3s Rydberg and cationic states of pyrazine studied by photoelectron spectroscopy

We have studied 3s(n^-1 and π^-1) Rydberg states and D₀(n^-1) and D₁(π^-1) cationic states of pyrazine [1,4diazabenzene] by picosecond (2 + 1) resonance-enhanced multiphoton ionization (REMPI), (2 + 1) REMPI photoelectron imaging, He(I) ultraviolet photoelectron spectroscopy (UPS), and vacuum ultraviolet pulsed field ionization photoelectron spectroscopy (VUV-PFI-PE). The new He(I) photoelectron spectrum of pyrazine in a supersonic jet revealed a considerably finer vibrational structure than a previous photoelectron spectrum of pyrazine vapor. We performed Franck-Condon analysis on the observed photoelectron and REMPI spectra in combination with ab initio density functional theory and molecular orbital calculations to determine the equilibrium geometries in the D₀ and 3s(n^-1) states. The equilibrium geometries were found to differ slightly between the D₀ and 3s states, indicating the influence of a Rydberg electron on the molecular structure. The locations of the D₁-D₀ and 3s(π^-1)-3s(n^-1) conical intersections were estimated. From the line width in the D₁ ← S₀ spectrum, we estimated the lifetime of D₁ to be 12 fs for pyrazine and 15 fs for fully deuterated pyrazine. A similar lifetime was estimated for the 3s(π^-1) state of pyrazine by REMPI spectroscopy. The vibrational feature of D₁ observed in the VUV-PFI-PE measurement differed dramatically from that in the UPS spectrum, which suggests that the high-n Rydberg (ZEKE) states converging to the D ₁ vibronic state are short-lived due to electronic autoionization to the D₀ continuum.