15min:

JAMES K. G. WATSON, Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, Canada K1A 0R6.

It is proposed that a set of diffuse interstellar bands (DIBs) can be attributed to the \widetilde B²-\widetilde X² electronic transitions of the cyanopolyyne cations, HC_2n+1N⁺, based on comparisons with laboratory observations in Ne matrices by Forney et al.¹ For HC₇N⁺ the DIB \lambda6614 is assigned as the 0₀⁰ origin band, and \lambda\lambda6426, 6196, 5982, and 5850 to transitions to upper-state fundamental vibrations. The three-peaked structure of \lambda6614 under high resolution^2,3 is assigned as P and R branches of the two subbands ²_1/2-²_1/2 and ²_3/2-²_3/2, with the central pair of branches overlapping. The 0₀⁰ band of HC₉N⁺ is assigned to \lambda7562, with vibrational bands at \lambda\lambda7358, 6919, and 6521, while the 0₀⁰ band of HC₁₁N⁺ is assigned to \lambda8531, with vibrational bands too weak to identify. These are all relatively sharp DIBs, with full widths at half maximum (FWHMs) in the range 1.7--3.1 cm^-1. Assignments are less satisfactory for shorter members of the series. The 0₀⁰ bands of HC₅N⁺ and HC₃N⁺ may be \lambda\lambda5797 and 5110, respectively, but the former seems to have a rather narrow PR separation under high resolution, although its FWHM of 2.9 cm^-1 lies in the above range, while the latter is much broader (FWHM = 45 cm^-1) than higher members of the series. The first member of the series, HCN⁺, does not have a \ldots³⁴\gets\ldots⁴³ electronic transition of this type.

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