Probing single particle and collective excitations in superconductors and related materials with femtosecond spectroscopy

Summary form only given. Time-domain femtosecond pump-probe spectroscopy is applied to probe the low-energy single particle and collective excitations in a number of correlated electron systems. Two examples, which are chosen as representative of their respective classes of material are YBaCuO and K/sub 0.3/MoO/sub 3/ (Bluebronze). We discuss the similarities and differences between the two systems, particularly their single-particle excitations. The development of models for the single-particle recombination is found to be crucial for the understanding of the experiments, which then reveal some qualitatively new features in the low-energy spectra of these materials.