Ž . Vibrational Spectroscopy 19 1999 23–31 Femtosecond pump-probe and four-wave mixing spectroscopies applied to simple molecules T. Chen, V. Engel, M. Heid, W. Kiefer ) , G. Knopp, A. Materny, S. Meyer, R. Pausch, M. Schmitt, H. Schwoerer, T. Siebert Institut fur Physikalische Chemie der UniÕersitat Wurzburg, Am Hubland, D-97074 Wurzburg, Germany ¨ ¨ ¨ ¨ Received 9 July 1998; revised 22 September 1998; accepted 24 September 1998 Abstract Ž . A femtosecond three color pumprdump-probe scheme in combination with a time-of-flight TOF mass selective detection unit has been applied to study vibrational wave packet dynamics in the electronic ground state of cold K 2 molecules. A spectral analysis of the time domain signal reveals two different wave packet contributions: one from a stimulated Raman process and one from stimulated emission pumping. Also femtosecond time-resolved degenerate Ž . four-wave mixing DFWM spectroscopy is performed in order to investigate molecular dynamics in iodine molecules in the gas phase. Depending on the timing of the laser pulses different dynamics are reflected in the DFWM transient signal. By the use of time-evolution diagrams, the varying contribution of ground and excited state dynamics can be explained conclusively. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Femtochemistry; Nonlinear spectroscopy; DFWM; Femtosecond spectroscopy; Resonance enhanced multiphoton ionization Ž . REMPI 1. Introduction Time-domain spectroscopy using ultrashort laser pulses with a duration of less than about 100 fs enables spectroscopists to directly access molecular w x and reaction dynamics 1,2 . Due to the inherent Ž . spectral width of the femtosecond fs pulses, rota- tional and vibrational superposition states can be prepared resulting in coherent wave packet motion on the potential energy surfaces excited by the inter- ) Corresponding author. Tel.: q49-931-888-6330; Fax: q49- 931-888-6332; E-mail: wolfgang.kiefer@mail.uni-wuerzburg.de action with the laser light. The rovibrational coher- ence can be probed by different means opening the possibility to follow the dynamics in real-time. The common technique for doing fs spectroscopy is a w x two-pulse pump-probe scheme 1–3 . The signal de- tection is possible in many ways. Examples are Ž . w x laser-induced fluorescence LIF detection 4–6 , ab- w x sorption 7–9 , resonant impulsive stimulated Raman Ž . w x scattering RISRS 10 , stimulated emission pump- w x Ž . w x ing 11 , multiphoton ionization MPI 12,13 , w x photoelectron kinetic energy 14 and zero kinetic Ž . w x energy ZEKE 15 , and femtosecond up-conversion w x 16 . 0924-2031r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0924-2031 98 00053-8