On the Investigation of Excited State Dynamics with (Pump-)Degenerate Four Wave Mixing

Abstract

Multidimensional time-resolved spectroscopy allows disentangling particular aspects of the molecular dynamics, which are normally hidden from linear techniques. In this chapter, we show how third- and fifth-order techniques using sub-20 fs pulses can be applied to address coherence and population dynamics in the excited states of biomolecules. In particular, broadband four wave mixing is combined with an initial pump pulse to promote population to the excited state. With this approach, it is possible to interrogate the potential surface of the excited and ground states during the excited state evolution with a time resolution better than 20 fs. Three general aspects of the excited state dynamics are discussed. (1) The assignment of vibrational coherence to the respective excited state potential is illustrated for retinal in solution and in the protein environment. By changing the excitation wavelength and comparing low- and high-frequency vibrational coherence content, it is shown that low-frequency modes are predominantly originated in the excited state, while high-frequency modes belong to the ground state. (2) The temporal resolution of dark electronic states in lycopene is investigated with pump-DFWM. Contrasting to lower-order techniques, pump-DFWM allows to snapshot the ultrafast population relaxation directly after the excitation of the S2 electronic state. (3) The evolution of the vibrational coherence in the excited state is demonstrated for β-carotene. This gives accurate information on the instantaneous frequency, populations and even anharmonicities of all relevant vibrational modes on the potential surface of the excited state.