A study of the unimolecular decomposition of internal-energy-selected furan molecular ions by threshold-photoelectron-photoion coincidence spectroscopy

The unimolecular decomposition of internal-energy-selected furan molecular ions has been studied by means of threshold-photoelectron-photoion coincide nce spectroscopy. Monochromatic synchrotron radiation was used as the ionis ation source, and the molecular ion internal energy was established through the detection of a threshold electron. A pulsed electric field was applied to extract the ions from the interaction region and direct them towards a time-of-flight mass spectrometer. Breakdown curves were measured for photon energies up to 30 eV, and these have allowed appearance energies for a wid e range of fragment ions to be determined. In the threshold region the brea kdown curves have been measured for various ion residence times by introduc ing electronic delays between the detection of the threshold electron and t he application of the ion extraction field. The breakdown curves have been modelled using the RRKM (Rice, Ramsperger, Kassel and Marcus)/QET (quasi-eq uilibrium theory) approach, and this has allowed activation energies and tr ansition state geometries to be deduced. The threshold photoelectron spectra of furan-hp and furan-d(4) have been me asured from the ionisation threshold to 78 eV, and vibrational structure ha s been observed and assigned in the bands due to the (X) over tilde (2)A(2) , the (A) over tilde B-2(1) and the (G) over tilde (2)A(1) states. Vibratio nal progressions discernible between 16.2 and 17.3 eV have been attributed to autoionisation from a p-type Rydberg series converging onto the (G) over tilde (2)A(1) state ionisation threshold. (C) 2001 Elsevier Science B.V. A ll rights reserved.