Dissociation pathways in the cysteine dication after site-selective core ionization.

A photoelectron-ion-ion coincidence experiment has been carried out on the amino acid molecule cysteine after core-ionization of the O 1s, N 1s, C 1s, and S 2p orbitals. A number of different dissociation channels have been identified. Some of them show strong site-selective dependence that can be attributed to a combination of nuclear motion in the core-ionized state and Auger processes that populate different final electronic states in the dication.

Development and characterization of a multiple-coincidence ion-momentum imaging spectrometer.

The design and performance of a high-resolution momentum-imaging spectrometer for ions which is optimized for experiments using synchrotron radiation is presented. High collection efficiency is achieved by a focusing electrostatic lens; a long drift tube improves mass resolution and a position-sensitive detector enables measurement of the transverse momentum of ions. The optimisation of the lens for particle momentum measurement at the highest resolution is described. We discuss the overall performance of the spectrometer and present examples demonstrating the momentum resolution for both kinetics and for angular measurements in molecular fragmentation for carbon monoxide and fullerenes. Examples are presented that confirm that complete space-time focussing is possible for a two-field three-dimensional imaging spectrometer.

Size selective spectroscopy of Se microclusters.

The electronic structure and photofragmentation in outer and inner valence regions of Se(n) (n ≤ 8) clusters produced by direct vacuum evaporation have been studied with size-selective photoelectron-photoion coincidence technique by using vacuum-ultraviolet synchrotron radiation. The experimental ionization potentials of these clusters were extracted from the partial ion yield measurements. The calculations for the possible geometrical structures of the Se(n) microclusters have been executed. The ionization energies of the clusters have been calculated and compared with the experimental results. In addition, theoretical fragment ion appearance energies were estimated. The dissociation energies of Se(n) clusters were derived from the recurrent relation between the gas phase enthalpies of the formation of corresponding cationic clusters and experimental ionization energies.

Role of the Renner-Teller effect after core hole excitation in the dissociation dynamics of carbon dioxide dication.

The fragmentation of the doubly-charged carbon dioxide molecule is studied after photoexcitation to the C 1s(1)2π(u) and O 1s(1)2π(u) states using a multicoincidence ion-imaging technique. The bent component of the Renner-Teller split states populated in the 1s→ π* resonant excitation at both the carbon and oxygen 1s ionization edges opens pathways to potential surfaces in highly bent geometries in the dication. Evidence for a complete deformation of the molecule is found in the coincident detection of C(+) and O(2)(+) ions. The distinct alignment of this fragmentation channel indicates rapid deformation and subsequent fragmentation. Investigation of the complete atomization dynamics in the dication leading to asymmetric charge separation shows that the primary dissociation mechanisms, sequential, concerted, and asynchronous concerted, are correlated to specific fragment kinetic energies. The study shows that the bond angle in fragmentation can extend below 20°.

Nuclear motion in carbonyl sulfide induced by resonant core electron excitation.

The angular anisotropy for selected dissociation channels is measured at resonantly excited states of Σ and Π symmetries at the C and O K-shell ionization edges of carbonyl sulfide. While the kinetic energy released in the reaction is mainly independent of the excitation energy, the angular anisotropy and momentum correlation clearly show deformation of the OCS molecule in the C 1s(-1)π(∗1) state. The discovery of a two-body fragmentation channel SO(+)/C(+) with a well defined angular anisotropy indicates the rapid formation of the CSO isomeric species.

Dynamics of proton migration and dissociation in core-excited ethyne probed by multiple ion momentum imaging.

The study focuses on the rapid geometry change in ethyne excited near the carbon 1s edge. Core excitation and ionization lead to population of dicationic states in ethyne. We study three competing dissociation pathways associated with deprotonation in the linear ethyne molecule, and two cases of rapid proton migration. We investigate the alignment of the molecule in the excited state and find startling differences in these three cases. We present evidence for a strong anisotropy in the production of H(2)(+)/C(2)(+) fragments through a rapid deformation of the molecule to a dibridged conformation with the transition dipole moment parallel to the polarization of the exciting radiation.