Photoelectron spectroscopy of pyrazine anion clusters

We studied the energetics and character of electron binding in the pyrazine anion clusters by mass spectrometry, photoelectron spectroscopy, and theoretical calculations. The mass distribution showed that the minimum number of molecules in a neat cluster of pyrazine to form an anion was two, with a single pyrazine molecule incapable of accommodating an excess electron. On the other hand, even the addition of a very weak solvent such as Ar sufficed to bring the affinity level of pyrazine below the vacuum level. Photoelectron spectra of some pyrazine-containing anion clusters, (Pz)(1)(-)-Ar-n (n=1-10) and (Pz)(1)(-)-S-1 (S=pyrazine, benzene, and water), were obtained. A vibrational progression was observed in the photoelectron spectra of (Pz)(1)(-)-Ar-n. The electron affinity of pyrazine was determined to be -0.01+/-0.01 eV from extrapolation. A small drop in incremental electron affinity was observed from (Pz)(1)(-)-Ar-4 to (Pz)(1)(-)-Ar-5, indicating closure of the first solvation shell by four Ar atoms. The pyrazine dimer anion was found to exist in two isomeric forms of comparable energy, one with a symmetric structure and the other with an asymmetric one. The hydrogen bonding with the ring nitrogen strongly affects the binding of the pyrazine dimer anions. The electron affinity of the pyrazine dimer was estimated to be 0.13-0.15 eV. (C) 2002 American Institute of Physics.