Rydberg states of cyanoacetylene investigated by (3+1) REMPI spectroscopy in the 77,000-90,000 cm-1 energy range

(3 + 1) resonantly enhanced multiphoton ionization (REMPI) spectroscopy coupled to photoelectron spectroscopy (REMPI-PES) has been carried out to study the Rydberg states of HC3N in the 77,000-90,000 cm(-1) region. Ab initio calculations (energies and optimized equilibrium geometries) have been performed for the first time for the low-lying (X) over tilde (2)Pi, (A) over tilde (2)Sigma(+) and (B) over tilde (2)Pi states of the cation HC3N+ in order to help the analysis. Thanks to the combination of the three-photon REMPI spectra, one-photon spectrum and photoelectron spectra, unambiguous assignments of the Rydberg series and their vibrationally excited members are proposed. The electronic Rydberg structure of cyanoacetylene is very similar to that of C2H2 and HCN (almost identical quantum defects), fully supporting the present analysis. New three-photon allowed Rydberg series are identified belonging to ns and nd series. The three-photon vibrational band assignments, confirmed by the photoelectrons spectra, reveal excitation of only one or two quanta of the nu(2) (C N) mode. Apparent discrepancies between the three-photon REMPI spectrum and the one-photon absorption spectrum are removed via a minor re-assignment of the absorption spectrum previously analysed by Connors et al. J. Chem. Phys. 60(12), 5011 (1974). Finally the observed analogy with C2H2 and HCN can be rationalized by a partial relocalization of the 2 pi electrons upon excitation to Rydberg states converging to the (X) over tilde (2)Pi state of HC3N+, as predicted by the present ab initio calculations on the cation core.