A SURVEY OF C4H, C6H, AND C6H- WITH THE GREEN BANK TELESCOPE

A survey of the carbon chain negative ion C6H- and the structurally similar radicals C4H and C6H has been done with the 100 m Green Bank Telescope toward 24 galactic molecular sources. The most readily observed molecular anion to date, C6H- was detected in two new sources: the dark clouds L1544 and L1521F, where it was found to be present at the level of 2.5% +/- 0.8% and 4% +/- 1%, respectively relative to neutral C6H. C4H was detected in nearly all dark clouds surveyed ( in six for the first time), and C6H was detected in five dark clouds and one translucent cloud. The observed C6H-/C6H( 1%-4%) and C6H/C4H(0.2%-1%) ratios in these sources suggest that C6H- may be close to detection in many other dark clouds. The fractional ionization derived from the C6H-/C6H ratio in L1544 and L1521F of 10(-8) to 10(-7) is comparable to earlier estimates from positive molecular ions.