Forschungszentrum Karlsruhe
Lecture Hall, Bldg. 640

14.07.2009

Dr. Wolfgang Christen
Institut für Chemie
Humboldt-Universität zu Berlin, Germany

15:00

Abstract: Supersonic molecular beams continue to constitute an extremely versatile and rather popular tool in modern chemical physics. They are of prime importance in basic as well as applied research fields such as analytical chemistry, cluster science, optical spectroscopy, surface science, and thin film growth. Supercritical fluids have received considerable attention in materials science and engineering. In particular, jet expansions from the supercritical state are used to grow nanoparticles or to transfer molecules of pharmaceutical interest into the gas phase.
 Focusing on the numerous applications, more fundamental topics of supersonic jet expansions such as the real gas behavior have not been investigated in great detail. Until now, in most studies the characteristics of supersonic jets are treated in the approximation of ideal gas expansions. Here, particle associations are completely ignored, although they are responsible for the formation of clusters, both in the stagnation reservoir at elevated pressures and during the jet expansion.
 In continuation of our earlier work on supersonic beams of supercritical fluids* we investigate - both experimentally and theoretically - the influence of thermodynamic quantities on the resulting beam velocity, beam temperature, and cluster size. The assumptions of an ideal gas treatment and its shortcomings are reviewed. A straightforward thermodynamic approach is presented that is capable to overcome most of those difficulties. It is shown that the experimental beam velocities are well described by a consideration of the initial and final fluid enthalpies. These are calculated using high accuracy equations of state explicit in the Helmholtz energy. The model calculations are complemented by a comprehensive set of experimental data covering a wide range of stagnation conditions. Specifically we demonstrate that phase transitions of a fluid system can be remotely probed using high-precision time-of-flight spectroscopy of supersonic molecular beams.
 *Int. J. Mass. Spectrom. 277, 305 (2008). Phys. Rev. A 77, 012702 (2008). Rev. Sci. Instrum. 78, 073106 (2007). J. Chem. Phys. 125, 174307 (2006). Rev. Sci. Instrum. 75, 5048 (2004).