Ultrafast internal conversion in ethylene. I. The excited state lifetime

Abstract

Using a combined theoretical and experimental approach, we investigate the non-adiabatic dynamics of the prototypical ethylene (C(2)H(4)) molecule upon π → π∗ excitation. In this first part of a two part series, we focus on the lifetime of the excited electronic state. The femtosecond time-resolved photoelectron spectrum (TRPES) of ethylene is simulated based on our recent molecular dynamics simulation using the ab initio multiple spawning method with multi-state second order perturbation theory [H. Tao, B. G. Levine, and T. J. Martinez, J. Phys. Chem. A 113, 13656 (2009)]. We find excellent agreement between the TRPES calculation and the photoion signal observed in a pump-probe experiment using femtosecond vacuum ultraviolet (hν = 7.7 eV) pulses for both pump and probe. These results explain the apparent discrepancy over the excited state lifetime between theory and experiment that has existed for ten years, with experiments [e.g., P. Farmanara, V. Stert, and W. Radloff, Chem. Phys. Lett. 288, 518 (1998) and K. Kosma, S. A. Trushin, W. Fuss, and W. E. Schmid, J. Phys. Chem. A 112, 7514 (2008)] reporting much shorter lifetimes than predicted by theory. Investigation of the TRPES indicates that the fast decay of the photoion yield originates from both energetic and electronic factors, with the energetic factor playing a larger role in shaping the signal.

Ab initio multiple cloning algorithm for quantum nonadiabatic molecular dynamics

“…The AIMS results for ethylene have been previously verified by direct comparison to experiment using CASPT2 for the electronic structure. 52 Gratifyingly, the AIMC and AIMS methods agree almost quantitatively on the population dynamics in this case, and are certainly indistinguishable within the error bars shown.…”

“…As ethylene is the simplest molecule with a C=C double bond, it has been extensively studied both experimentally and computationally. 22,[50][51][52][53][54][55][56][57][58] The AIMC and AIMS simulations were carried out with a modified version of AIMS-MOLPRO, 36 which was extended to include Ehrenfest dynamics for the TBFs. Electronic structure calculations were performed with the CASSCF method using an active space of two electrons in two orbitals and equally weighted stateaveraging over the three lowest singlet states, i.e., SA-3-CAS(2/2).…”

Photoelectron spectra of 2-thiouracil, 4-thiouracil, and 2,4-dithiouracil

“…In the case of single-photon ionization, the norms of Dyson orbitals have been shown to be a good approximation for relative ionization probabilities. [34][35][36][37] An excellent introduction into the description of photoionization using Dyson orbitals can be found in references 37-39, so here we only provide a brief overview.…”

Coherent phase control of internal conversion in pyrazine

“…A similar situation was encountered in the transient transparency of ethylene, [33] where the probe photon energy exceeded the adiabatic ionization potential by 4.9 eV. The question we must therefore address is how such a WP could be localized.…”