Thomas Hornung 1 Observation and control of all-trans-β- carotene wavepacket motion using pump- degenerate four-wave mixing Thomas Hornung 1,2) , Hrvoje Skenderović 1) , Karl-Ludwig Kompa 1) and Marcus Motzkus 1,3) 1) Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, D-85748 Garching, Germany 2) Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge MA 02139-4307, USA 3) Philipps-Universität, Hans-Meerwein-Str., D-35032 Marburg, Germany E-mail:thornung@mit.edu, motzkus@staff.uni-marburg.de Abstract. Wavepacket dynamics on the ground and optically dark, first electronic state of all-trans-β-carotene are studied with 16 fs time resolution using pump-degenerate four- wave mixing spectroscopy. Moreover control over the vibrational ground state modes is shown. Carotenoids, being part of light-harvesting complexes (LH) play an important role in photosynthesis. They absorb light in blue-green and efficiently transfer energy to chlorophylls. The electronic states of interest to this study are the electronic ground 1 1 A g - (S 0 ), the first optically allowed 1 1 B u + (S 2 ) and the optically dark state 2 1 A g - (S 1 ) [1]. The long time kinetics of β-carotene have been studied extensively [1,2] and only very recent work has been successful in monitoring the coherent dynamics of the very high frequency modes of S 0 using transient absorption [3]. To observe the wavepacket dynamics in S 1 poses an extra challenge besides the short pulse duration required, since this state is not directly optically accessible from the ground state, but only via the population transfer from S 2 to S 1 . This pathway seems to be however incoherent [3], thus making simple pump probe measurements impossible. Recently it has been realized that by a pulse sequence that incorporates stimulated emission pumping (SEP) coherent motion in dark states can be observed [4]. FSRS is able to collect vibrational spectra of S 1 with high temporal and spectral resolution, but it is not sensitive to the relative phase between modes, thus a wavepacket motion cannot be reconstructed from this data. In this work we apply degenerate four-wave mixing spectroscopy (DFWM) combined with narrowband detection (1nm spectral resolution) to probe and control the wavepacket dynamics in S 0 . In combination with a pump pulse (pump- DFWM) we are able to monitor coherent dynamics of the dark S 1 state. The vibrational spectrum of S 0 and S 1 has been observed in several picosecond resonance Raman studies and has been recently studied with <100 fs time resolution with femtosecond-stimulated Raman spectroscopy (FSRS) [5]. Compressed output pulses of 10 µJ from a NOPA are frequency tunable from 400-700nm and have a typical pulse duration of 16 fs. The output of one NOPA is split into the 3 DFWM beams. The pulses within two of the beams are passed