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

Abstract

The photoelectron spectra of the X^{+ 2}Π_g ← X ¹Σ⁺_g 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⁺₀ = −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.