Femtosecond x-ray spectroscopy of an electrocyclic ring-opening reaction

Abstract

The ultrafast light-activated electrocyclic ring-opening reaction of 1,3-cyclohexadiene is a fundamental prototype of photochemical pericyclic reactions. Generally, these reactions are thought to proceed through an intermediate excited-state minimum (the so-called pericyclic minimum), which leads to isomerization via nonadiabatic relaxation to the ground state of the photoproduct. Here, we used femtosecond (fs) soft x-ray spectroscopy near the carbon K-edge (~284 electron volts) on a tabletop apparatus to directly reveal the valence electronic structure of this transient intermediate state. The core-to-valence spectroscopic signature of the pericyclic minimum observed in the experiment was characterized, in combination with time-dependent density functional theory calculations, to reveal overlap and mixing of the frontier valence orbital energy levels. We show that this transient valence electronic structure arises within 60 ± 20 fs after ultraviolet photoexcitation and decays with a time constant of 110 ± 60 fs.

Multireference Electron Correlation Methods: Journeys along Potential Energy Surfaces

“…With the full-valence active space and XMS-CASPT2, the correct state ordering is obtained. Polyak et al observed the ultrafast decay to the ground state (89 ± 9 fs) with large quantum yield (47 ± 8 %) of hexatriene, which was comparable to experimental results (80 to 170 fs lifetime [153][154][155] and 30% to 40% quantum yield). 156 By mapping the distribution of the hopping geometries in the simulations, these lifetime and quantum yields are ascribed to the extended conical intersection seam along the reaction coordinate.…”

Efficient table-top dual-wavelength beamline for ultrafast transient absorption spectroscopy in the soft X-ray region

“…A temporal resolution of 11 fs is measured in a pump-probe experiment. This duration is already 8/10 enough to consider the observation of ultrafast chemical dynamics at the C K-edge 30,31,52,58 . Improved CEP stability and feedback control over the delay will allow attosecond transient absorption experiments in the SXR in the future.…”

Effects of probe energy and competing pathways on time-resolved photoelectron spectroscopy: the ring-opening of 1,3-cyclohexadiene

“…It is clear from our analysis that it is difficult to disentangle the individual contributions to the total signal, a similar conclusion to the one drawn in a recent paper by Ruckenbauer et al 84 This strengthens the argument in favour of combining data from complementary experimental techniques, 59 which can constrain calculations and inform reduced dimensionality models. The issues associated with overlapping features and spectral congestion are also evident in other examples of global probes such as those based on diffraction 45 or NEXAFS, 58 with both studies using theoretically calculated dynamics to deconvolute and interpret the data. These examples, and the results in this article, support the case for accurate simulations of dynamics, but also demonstrate the need for accurate predictions of how the dynamics projects onto the measurement.…”

“…This includes coherent control, both experiment 43,48,49 and theory, 7,50,51 and a range of new ultrafast imaging techniques 52 such as electron diffraction, [53][54][55][56] X-ray scattering, 44,45 Coulomb explosion, 57 and near-edge X-ray absorption fine structure (NEXAFS). 58 The variety of complementary observations generated by these techniques holds great promise for tying down a complete understanding of this important and complex reaction, for instance by combining X-ray scattering and photoelectron data such as in ref. 59.…”