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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 photolysis of pyrrole has been studied in a molecular beam at wavelengths of 250, 240, and 193.3 nm, using two different carrier gases, He and Xe. A broad bimodal distribution of H-atom fragment velocities has been observed at all wavelengths. Near threshold at both 240 and 250 nm, sharp features have been observed in the fast part of the H-atom distribution. Under appropriate molecular beam conditions, the entire H-atom loss signal from the photolysis of pyrrole at both 240 and 250 nm (including the sharp features) disappear when using Xe as opposed to He as the carrier gas. We attribute this phenomenon to cluster formation between Xe and pyrrole, and this assumption is supported by the observation of resonance enhanced multiphoton ionization spectra for the (Xe...pyrrole) cluster followed by photofragmentation of the nascent cation cluster. Ab initio calculations are presented for the ground states of the neutral and cationic (Xe...pyrrole) clusters as a means of understanding their structural and energetic properties.

High resolution time-of-flight (TOF) measurements of the D atom fragments arising in the near ultraviolet (UV) photodissociation of deuterated pyrrole are reported. Structures evident in the measured TOF spectra are all interpretable in terms of N-D bond fission, and population of selected vibrational states of the pyrrolyl-d(4) co-fragment -- thereby clarifying previous uncertainties regarding the branching into different vibronic states of the pyrrolyl radical following UV excitation of pyrrole.

Photoelectron imaging and time of flight mass spectrometry are used to study the multiphoton ionization and dissociation of pyrrole and its cation following excitation at 243 nm and at 364.7 nm. Our results confirm the 8.2 eV ionization potential of pyrrole and the 9.2 eV ionization threshold for forming the 2B1 first excited state of the cation. Prompt photolysis of the N-H bond in neutral pyrrole following one-photon excitation to its 1 1A2 neutral excited state is inferred from analysis of the two-photon photoelectron spectrum recorded at 243 nm, confirming the findings of recent translational spectroscopy studies. Facile dissociation of the pyrrole cation is also observed following excitation at 243 nm; analysis of the fragment cations indicates the operation of a complex dissociation mechanism involving dual bond fission and possible migration of the H atom originally bonded to the nitrogen heteroatom.