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

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Abstract
Photofragment velocity map imaging was used to study the H atom elimination mechanism in the first excited state of pyrrole at λ = 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 ≈72% of the available energy is transferred into relative fragment translation. The observed angular recoil distribution which is described by an anisotropy parameter of β = −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 D0(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 πσ* anti-bonding character and is repulsive with respect to the stretching of the N–H bond.

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