Core level excitation of atomic and molecular clusters

Abstract

Core level excitation of weakly bonded atomic and molecular species is reported. Excitations into unoccupied orbitals with valence or Rydberg character as well as into the corresponding K-and L-continua are compared with the isolated and condensed species. Core level excitation of clusters is also accompanied with extensive fragmentation of singly and doubly charged clusters. Coincidence techniques such as photoelectronphotoion-coincidences (PEPICO) and photoion-photoion-coincidences (PIPICO) are used to study the photochemical fragmentation pathways of core excited clusters. Kinetic energy releases related to charge separation of cluster dications (Coulomb explosion) are derived from PIPICO spectra. The fragmentation energetics is discussed in relation to cluster dication threshold energies.

Site-Dependent Spectral Shifts in Core-to-π* Excitations of Pyridine Clusters

“…The structures of pyridine dimers and clusters will be shown further below. Furthermore, it is well-known that it is difficult to estimate the average cluster size of molecular clusters from the expansion conditions. , The cluster size contributing to the experimental spectra shown in Figures and is estimated similarly to previous work on benzene clusters . Core-level excitation produces efficiently doubly charged clusters via Auger decay processes.…”

“…An efficient way to probe size effects in electronic structure is to change the expansion conditions, as done in the present and previous work . The experiments cover the regimes of the C 1s and N 1s absorption edges, similarly to previous work. – ,, This provides photoion yields, which reflect changes in electronic properties upon cluster formation, as outlined in our previous work on benzene clusters …”

“…Molecular properties are measured simultaneously by using the m / z ≈ 50 mass signal. This approach is known to yield reliably spectral shifts in the core level excitation regime. ,,, The results are shown in Figures –.…”

“…The experimental setup consists of a supersonic jet expansion that is used for cluster production and a time-of-flight mass spectrometer for cation separation and detection. Details of this setup have been published elsewhere. , This setup is mounted at the UE52-SGM beam line of the storage ring BESSY (Berlin, Germany), where monochromatic synchrotron radiation in the soft X-ray regime is delivered for photoexcitation and photoionization of neutral, variable-size pyridine clusters. The energy resolving power E /Δ E is set to ≥6 000.…”

“…Cations are formed by photoionization in the soft X-ray regime. They are separated in a time-of-flight mass spectrometer . Photoion yields of the molecular fragments that are centered in the mass range near m / z ≈ 50, corresponding to C 3 H x N + /C 4 H x + , are not affected by fragmentation of clusters.…”