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

Abstract

The internal conversion from the optically bright S2 ( 1 B2u, ππ*) state to the dark S1 ( 1 B3u, 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 ( 1 Au, nπ*) and S4 ( 1 B2g, nπ*) to the decay of S2. We have previously reported the results of nuclear wave packet simulations [Phys. Chem. Chem. Phys. 17, 2012] and photoelectron spectrum calculations [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 timeresolved spectrum [T. Horio et al., J. Chem. Phys. 145, 044306 (2016)] such as a rapid change from a three-band to two-band structure. This establishes the existence and character of the widely accepted pathway (S2 → S1) of ultrafast internal conversion in pyrazine.

Ab Initio Quasiclassical Simulation of Femtosecond Time-Resolved Two-Dimensional Electronic Spectra of Pyrazine

“…Experimentally, the photodynamics of pyrazine was studied by Suzuki and co-workers with femtosecond time-resolved photoelectron spectroscopy (TRPES) in the UV − and very recently by Leone and co-workers with femtosecond X-ray absorption spectroscopy . The transient population of the dark A u state in the photophysics of pyrazine has become a controversially discussed topic in the recent literature. ,− While Suzuki and co-workers could not find evidence for the intermediate population of the A u state, − Leone and co-workers reported detection of population in the A u state with a time delay of 200 fs . A recent computational simulation of the TRPES of pyrazine confirmed that the A u population indeed cannot be clearly distinguished from the B 3u population in TRPES .…”

Combined Surface-Hopping, Dyson Orbital, and B-Spline Approach for the Computation of Time-Resolved Photoelectron Spectroscopy Signals: The Internal Conversion in Pyrazine

“…19,20 In particular, in their very recent study, Kanno et and Q 10a modes and found that this two-state model reproduces well the main features of the experimental timeresolved spectrum. 20 As Sala et al found that several modes of B 2g and B 3g symmetry need to be included in the three-state model to couple the 1 A u (nπ*), 1 B 2u (ππ*), and 1 B 3u (nπ*) states, 8 the minimum model of Kanno et al may have resulted in an incomplete description of the photophysics of pyrazine. The three-state ( 1 B 2u (ππ*), 1 B 3u (nπ*), 1 A u (nπ*)) nine-mode model of Sala et al 8 has been used to simulate time-and frequency-resolved fluorescence, transient absorption, and electronic two-dimensional (2D) signals of pyrazine.…”

“…Note, however, that we have not employed any scaling of the spectra in the energy domain. In contrast to the work of ref , in which plane-wave continuum orbitals were used, the static-exchange continuum orbitals used in the present work take the long-range Coulomb potential of the molecular cation into account. As a consequence, the photoionization cross section attains a finite value at the threshold, in contrast to the plane-wave cross section. , …”

“…The photoionization signature of the 1 A u ( n π*) state has been the subject of several theoretical investigations. While Tomasello et al were not able to unequivocally identify the signature of the 1 A u ( n π*) state in the simulated time-resolved photoelectron spectrum and photoelectron angular distributions (PADs), Kanno and co-workers did not find evidence that the 1 A u ( n π*) state is involved in the excited-state decay of pyrazine. , In particular, in their very recent study, Kanno et al used a minimum-vibrational-mode model encompassing the 1 B 2 u (ππ*) and 1 B 3 u ( n π*) states and the Q 6 a and Q 10 a modes and found that this two-state model reproduces well the main features of the experimental time-resolved spectrum . As Sala et al found that several modes of B 2 g and B 3 g symmetry need to be included in the three-state model to couple the 1 A u ( n π*), 1 B 2 u (ππ*), and 1 B 3 u ( n π*) states, the minimum model of Kanno et al may have resulted in an incomplete description of the photophysics of pyrazine.…”

Density matrix via few dominant observables for the ultrafast non-radiative decay in pyrazine

“…In the present work we apply the SVD-based surprisal analysis 31 to identify the set of dominant observables that drive the S2-S1 non-radiative transitions in pyrazine. Internal conversion in pyrazine is extensively studied by various theoretical [42][43][44][45][46][47][48][49][50][51][52][53] and experimental methods. [54][55][56][57][58][59][60][61] Similar to other heteroaromatic organic chromophores, 13 strong non-adiabatic…”