Volume 127, 2004

Photodissociation dynamics of pyrrole: Evidence for mode specific dynamics from conical intersections

Abstract

The H and D atom elimination mechanisms in the photodissociation of jet cooled pyrrole and pyrrole-d1 have been studied by photofragment velocity map imaging. The molecules were excited to the 1 1A2 (πσ*) state at λ = 243 nm and to the 1 1B2 (ππ*) state at λ = 217 nm. H/D atoms were detected by (2 + 1) resonance enhanced multiphoton ionization (REMPI) at λ = 243 nm. The analysis of the images and the resulting translational energy distributions from the 1 1A2 state demonstrates the existence of two decay pathways, fast mode-specific cleavage of the NH bond in the excited state (channel A) and internal conversion (IC) to the electronic ground state (S0) followed by unimolecular decomposition of the vibrationally hot S0 molecules (channel B). The angular distributions of the H/D atoms from the direct dissociation in the excited state are strongly anisotropic, whereas the decay of the S0 molecules leads to spatially isotropic distributions. The results at λ = 217 nm indicate that the 1 1B2 state undergoes an ultrafast radiationless transition to 1 1A2 followed by the abovementioned direct mode-specific NH bond fission on the 1 1A2 potential energy surface (channel A′) or conversion to S0 and subsequent unimolecular decomposition (channel B′). The latter pathway may also be initiated by a direct relaxation from 1 1B2 to S0. The anisotropy parameter of β ≈ −1 for the direct NH bond fission at λ = 217 nm is in accordance with the expectations for a perpendicular electronic excitation and a dissociation lifetime that is short compared to the rotational period of the molecules. The fast decay dynamics of both excited electronic states can be rationalized with reference to the theoretically predicted conical intersections between the ππ*, πσ*, and S0 potential energy surfaces and the antibonding nature of the πσ* potential energy surface with respect to the NH bond [A. L. Sobolewski, W. Domcke, C. Dedonder-Lardeux and C. Jouvet, Phys. Chem. Chem. Phys. 2002, 4, 1093].

Article information

Article type
Paper
Submitted
01 Mar 2004
Accepted
03 Mar 2004
First published
07 May 2004

Faraday Discuss., 2004,127, 267-282

Photodissociation dynamics of pyrrole: Evidence for mode specific dynamics from conical intersections

J. Wei, J. Riedel, A. Kuczmann, F. Renth and F. Temps, Faraday Discuss., 2004, 127, 267 DOI: 10.1039/B403191C

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