Rovibrationally selected and resolved pulsed field ionization-photoelectron study of propyne: Ionization energy and spin-orbit interaction in propyne cation

By using a high-resolution infrared (IR) laser to prepare propyne (C₃H₄) in selected rotational levels of the excited v₁ (acetylenic C-H stretching) vibration mode prior to vacuum ultraviolet (VUV) laser pulsed field ionization-photoelectron (PFI-PE) measurements, we have obtained rotationally resolved VUV-PFI-PE spectra for the C₃H₄⁺ (X̃²E_3/2,1/2, v₁⁺=1) band. The analysis of these PFI-PE spectra leads to the determination of the spin-orbit constant of A=-13.0±0.2 cm^-1 for the C₃H₄+ (X̃²E_3/2,1/2, v₁⁺=1) state. Using this A constant and the relative rotationally selected and resolved state-to-state photoionization cross sections thus measured, we have obtained an excellent simulation for the VUV-PFI-PE origin band of C₃H₄⁺ (X̃²E_3/2,1/2), yielding a value of 83 619.0±1.0 cm^-1 (10.367 44±0.000 12 eV) for the adiabatic ionization energy of C₃H₄ [IE (C₃H₄)]. The present two-color IR-VUV-PFI-PE study has also made possible the determination of the C-H stretching frequencies v₁⁺=3217.1±0.2 cm^-1 for C₃H₄+ (X̃²E_3/2,1/2). The spectral assignment and simulation were guided by high-level ab initio calculations on the IE (C₃H₄), Franck-Condon factors for photoionization transitions, and rotational constants and vibrational frequencies for C₃H₄⁺. © 2008 American Institute of Physics.