Photofragment slice imaging studies of pyrrole and the Xe⋯pyrrole cluster

Abstract

The photolysis of pyrrole has been studied in a molecular beam at wavelengths of 250, 240, and 193.3 nm, using two different carrier gases, He and Xe. A broad bimodal distribution of H-atom fragment velocities has been observed at all wavelengths. Near threshold at both 240 and 250 nm, sharp features have been observed in the fast part of the H-atom distribution. Under appropriate molecular beam conditions, the entire H-atom loss signal from the photolysis of pyrrole at both 240 and 250 nm (including the sharp features) disappear when using Xe as opposed to He as the carrier gas. We attribute this phenomenon to cluster formation between Xe and pyrrole, and this assumption is supported by the observation of resonance enhanced multiphoton ionization spectra for the (Xe...pyrrole) cluster followed by photofragmentation of the nascent cation cluster. Ab initio calculations are presented for the ground states of the neutral and cationic (Xe...pyrrole) clusters as a means of understanding their structural and energetic properties.

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

“…This suggests that when pump wavelengths of 236 and 242 nm are used either no excited state is involved or that the lifetime of the excited state is shorter than 20 fs. Based on the absorption spectrum of pyrrole 23 and previous photodissociation studies of pyrrole using similar pump wavelengths, 1,4-8, 15 we suspect that there is some excited state with a lifetime shorter than 20 fs involved when pump wavelengths of 236 and 242 nm are used. In Sec.…”

“…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.…”

UV Photodissociation of Pyrroles: Symmetry and Substituent Effects

“…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. The product energy disposals and recoil anisotropies have been rationalised in terms of vibronically induced S 1 S 0 excitation, followed by prompt N-H bond fission -a view confirmed in a number of ultrafast pump-probe studies.…”

Cluster‐Controlled Photofragmentation: The Case of the Xe–Pyrrole Cluster

“…The additional excess energy is channelled into the pyrrolyl photofragments as internal energy, and vibrational mode selective excitation (out-ofplane skeletal vibrations) has been reported experimentally. [8,9] The origin of the slow H-atom component remains controversial, as several mechanisms have been proposed. Concerning the excitation at long wavelengths, some have suggested that this channel results from the dissociation on the S 0 (X 1 A 1 ) ground state following internal conversion from the initially excited 1 ps* state [15] or from adiabatic dissociation on the 1 ps* state.…”