; 6 ?> combination levels ( 2 &Sgr; ?> symmetry) on the basis of the derived spectroscopic constants, relative intensities in our discharge source, and published laser spectroscopic and quantum calculations. Prior spectroscopic constants in the 1 &ngr; 7 ?> and 2 &ngr; 7 ?> levels were refined. Also presented are interferometric maps of the ground state and the 1 &ngr; 7 ?> level obtained with the Submillimeter Array (SMA) near 257 GHz which show that C4H is present near the central star in IRC+10216. We found no evidence with the SMA for the new vibrationally excited levels of C4H at a peak flux density averaged over a 3 ″ ?> synthesized beam of ≥ 0.15 ?> Jy/beam in the 294–296 and 304–306 GHz range, but it is anticipated that rotational lines in the new levels might be observed in IRC+10216 when ALMA attains its full design capability.