Probing ultrafast dynamics in photoexcited pyrrole: timescales for pi sigma* mediated H-atom elimination

The heteroaromatic ultraviolet chromophore pyrrole is found as a subunit in a number of important biomolecules: it is present in heme, the non-protein component of hemoglobin, and in the amino acid tryptophan. To date there have been several experimental studies, in both the time- and frequency-domains, which have interrogated the excited state dynamics of pyrrole. In this work, we specifically aim to unravel any differences in the H-atom elimination dynamics from pyrrole across an excitation wavelength range of 250-200 nm, which encompasses: (i) direct excitation to the (formally electric dipole forbidden) 1(1)pisigma* (1A2) state; and (ii) initial photoexcitation to the higher energy 1 pipi* (1B2) state. This is achieved by using a combination of ultrafast time-resolved ion yield and time-resolved velocity map ion imaging techniques in the gas phase. Following direct excitation to 1(1)pisigma* (1A2) at 250 nm, we observe a single time-constant of 126 +/- 28 fs for N-H bond fission. We assign this to tunnelling out of the quasi-bound 3s Rydberg component of the 1(1)pisigma* (1A2) surface in the vertical Franck-Condon region, followed by non-adiabatic coupling through a 1(1)pisigma*/S(0) conical intersection to yield pyrrolyl radicals in their electronic ground state (C4H4N(X)) together with H-atoms. At 238 nm, direct excitation to, and N-H dissociation along, the 1(1)pisigma* (1A2) surface is observed to occur with a time-constant of 46 +/- 22 fs. Upon initial population of the 1pipi* (1B2) state at 200 nm, a rapid 1pipi* (1B2) --> 1(1)pisigma* (1A2) --> N-H fission process takes place within 52 +/- 12 fs. In addition to ultrafast N-H bond cleavage at 200 nm, we also observe the onset of statistical unimolecular H-atom elimination from vibrationally hot S(0) ground state species, formed after the relaxation of excited electronic states, with a time-constant of 1.0 +/- 0.4 ns. Analogous measurements on pyrrole-d1 reveal that these statistical H-atoms are released only through C-H bond cleavage.