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

Abstract

We have studied 3s(n-1 and pi-1) Rydberg states and D0(n-1) and D1(pi-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 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(pi-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(pi-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 D1 vibronic state are short-lived due to electronic autoionization to the D0 continuum.

Time-resolved photoelectron imaging using a femtosecond UV laser and a VUV free-electron laser

“…2(a)] shows the clear advantage of using VUV radiation over 198-nm DUV light [red curve in Fig. 2(a) [20], 264-nm UV pump and 198-nm DUV probe experiment (red) [16] and 260-nm UV pump and 161-nm VUV probe (blue) (this experiment). (b) Schematic energy diagram of ionization process.…”

“…In contrast, 161-nm radiation, with 1.5 eV higher probe photon energy, allows observation of the entire Franck-Condon envelope. Also shown in the figure is the He(I) photoelectron spectrum of jet-cooled pyrazine adopted from Oku et al [20]. The 260-nm excitation prepares an S 2 vibronic state, which rapidly dephases, within 23 fs [16], to isoenergetic S 1 vibronic states under the collision-free condition.…”

Elastic and inelastic low-energy electron collisions with pyrazine

“…The electronic excited states of pyrazine, important for the inelastic part of our calculations, have been studied extensively by experimental [29][30][31] and theoretical quantum chemistry methods. [32][33][34][35][36] The latter provide a basis for the description of the electronic excited states in our scattering calculations.…”

Visualization of chemical reaction dynamics: Toward understanding complex polyatomic reactions

“…9) and these vertical ionization energies are almost identical. 64,65) In other words, removal of a π* electron always requires the same energy.…”