Ultrafast Excited-State Dynamics of 2,4-Dimethylpyrrole

The dynamics of photoexcited 2,4-dimethylpyrrole (DMP) were studied using time-resolved velocity map imaging spectroscopy over a range of photoexcitation wavelengths (276238 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 similar to 120 fs producing H atoms with high kinetic energies centered around 50007000 cm(1) and a second channel with a time constant of similar to 3.5 ps producing H atoms with low kinetic energies centered around 25003000 cm(1). The first of these channels is attributed to direct excitation from the ground electronic state (S-0) to the dissociative 1(1)pi sigma* state (S-1) and subsequent NH bond fission, moderated by a reaction barrier in the NH stretch coordinate. In contrast to analogous measurements in pyrrole (Roberts et al. Faraday Discuss. 2013, 163, 95116), the NH dissociation times are invariant with photoexcitation wavelength, implying a relatively flatter potential in the vertical FranckCondon region of the 1(1)pi sigma* 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)pi sigma*(S-2)] which, following vibronic coupling into lower-lying intermediary states (namely, S-1 or S-0), leads to prompt NH bond fission.