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Following on from previous experimental and theoretical work [Neville et al., Nat. Commun. 7, 11357 (2016)], we report the results of a combined electronic structure theory and quantum dynamics study of the excited state dynamics of the pyrrole dimer following excitation to its first two excited states. Employing an exciton-based analysis of the à (π3s/σ*) and B ̃ (π 3 s / 3 p / σ * ) states, we identify an excited-state electron transfer pathway involving the coupling of the à (π3s/σ*) and B ̃ (π 3 s / 3 p / σ * ) states and driven by N–H dissociation in the B ̃ (π 3 s / 3 p / σ * ) state. This electron transfer mechanism is found to be mediated by vibronic coupling of the B ̃ state, which has a mixed π3s/3p Rydberg character at the Franck-Condon point, to a high-lying charge transfer state of the πσ* character by the N–H stretch coordinate. Motivated by these results, quantum dynamics simulations of the excited-state dynamics of the pyrrole dimer are performed using the multiconfigurational time-dependent Hartree method and a newly developed model Hamiltonian. It is predicted that the newly identified electron transfer pathway will be open following excitation to both the à (π3s/σ*) and B ̃ (π 3 s / 3 p / σ * ) states and may be the dominant relaxation pathway in the latter case. [ABSTRACT FROM AUTHOR]

The dynamics of N-methylpyrrole following excitation at wavelengths in the range 241.5-217.0 nm were studied using a combination of time-resolved photoelectron spectroscopy (TRPES), ab initio quantum dynamics calculations using the multi-layer multi-configurational time-dependent Hartree method, as well as high-level photoionization cross section calculations. Excitation at 241.5 and 236.2 nm results in population of the A2(πσ*) state, in agreement with previous studies. Excitation at 217.0 nm prepares the previously neglected B1(π3py) Rydberg state, followed by prompt internal conversion to the A2(πσ*) state. In contrast with the photoinduced dynamics of pyrrole, the lifetime of the wavepacket in the A2(πσ*) state was found to vary with excitation wavelength, decreasing by one order of magnitude upon tuning from 241.5 nm to 236.2 nm and by more than three orders of magnitude when excited at 217.0 nm. The order of magnitude difference in lifetimes measured at the longer excitation wavelengths is attributed to vibrational excitation in the A2(πσ*) state, facilitating wavepacket motion around the potential barrier in the N-CH3 dissociation coordinate. [ABSTRACT FROM AUTHOR]

The dynamics of pyrrole excited at wavelengths in the range 242-217 nm are studied using a combination of time-resolved photoelectron spectroscopy and wavepacket propagations performed using the multi-configurational time-dependent Hartree method. Excitation close to the origin of pyrrole's electronic spectrum, at 242 and 236 nm, is found to result in an ultrafast decay of the system from the ionization window on a single timescale of less than 20 fs. This behaviour is explained fully by assuming the system to be excited to the A2(πσ*) state, in accord with previous experimental and theoretical studies. Excitation at shorter wavelengths has previously been assumed to result predominantly in population of the bright A1(ππ*) and B2(ππ*) states. We here present time-resolved photoelectron spectra at a pump wavelength of 217 nm alongside detailed quantum dynamics calculations that, together with a recent reinterpretation of pyrrole's electronic spectrum [S. P. Neville and G. A. Worth, J. Chem. Phys. 140, 034317 (2014)], suggest that population of the B1(πσ*) state (hitherto assumed to be optically dark) may occur directly when pyrrole is excited at energies in the near UV part of its electronic spectrum. The B1(πσ*) state is found to decay on a timescale of less than 20 fs by both N-H dissociation and internal conversion to the A2(πσ*) state. [ABSTRACT FROM AUTHOR]

The non-radiative relaxation dynamics of pyrrole have been investigated using time-resolved photoelectron spectroscopy and quantum dynamics simulations. Following excitation of the A 2 ( 1 1 π σ ∗ ) state, we observe population flow out of the Franck-Condon region on a ≲ 50 fs timescale. Following excitation of the B 2 ( 2 1 π π ∗ ) state, we observe population being transferred to the A 2 ( 1 1 π σ ∗ ) state on a <50 fs timescale and subsequently out of the Franck-Condon region, also on a <50 fs timescale. Quantum dynamics calculations suggest that population is transferred from the B 2 ( 2 1 π π ∗ ) state through the A 2 ( 1 π 3 p z ) state to the B 1 ( 2 1 π σ ∗ ) state before being transferred to the A 2 ( 1 1 π σ ∗ ) state. [ABSTRACT FROM AUTHOR]