Spin-orbit interaction and Renner-Teller effect in HCCCCH⁺ studied by high-resolution photoelectron spectroscopy

The photoelectron spectra of the X^{+ 2}Π_g ← X ¹Σ photoionizing transition of diacetylene (HCCCCH) and d2-diacetylene (DCCCCD) have been recorded at high resolution using the technique of pulsed-field-ionization zero-kinetic-energy (PFI-ZEKE) photoelectron spectroscopy. The partially resolved rotational structure of the origin band of the spectra of HCCCCH and DCCCCD has enabled the determination of the adiabatic ionization energies of HCCCCH [E_ad = 82072.2(5) cm^-1] and DCCCCD [E_ad = 82090.0(10) cm^-1] and of the spin-orbit coupling constant [A = -31.1(4) cm^-1] of the ground vibronic state of HCCCCH⁺, which is smaller than the value of -33.3 cm^-1 commonly used since the work of Callomon (J. H. Callomon, Can. J. Phys., 1956, 34, 1046). Several excited vibrational levels of HCCCCH⁺ and DCCCCD⁺, including some affected by the Renner-Teller effect and Fermi interactions, have been observed and the fundamental wavenumber of the mode ν₉ has been determined in both HCCCCH⁺ (200.0(10) cm^-1) and DCCCCD⁺ (192.6(20) cm^-1). Possible assignments for several of these levels are discussed and deficiencies in the current understanding of the energy-level structure of the radical cation of diacetylene are pointed at.