The Role of the nπ* 1Au State in the Photoabsorption and Relaxation of Pyrazine

Vibrational Motions in Ultrafast Electronic Relaxation of Pyrazine

Ultrafast internal conversion via a conical intersection is ubiquitous in highly efficient photochemical reactions. Internal conversion from the 1ππ* to the 1nπ* state of pyrazine is the paradigm for this phenomenon; however, the relaxation occurs in such a short time (<20 fs) that the nuclear motion is difficult to observe in real time. The present study precisely measures the vibrational coherence transferred from the 1ππ* state to the 1nπ* state using time-resolved photoelectron spectroscopy with an unprecedented time resolution of 13.3 fs and reveals the key nuclear motions that drive the internal conversion.

Identification of an ultrafast internal conversion pathway of pyrazine by time-resolved vacuum ultraviolet photoelectron spectrum simulations

The internal conversion from the optically bright S₂ (¹B_2u, ππ*) state to the dark S₁ (¹B_3u, nπ*) state in pyrazine is a standard benchmark for experimental and theoretical studies on ultrafast radiationless decay. Since 2008, a few theoretical groups have suggested significant contributions of other dark states S₃ (¹A_u, nπ*) and S₄ (¹B_2g, nπ*) to the decay of S₂. We have previously reported the results of nuclear wave packet simulations [Kanno et al., Phys. Chem. Chem. Phys. 17, 2012 (2015)] and photoelectron spectrum calculations [Mignolet et al., Chem. Phys. 515, 704 (2018)] that support the conventional two-state picture. In this article, the two different approaches, i.e., wave packet simulation and photoelectron spectrum calculation, are combined: We computed the time-resolved vacuum ultraviolet photoelectron spectrum and photoelectron angular distribution for the ionization of the wave packet transferred from S₂ to S₁. The present results reproduce almost all the characteristic features of the corresponding experimental time-resolved spectrum [Horio et al., J. Chem. Phys. 145, 044306 (2016)], such as a rapid change from a three-band to two-band structure. This further supports the existence and character of the widely accepted pathway (S₂ → S₁) of ultrafast internal conversion in pyrazine.

Ab initio quantum dynamical analysis of ultrafast nonradiative transitions via conical intersections in pyrazine

Ab initio quantum dynamical calculations reveal the contributions of the optically dark S₁ (¹B_3u), S₃ (¹A_u), and S₄ (¹B_2g) states to ultrafast nonradiative transition from the optically bright S₂ (¹B_2u) state in pyrazine and suggest a much faster transition rate than previously believed.