1. Present address: Max Planck Institute for Biophysical Chemistry, Am Fassberg 11, 37077 Göttingen, Germany
2. Instituto de Ciencia de Materiales, Universidad de Valencia, P. O. Box 2085, 46071 Valencia, Spain
3. Instituto de Microelectrónica de Madrid, Isaac Newton 8, 28760 Tres Cantos, Spain
4. European Laboratory for Non‐linear Spectroscopy, Via Carrara 1, Sesto‐Fiorentino, Italy