Evidence of decay channel dependent linewidth narrowing in K-shell photoexcitation spectroscopy of N2 and CO A. Hempelmann, N. Saito*, F. Heiser, O. Hemmers, K. Wieliczek, J. Viefhaus, and U. Becker Fritz-Haber-Institut, Faradayweg 4-6, 14195 Berlin, Germany;*Electrontechnical Laboratory, Umezono, Tsukuba-shi, Ibaraki, Japan Vibrationally resolved coincident partial ion yield spectra of N2 and CO were measured after excitation of the 1s to pi* resonance with synchrotron radiation. The linewidths of the ion yield spectra becomes more narrow when the number of ejected electrons increases. The lifetimes of the singly charged molecular ions N2+ and CO+ are longer than the natural lifetime of the core excited states measured in absorption or via total yield.