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 (pi sigma*) state at lambda = 243 nm and to the 1 1B2 (pi pi*) state at lambda = 217 nm. H/D atoms were detected by (2 + 1) resonance enhanced multiphoton ionization (REMPI) at lambda = 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 lambda = 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 beta approximately -1 for the direct NH bond fission at lambda = 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 pi pi*, pi sigma*, and S0 potential energy surfaces and the antibonding nature of the pi sigma* 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].

Non-adiabatic dynamics of pyrrole: Dependence of deactivation mechanisms on the excitation energy

“…Most of experimental investigations on pyrrole photolysis have been performed either close to the band origin near 248 nm or at longer wavelengths near 193 nm [4][5][6]24,26]. The absorption cross section at the region of the long wavelength is only approximately 3 Â 10 À4 Å 2 molecule À1 , about 300 times smaller than at the short wavelength [2].…”

“…Photofragment spectroscopy has revealed the presence of the large fragments HCN, C 3 H 4 , C 3 H 3 , and CNH 2 in different fractions depending on the pump wavelength [4,5,26,27] The appearance of such fragments also seems to imply the existence of other internal conversion mechanisms occurring in competition with the NH-stretching. It has been proposed that such mechanisms would involve ring puckering [28] and ring opening [23,29] processes ( Fig.…”

UV Photodissociation of Pyrroles: Symmetry and Substituent Effects

“…12 The early PTS result was confirmed by a velocity map imaging (VMI) study of the H atoms from pyrrole photolysis at  = 243.1 nm. 13,14 Higher resolution TKER spectra of the H + pyrrolyl fragments formed following near UV excitation to the S 1 state of pyrrole show structure attributable to population of specific vibrational levels of the radical product. 15,16 The relative branching into these levels varies with photolysis wavelength, but all of the populated levels (apart from the v = 0 level) are found to involve non-totally symmetric nuclear motions.…”

Excited state non-adiabatic dynamics of pyrrole: A time-resolved photoelectron spectroscopy and quantum dynamics study

“…Using photofragment translational spectroscopy (PTS), Blank et al found upon excitation at 193 and 248 nm that the H elimination channel is the main dissociation channel. 1 Later, higher energy-resolution experiments [4][5][6][7][8]15 in the wavelength range 193.3-254.0 nm observed two components in the H product translational energy distributions: a "fast" component with a higher, sharp kinetic energy distribution and a "slow" component with a lower, broad kinetic energy distribution. A picture was proposed as follows: upon the approach of the wavepacket to the conical intersection (CI) between the πσ * and ground states, the wavepacket either evolves to the ground state and results in a statistical dissociation of the hot ground state, leading to nascent H-atoms with lower kinetic energies, or evolves diabatically leading to a direct cleavage of the N-H bond to yield H-atoms with higher kinetic energies.…”