title: Ultrafast excited-state dynamics of 2,4-dimethylpyrrole.
abstract: 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.
authors.affiliation: Department of Chemistry, University of Warwick , Library Road, Coventry, CV4 7AL, U.K.
authors.forename: Michael
authors.initials: M
authors.lastname: Staniforth
authors.forename: Jamie D
authors.initials: JD
authors.lastname: Young
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Cole
authors.forename: Tolga N V
authors.initials: TN
authors.lastname: Karsili
authors.forename: Michael N R
authors.initials: MN
authors.lastname: Ashfold
authors.forename: Vasilios G
authors.initials: VG
authors.lastname: Stavros
mesh_terms: D007700:Kinetics; D015394:Molecular Structure; D055668:Photochemical Processes; D011758:Pyrroles / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: C000599305:2,4-dimethylpyrrole; D011758:Pyrroles