Effect of electron correlation and shape resonance on photoionization from the S1 and S2 states of pyrazine

Abstract

In a previous study [T. Horio, T. Fuji, Y.-I. Suzuki, and T. Suzuki, J. Am. Chem. Soc. 131, 10392 (2009)], we demonstrated that the time-energy map of photoelectron angular anisotropy enables unambiguous identification of ultrafast S(2)(ππ*)-S(1)(nπ*) internal conversion in pyrazine. A notable characteristic of this map is that the forbidden ionization process of D(0)(n(-1)) ← S(2)(ππ*) gives a negative photoelectron anisotropy parameter. In the present study, we elucidate the mechanism of this process by calculating the photoionization transition dipole moments and photoelectron angular distribution using the first-order configuration interaction method and the continuum multiple scattering Xα approximation; these calculations at the S(0) equilibrium geometry reproduce the observed anisotropy parameters for D(0) ← S(2) and D(0) ← S(1) ionizations, respectively. On the other hand, they do not reproduce the small difference in the photoelectron anisotropy parameters for the D(1)(π(-1)) ← S(2) and D(0) ← S(1) ionizations, both of which correspond to removal of an electron from the same π* orbital in the excited states. We show that these ionizations are affected by the ka(g) shape resonance and that the difference between their photoelectron anisotropy parameters originates from the difference in the molecular geometry in D(1) ← S(2) and D(0) ← S(1).