9 On the Investigation of Excited State Dynamics with (Pump-)Degenerate
207
Fig. 9.1 Interaction scheme of (a) pump-DFWM, (b) electronically resonant DFWM and (c) nonresonant DFWM. In pump-DFWM (a), the excited potential can be exclusively investigated by
pre-exciting the ground state population with the initial pump. This is not the case when using
DFWM only: Resonant DFWM (b) excites and probes dynamics in both potential surfaces, i.e.,
excited and ground state dynamics
the pulses of the DFWM sequence generate both CARS (and its Stokes counterpart
CSRS) and TG contributions.
Three fundamental aspects of the excited state dynamics found in several molecular systems are discussed using such nonlinear experiments. Firstly, the assignment
of vibrational coherence to the respective electronic states is shown in the case of
Retinal Protonated Schiff Base (RPSB) using DFWM (Fig. 9.1(b) and (c)) with sub20 fs time resolution (Sect. 9.3.1). Tuning the excitation spectrum of the DFWM
sequence is exploited with its high sensitivity to disentangle the origin of low(<800 cm −1 ) and high-frequency (>800 cm −1 ) molecular modes. The second aspect is the detection of short-living dark electronic states, whose excitation via onephoton is forbidden. In Sect. 9.3.2, pump-DFWM (Fig. 9.1(a)) is applied to lycopene
in order to unravel the presence of additional electronic states after the relaxation of
the S 2 state. The focus is the unique capability of pump-DFWM to detect additional
relaxation pathways due to its higher dimensionality. The experimental results are
further supported by a simulation in the response function formalism. In the last experimental Sect. 9.3.3, vibrational coherences in a vibrationally excited electronic
state are addressed with pump-DFWM. In particular the vibrational evolution of
high-frequency modes are resolved with a high time and spectral resolution. This is
demonstrated for β-carotene during the internal conversion between S 2 and S 1 .
9.2 Pump-Degenerate Four Wave Mixing
9.2.1 Signal Generation
Pump-DFWM is an ultrafast technique able to obtain the complete vibrational spectra as well as electronic population and coherence relaxation in excited state poten-
207
Fig. 9.1 Interaction scheme of (a) pump-DFWM, (b) electronically resonant DFWM and (c) nonresonant DFWM. In pump-DFWM (a), the excited potential can be exclusively investigated by
pre-exciting the ground state population with the initial pump. This is not the case when using
DFWM only: Resonant DFWM (b) excites and probes dynamics in both potential surfaces, i.e.,
excited and ground state dynamics
the pulses of the DFWM sequence generate both CARS (and its Stokes counterpart
CSRS) and TG contributions.
Three fundamental aspects of the excited state dynamics found in several molecular systems are discussed using such nonlinear experiments. Firstly, the assignment
of vibrational coherence to the respective electronic states is shown in the case of
Retinal Protonated Schiff Base (RPSB) using DFWM (Fig. 9.1(b) and (c)) with sub20 fs time resolution (Sect. 9.3.1). Tuning the excitation spectrum of the DFWM
sequence is exploited with its high sensitivity to disentangle the origin of low(<800 cm −1 ) and high-frequency (>800 cm −1 ) molecular modes. The second aspect is the detection of short-living dark electronic states, whose excitation via onephoton is forbidden. In Sect. 9.3.2, pump-DFWM (Fig. 9.1(a)) is applied to lycopene
in order to unravel the presence of additional electronic states after the relaxation of
the S 2 state. The focus is the unique capability of pump-DFWM to detect additional
relaxation pathways due to its higher dimensionality. The experimental results are
further supported by a simulation in the response function formalism. In the last experimental Sect. 9.3.3, vibrational coherences in a vibrationally excited electronic
state are addressed with pump-DFWM. In particular the vibrational evolution of
high-frequency modes are resolved with a high time and spectral resolution. This is
demonstrated for β-carotene during the internal conversion between S 2 and S 1 .
9.2 Pump-Degenerate Four Wave Mixing
9.2.1 Signal Generation
Pump-DFWM is an ultrafast technique able to obtain the complete vibrational spectra as well as electronic population and coherence relaxation in excited state poten-