Chapter 9
On the Investigation of Excited State Dynamics
with (Pump-)Degenerate Four Wave Mixing
Tiago Buckup, Jan P. Kraack, Marie S. Marek, and Marcus Motzkus
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 sub20 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 pumpDFWM. Contrasting to lower-order techniques, pump-DFWM allows to snapshot
the ultrafast population relaxation directly after the excitation of the S 2 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.
9.1 Introduction
The evolution of photochemical transformations has been in the focal point of ultrafast time-resolved spectroscopy since its advent [1–6]. In this regard, the ability
to follow the population and coherence dynamics in the femtosecond time scale
T. Buckup · J.P. Kraack · M.S. Marek · M. Motzkus (B)
Physikalisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 229,
69120 Heidelberg, Germany
e-mail: marcus.motzkus@pci.uni-heidelberg.de
R. de Nalda, L. Bañares (eds.), Ultrafast Phenomena in Molecular Sciences,
Springer Series in Chemical Physics 107, DOI 10.1007/978-3-319-02051-8_9,
© Springer International Publishing Switzerland 2014
205
On the Investigation of Excited State Dynamics
with (Pump-)Degenerate Four Wave Mixing
Tiago Buckup, Jan P. Kraack, Marie S. Marek, and Marcus Motzkus
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 sub20 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 pumpDFWM. Contrasting to lower-order techniques, pump-DFWM allows to snapshot
the ultrafast population relaxation directly after the excitation of the S 2 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.
9.1 Introduction
The evolution of photochemical transformations has been in the focal point of ultrafast time-resolved spectroscopy since its advent [1–6]. In this regard, the ability
to follow the population and coherence dynamics in the femtosecond time scale
T. Buckup · J.P. Kraack · M.S. Marek · M. Motzkus (B)
Physikalisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 229,
69120 Heidelberg, Germany
e-mail: marcus.motzkus@pci.uni-heidelberg.de
R. de Nalda, L. Bañares (eds.), Ultrafast Phenomena in Molecular Sciences,
Springer Series in Chemical Physics 107, DOI 10.1007/978-3-319-02051-8_9,
© Springer International Publishing Switzerland 2014
205