HC_9N {nu}_19 overtones in IRC +10216

The newly found series of rotational lines appears in seven groups with rotational constants (B) between 291.252 and 293.465MHz, with integer quantum numbers from J_up_=54 to 81 and upper-level excitation energies up to ~600K, well above what is typical of lines found in the outer part of this circumstellar envelope.

All evidence, especially the spectral splitting of the l-doubling components and the spacing between B values, unambiguously points toward rotational lines within nu_19_=2-8 that appear surprisingly strong, possibly due to very efficient infrared pumping and slow decay via the nu_19_ mode. The detection of nu_19_=5, 6, 7, and 8 has been possible thanks to collapsed l-doubling components to a single or only two spectral features per rotational transition.