The dynamics of photoexcited 2,4-dimethylpyrrole (DMP) were studied using time-resolved velocity map imaging spectroscopy over a range of photoexcitation wavelengths (276-238 nm). Two dominant H atom elimination channels were inferred from the time-resolved total kinetic energy release spectra, one which occurs with a time constant of ∼120 fs producing H atoms with high kinetic energies centered around 5000-7000 cm(-1) and a second channel with a time constant of ∼3.5 ps producing H atoms with low kinetic energies centered around 2500-3000 cm(-1). The first of these channels is attributed to direct excitation from the ground electronic state (S0) to the dissociative 1(1)πσ* state (S1) and subsequent N-H bond fission, moderated by a reaction barrier in the N-H stretch coordinate. In contrast to analogous measurements in pyrrole (Roberts et al. Faraday Discuss. 2013, 163, 95-116), the N-H dissociation times are invariant with photoexcitation wavelength, implying a relatively flatter potential in the vertical Franck-Condon region of the 1(1)πσ* state of DMP. The origins of the second channel are less clear-cut, but given the picosecond time constant for this process, we posit that this channel is indirect and is likely a consequence of populating higher-lying electronic states [e.g., 2(1)πσ* (S2)] which, following vibronic coupling into lower-lying intermediary states (namely, S1 or S0), leads to prompt N-H bond fission.
The linear stochastic estimation is a powerful technique that provides a means of estimating conditional eddies given unconditional two-point correlation data. This procedure was used to reconstruct estimates of multipoint conditional averages of the dominant structures in the jet mixing layer of Glauser and George (Proceedings of the Sixth Symposium on Turbulent Shear Flows, Toulouse, France, 1987). The pseudodynamic evolution of these conditional eddies was systematically compared to the instantaneous velocity fields and the results were quantified in terms of the percentage of the energy captured by the multipoint stochastic estimates. It was found that the single-point estimates do not yield adequate representations of the instantaneous velocity field, but that two reference points located on opposite sides of the shear layer yield realistic estimates, with little gained by adding more reference points.
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