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.
Subject(s)
Light , Photochemistry/methods , Photolysis , Pyrroles/chemistry , Xenon/chemistry , Cations , Chemistry, Physical/methods , Cluster Analysis , Electrons , Helium/chemistry , Hydrogen/chemistry , Models, Theoretical , Molecular Conformation , PhotonsABSTRACT
High-resolution measurements of the kinetic energies of hydrogen atom fragments formed during ultraviolet photolysis of imidazole, pyrrole, and phenol in the gas phase confirm that N(O)-H bond fission is an important nonradiative decay process from their respective 1pisigma* excited states. The measurements also reveal that the respective cofragments (imidazolyl, pyrrolyl, and phenoxyl) are formed in very limited subsets of their available vibrational states. Identification of these product states yields uniquely detailed insights into the vibronic couplings involved in the photoinduced evolution from parent molecule to ultimate fragments.
ABSTRACT
The fragmentation dynamics of indole molecules following excitation at 193.3 nm, and at a number of different wavelengths in the range 240 < or = lambda(phot) < or = 286 nm, have been investigated by H Rydberg atom photofragment translational spectroscopy. The longer wavelength measurements have been complemented by measurements of excitation spectra for forming parent and fragment ions by two (or more) photon ionisation processes. Analysis identifies at least three distinct contributions to the observed H atom yield, two of which are attributable to dissociation of indole following radiationless transfer from the 1pi pi* excited states (traditionally labelled 1L(b) and 1L(a)) prepared by UV single photon absorption. The structured channel evident in total kinetic energy release (TKER) spectra recorded at lambda(phot) < or = 263 nm is rationalised in terms of N-H bond fission following initial pi* <-- pi excitation and subsequent coupling to the 1pi sigma* potential energy surface via a conical intersection between the respective surfaces--thereby validating recent theoretical predictions regarding the importance of this process (Sobolewski et al., Phys. Chem. Chem. Phys., 2002, 4, 1093). Analysis provides an upper limit for the N-H bond strength in indole: D0(H-indolyl) < or = 31,900 cm(-1). Unimolecular decay of highly vibrationally excited ground state molecules formed by internal conversion from the initially prepared 1pi pi* states is a source of (slow) H atoms but their contribution to the TKER spectra measured in the present work is dwarfed by that from H atoms generated by one or more (unintended but unavoidable) multiphoton processes.