9 On the Investigation of Excited State Dynamics with (Pump-)Degenerate
223
rates depending on the conjugation length could only be explained by the existence
of an additional state located between S 2 and S 1 . Similar to studies on lutein and
β-carotene [46], the longer lifetime of about 100 fs was assigned to the S 2 state
while the identical rise time of the S 1 state was explained with a very fast S x –S 1
relaxation (10 fs). The dark nature of the additional state in combination with the
extremely rapid relaxation from S 2 to S 1 in carotenoids makes it very difficult to
observe this state in spectroscopic studies using transient absorption and to differentiate between the time constants of the S 2 state and the dark state. Using the
method of pump-DFWM, however, we can distinguish between these two adjacent
states and unambiguously assign of the longer lifetime of 110 fs to the dark state
due to its intense stimulated emission that leads to a unique signal in this technique.
9.3.3 Vibrational Coherence Evolution in the Excited State
9.3.3.1 Introduction
Among the incoherent population relaxation (Sect. 9.3.2), the evolution of the ultrafast vibrational coherence in the excited state is an additional but very important
aspect in understanding chemical transformations. Not just the frequency of oscillatory phenomena can give significant hints on the flow of a given reaction, but
also how the frequencies change during the relaxation process may also help to
re-construct the relaxation pathway and the participating molecular degrees of freedom. The high temporal and spectral resolution of pump-DFWM make this technique an ideal tool for the analysis of the shift of the vibrational frequencies in time.
Another important aspect of the vibrational dynamics is the population time of individual modes. Rise and decay times of vibrational mode amplitudes present us
a better picture on how modes are populated and depopulated and, sometimes, allow for the identification of vibrational modes specific for a given electronic state.
In pump-DFWM, the evolution of the vibrational population can be measured by
detecting the vibrational mode intensity for several initial pump delays T .
In this section, we apply pump-DFWM to resolve the evolution of the vibrational
coherence in the excited states of carotenoids. Similar to lycopene investigated on
the last section, the S 1 state will be populated with excess vibrational energy, given
the considerable difference in zero-point energies between S 1 and S 2 . McCamant
et al. [54] proposed a two-stage mechanism for internal vibrational redistribution
(IVR) on S 1 for β-carotene in solution. The authors employed femtosecond stimulated Raman scattering and explained their results without invoking any intermediate states between S 1 and S 2 in β-carotene. Rondonuwu et al. [55] however, investigated polyenes of similar conjugation length by sub-picosecond time-resolved
Raman scattering. The authors suggested the following energetic pathway upon excitation of S 2 (1B +
u ): 1B +
u → 3A −
g → 1B −
u → 2A −
g → 1A −
g . Hence, the puzzle of
carotenoid’s deactivation network is not resolved by either time dependent vibrational or transient absorption spectroscopy.
223
rates depending on the conjugation length could only be explained by the existence
of an additional state located between S 2 and S 1 . Similar to studies on lutein and
β-carotene [46], the longer lifetime of about 100 fs was assigned to the S 2 state
while the identical rise time of the S 1 state was explained with a very fast S x –S 1
relaxation (10 fs). The dark nature of the additional state in combination with the
extremely rapid relaxation from S 2 to S 1 in carotenoids makes it very difficult to
observe this state in spectroscopic studies using transient absorption and to differentiate between the time constants of the S 2 state and the dark state. Using the
method of pump-DFWM, however, we can distinguish between these two adjacent
states and unambiguously assign of the longer lifetime of 110 fs to the dark state
due to its intense stimulated emission that leads to a unique signal in this technique.
9.3.3 Vibrational Coherence Evolution in the Excited State
9.3.3.1 Introduction
Among the incoherent population relaxation (Sect. 9.3.2), the evolution of the ultrafast vibrational coherence in the excited state is an additional but very important
aspect in understanding chemical transformations. Not just the frequency of oscillatory phenomena can give significant hints on the flow of a given reaction, but
also how the frequencies change during the relaxation process may also help to
re-construct the relaxation pathway and the participating molecular degrees of freedom. The high temporal and spectral resolution of pump-DFWM make this technique an ideal tool for the analysis of the shift of the vibrational frequencies in time.
Another important aspect of the vibrational dynamics is the population time of individual modes. Rise and decay times of vibrational mode amplitudes present us
a better picture on how modes are populated and depopulated and, sometimes, allow for the identification of vibrational modes specific for a given electronic state.
In pump-DFWM, the evolution of the vibrational population can be measured by
detecting the vibrational mode intensity for several initial pump delays T .
In this section, we apply pump-DFWM to resolve the evolution of the vibrational
coherence in the excited states of carotenoids. Similar to lycopene investigated on
the last section, the S 1 state will be populated with excess vibrational energy, given
the considerable difference in zero-point energies between S 1 and S 2 . McCamant
et al. [54] proposed a two-stage mechanism for internal vibrational redistribution
(IVR) on S 1 for β-carotene in solution. The authors employed femtosecond stimulated Raman scattering and explained their results without invoking any intermediate states between S 1 and S 2 in β-carotene. Rondonuwu et al. [55] however, investigated polyenes of similar conjugation length by sub-picosecond time-resolved
Raman scattering. The authors suggested the following energetic pathway upon excitation of S 2 (1B +
u ): 1B +
u → 3A −
g → 1B −
u → 2A −
g → 1A −
g . Hence, the puzzle of
carotenoid’s deactivation network is not resolved by either time dependent vibrational or transient absorption spectroscopy.