3s Rydberg and Cationic States of Pyrazine Studied by Photoelectron Spectroscopy.

We have studied 3s(n-1and -1) Rydberg states and D0(n-1) and D1(-1) cationic states of pyrazine 1,4-diazabenzene 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 D0and 3s(n-1) states. The equilibrium geometries were found to differ slightly between the D0and 3s states, indicating the influence of a Rydberg electron on the molecular structure. The locations of the D1−D0and 3s(-1)−3s(n-1) conical intersections were estimated. From the line width in the D1← S0spectrum, we estimated the lifetime of D1to be 12 fs for pyrazine and 15 fs for fully deuterated pyrazine. A similar lifetime was estimated for the 3s(-1) state of pyrazine by REMPI spectroscopy. The vibrational feature of D1observed in the VUV−PFI-PE measurement differed dramatically from that in the UPS spectrum, which suggests that the high-nRydberg (ZEKE) states converging to the D1vibronic state are short-lived due to electronic autoionization to the D0continuum. [ABSTRACT FROM AUTHOR]