Helena Levola

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.

Gas-phase study on uridine: Conformation and X-ray photofragmentation

Fragmentation of RNA nucleoside uridine, induced by carbon 1s core ionization, has been studied. The measurements by combined electron and ion spectroscopy have been performed in gas phase utilizing synchrotron radiation. As uridine is a combination of d-ribose and uracil, which have been studied earlier with the same method, this study also considers the effect of chemical environment and the relevant functional groups. Furthermore, since in core ionization the initial core hole is always highly localized, charge migration prior to fragmentation has been studied here. This study also demonstrates the destructive nature of core ionization as in most cases the C 1s ionization of uridine leads to concerted explosions producing only small fragments with masses ≤43 amu. In addition to fragmentation patterns, we found out that upon evaporation the sugar part of the uridine molecule attains hexagonal form.

Photofragmentation of Serine Following C 1s Core Ionization-Comparison with Cysteine.

Photofragmentation pathways of doubly ionized serine molecules are investigated and compared with those of cysteine. The main motivation for the study is to investigate if an atomic substitution within the same group of elements, namely, replacing sulfur (in cysteine) with oxygen (in serine), causes a major change in the C 1s core ionization induced dissociation pattern in the molecules of an otherwise identical structure. The results show that in serine there is a single completely dominant fragmentation channel producing the (CNH2–4+, COH1–3+) pairs, whereas in cysteine there are also many other fragmentation channels. The employed experimental method was the photoelectron–photoion–photoion coincidence (PEPIPICO) technique combined with synchrotron radiation tuned to ionize desired core levels. Molecular dynamics calculations were also carried out in order to extract information on the fragmentation and the neutral final fragments.

Photoinduced intermolecular dynamics and subsequent fragmentation in VUV-ionized acetamide clusters

Photofragmentation of small gas-phase acetamide clusters (CH3CONH2)n (n ≤ 10) produced by a supersonic expansion source has been studied using time-of-flight ion mass spectroscopy combined with tunable vacuum-ultraviolet (VUV) synchrotron radiation. Fragmentation channels of acetamide clusters under VUV photoionization resulting in protonated and ammoniated clusters formation were identified with the discussion about the preceding intramolecular rearrangements. Acetamide-2,2,2-d3 clusters were also studied in an experiment with a gas discharge lamp as a VUV light source; comparison with the main experiment gave insights into the mechanism of formation of protonated acetamide clusters, indicating that proton transfer from amino group plays a dominant role in that process. Geometry of the acetamide dimer was discussed and the most stable arrangement was concluded to be achieved when subunits of the dimer are connected via two N-H⋯O -C hydrogen bonds. Also, the influence of the photon energy on the stability of the clusters and their fragmentation channels has been examined.

X-ray induced fragmentation dynamics of doubly charged L-alanine in gas phase

The molecular fragmentation of doubly charged L-alanine in gas phase was studied in radiation synchrotron experiments. In this presentation, we summarize our theoretical study on the dynamics of this fragmentation, using various computational methods. We show that in practice the ground state MD simulations are able to statistically reproduce the experimental results of the photo-fragmentation initiated at the excited state.

Molecular dynamics of photodissociation : towards more complex systems

We present a combined experimental and theoretical study of the photodissociation of thiophene molecule using energy-resolved electron-ion-ion coincidence technique and self-consistent charge densi ...