THE PFI-ZEKE PHOTOELECTRON SPECTROSCOPY OF $C_{2}H_{2}$ AND $C_{2}D_{2}$ NEAR ITS IONIZATION THRESHOLD

$^{g}$S.T. Pratt, P. M. Dehmer, and J. L. Dehmer, J. Chem. Phys. 99, 6233 (1933). $^{h}$M. F. Jagod, M. R\""{o}sslein, C. M. Gabrys, B.D. Rehfuss, F. Scappini, M. W. Crofton, and T. Oka, J. Chem. Phys. 97, 7111 (1992).Author Institution: Institute of Atomic and Molecular Sciences Department of Chemistry, Academia Sinica National Taiwan UniversityThe ${1+1}^{\prime}$ two-colour pulsed-field induced zero-kinetic-energy (PFI-ZEKE) photoelectron spectra of $C_{2}H_{2}$ via a single rovibronic level of the $\bar{A}1 A_{u}$ state of acetylene were previously reported by Pratt $et al^{g}$. In this work, with similar approach the effects of the pulsed-field strengths and the rovibronic symmetries of the intermediate states of $C_{2}H_{2}$ and $C_{2}D_{2}$ on the PFI-ZEKE photoelectron spectra near the ionization threshold were investigated and will be reported. Although the number of observed transitions was limited, the spectroscopic constants of the zero-point energy levels of $C_{2}H_{2}^{+}$ obtained from the best fit are in good agreement with those reported by Jagod $et al^{h}$: $B=1.098 \pm 0.002 cm^{-1}, A_{80}$(spin-orbit constant)=$-30.84 \pm 0.05 cm^{-1}$, and $IP_{0}=91967.06\pm 0.03 cm^{-1}$. Those of $C_{2}D_{2}^{+}$ are $B=0.82\pm 0.03 cm^{-1}, A_{80}=-31.1\pm 0.3 cm^{-1}$, and $IP_{0}=92004.7\pm 0.3 cm^{-1}. IP_{0}$ is defined as the energy difference between the zero point levels of the ground states of the neutral and its cation. We propose to use $IP_{0}$ to denote the ionization potential of the molecule, of which the cation belongs to the Hund’s case(b) coupling