| ABSTRACT: |
Core-to-valence transitions of molecular clusters containing pyridine are investigated in the C 1s- and the N 1s-regime. The vibrationally resolved 1s _ ¡¦ bands show in clusters the same shape as in the isolated molecule, but in the case of C 1s-excited pyridine clusters there is a redshift relative to the bare molecules of 110 meV and 60 meV, respectively. In contrast, there is a blueshift of 60 meV in small N 1s-excited pyridine clusters.
The experiments have been performed at the UE52-SGM beam line of the electron storage ring BESSY II (Berlin, Germany). Free clusters are prepared by adiabatic expansion of gas mixtures, using the seeded beam technique [1]. The samples are expanded through a nozzle (d = 50 ¥·m) at room temperature (T0 = 300 K), so that most of the gas phase consists of seed gas. The cluster jet is shaped by a 500 ¥·m skimmer. Finally, it is crossed with the beam of monochromatic soft X-rays in the ionization region of a time-of-flight mass spectrometer, where the cations are separated and detected. This approach allows us to measure reliably small energy shifts between the molecules and clusters.
The experimental results are assigned in comparison with ab initio calculations were the core-excited molecules and clusters are calculated using the GSCF3 package [2, 3]. We found that each element-selectively excited site within a cluster contributes to a distinct energy shift relative to the bare molecule, where the structure of the cluster plays an important role. Summation over all sites of the same element allows us to make a comparison with the experimental energy shifts. It is found that these can be related to structural properties of clusters, where the number of intermolecular neighbors in a cluster contributes significantly to the energy shift of the absorbing site, which is similar to recent work on benzene clusters [4]. This allows us to derive an assignment of site-specific spectral shifts in molecular clusters containing aromatic molecules as well as gas-to-solid shifts.
References
[1] R.E. Smalley, L. Wharton, and D.H. Levy, Acc. Chem. Res. 10, 13, (1977).
[2] N. Kosugi and H. Koroda, Chem. Phys. Lett. 74, 490, (1980)
[3] N. Kosugi, Theoret. Chim. Acta, 72, 149 (1987)
[4] I.L. Bradeanu, R. Flesch, N. Kosugi, A.A. Pavlychev, and E. RÝÉl, Phys. Chem. Chem. Phys. 8, 1906 (2006). DOI: 10.1039/b517199g.
|
