1. University of Dortmund, Department of Chemistry, Physical Chemistry I, Otto-Hahn-Strasse 6, D-44227 Dortmund, Germany
2. TF Instruments, Im Neuenheimer Feld 515, D-69129 Heidelberg, Germany
3. Institute for High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland