Observation of all-trans-b-carotene wavepacket motion on the electronic ground and excited dark state using degenerate four-wave mixing (DFWM) and pump–DFWM Thomas Hornung a,b , Hrvoje Skenderovic ´ a,c , Marcus Motzkus a,d, * a Max-Planck-Institut fu ¨ r Quantenoptik, Hans-Kopfermann-Str. 1, D-85748 Garching bei Mu ¨ nchen b Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139-4307, USA c Institute of Physics, Bijenic ˇka cesta 46, 10000 Zagreb, Croatia d Institut fur Physikalische Chemie, Philipps-Universita ¨ t, Hans-Meerwein-Strasse, D-35032 Marburg, Germany Received 20 October 2004; in final form 28 October 2004 Available online 5 January 2005 Abstract Wavepacket dynamics on the ground and on the optically dark, first electronic state of all-trans-b-carotene is presented. Ground state dynamics are studied using resonant and non-resonant degenerate four-wave mixing spectroscopy with 16 fs time resolution. Wavepacket motion on the electronic dark state was monitored using pump-degenerate four-wave mixing spectroscopy. Ó 2004 Elsevier B.V. All rights reserved. 1. Introduction Complex molecular systems not only have several ex- cited states, but also dark excited states, that are inacces- sible by a direct one-photon process from the ground state. Four-wave-mixing (FWM) spectroscopy has been shown to be a versatile tool to study wavepacket motion on both ground and excited state simultaneously, since the process includes three laser pulse interactions and thus Feynman pathways that correspond to pump– dump schemes [1,2]. The freedom to choose a certain pulse ordering is a further advantage of this method compared to pump–probe, since it allows to isolate the wavepacket dynamics of interest [3]. Finally the degener- ate four-wave mixing (DFWM) signal is background- free. One essential requirement of the DFWM process is that the system stays coherent during the three laser pulse interactions. In complex systems however the wavepacket dynamics can evolve from the initially ex- cited states incoherently into other dark excited states. This excited state process can not be monitored by FWM alone, since it involves an incoherent step. In these situations a pulse scheme can help out, where a pump pulse precedes the FWM process. This method was first used to monitor the dissocation dynamics of Na–I and termed there pump–FWM [4]. The pump pulse transfers population to the first excited state that can then undergo an incoherent decay into a second ex- cited state. The subsequent FWM process creates a coherence or wavepacket in this incoherent population (as it does normally for the thermally equilibrated ground state population) and allows to determine the motion in the otherwise inaccessible dark excited states. In this Letter we will show how the use of these FWM techniques using 16-fs long pulses can reveal significant information about an important class of biological mol- ecules: the carotenoids. Carotenoids, being part of light-harvesting complexes (LH) play an important role in photosynthesis. They ab- sorb light in blue-green and efficiently transfer energy to chlorophylls. The most stable isomer of b-carotene is 0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.11.135 * Corresponding author. Fax: +49 6421 28 22542. E-mail address: motzkus@staff.uni-marburg.de (M. Motzkus). www.elsevier.com/locate/cplett Chemical Physics Letters 402 (2005) 283–288