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

Abstract

(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 ²Π, òΣ⁺ and ²Π 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 ν₂ (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 2π electrons upon excitation to Rydberg states converging to the ²Π state of HC₃N^+, as predicted by the present ab initio calculations on the cation core.

Keywords:

• cyanoacetylene
• propynenitrile
• propiolonitrile
• cyanoethyne
• HC₃N
• Rydberg states
• cationic states
• π electron delocalization
• REMPI spectroscopy
• REMPI-PES
• ab initio calculations of HC₃N⁺
• cyanopolyyne

Acknowledgements

The authors wish to thank S. Douin for his help during the cyanoacetylene synthesis stage, C. Le Bris for her technical support and M.-C. Gazeau and Y. Bénilan (LISA, Créteil) for providing the datafile of the one-photon absorption spectrum of HC₃N. L.V.M is grateful to the CAPES-COFECUB programme for a CAPES fellowship (contract # 525/06). J. L. acknowledges the financial support of the ‘Fonds National de la Recherche Scientifique’ and of the ‘Action de Recherches Concertées de la Communauté française de Belgique’.