Photofragment velocity map imaging of H atom elimination in the first excited state of pyrrole

Photofragment velocity map imaging was used to study the H atom elimination mechanism in the first excited state of pyrrole at lambda = 243.1 nm. Two major channels were observed. The first one (76%) produces very fast H atoms and appears to be due to a rapid direct N-H bond breaking in the excited electronic state. The respective H atom kinetic energy distribution has a strong narrow peak at high energies, showing that approximate to72% of the available energy is transferred into relative fragment translation. The observed angular recoil distribution which is described by an anisotropy parameter of beta = -0.37 +/- 0.05 indicates that the excited optical transition is preferentially perpendicular with respect to the N-H dissociation coordinate. From the maximal kinetic energy release, the value of the N-H bond dissociation energy was found to be D-0(N-H) = (32 400 +/- 400) cm(-1). The other channel (24%) leads to much slower H atoms with a very broad kinetic energy distribution, consistent with subsequent unimolecular decay reactions of the molecules in the ground electronic state after internal conversion. This conclusion was supported by similar experiments for N-methylpyrrole which showed only H atoms from the second channel and no fast component. The results corroborate the conclusion that the lowest electronic state of pyrrole has pisigma* anti-bonding character and is repulsive with respect to the stretching of the N-H bond.