218
T. Buckup et al.
has been demonstrated previously for RPSB [36]. An alternative activation mechanism of the low-frequency modes compared to a direct excitation through lightmatter interaction can be given as coherent internal vibrational energy redistribution
starting from directly excited high-frequency vibrational levels in the S 1 state [40]
(Fig. 9.7). Such an activation mechanism has been proposed for RPSB previously
based on the need for a huge amount of excess vibrational energy (∼ 5000 cm −1 ) in
order to observe the low-frequency modes [40]. However, our DFWM experiments
reveal that such a threshold can only be on the order of a few hundreds of wavenumbers (200–500 cm −1 ). In the context of the activation of the low-frequency modes, it
is important to note that very similar observations have been determined for the outof-plane modes for BR (Fig. 9.6). This observation is indicative for a conclusion that
both types of modes are acceptor modes for access vibrational energy deposition in
the excited state and are therefore responsible for ultrafast relaxation possibly along
the reaction coordinate. Their specific observation only for BR can be indicative for
the conclusion that these modes are responsible for the much faster isomerization in
the protein environment compared to the chromophore in solution and also for the
much higher selectivity and quantum yield for photoproduct formation [42].
9.3.2 Detection of Dark States
9.3.2.1 Introduction
The identification and characterization of dark electronic states is of the greatest
challenges in spectroscopy of excited dynamics. Since these states cannot be excited
directly, as the denotation ‘dark state’ already implies, identification of dark states
on photochemical processes in biological systems is often difficult. Nonetheless, the
processes that cannot be explained without the presence of dark states are numerous
and the examples range from photo-damage of DNA [43] to light-harvesting [44]. In
the latter case, a long standing and still ongoing debate is held on the possible participation of dark electronic states in the energy dissipation pathway in carotenoids.
It is well known that the first bright electronic singlet state (S 2 ) of these natural pigments that is excited via absorption of light in the green-blue region of the visible
spectrum is not the lowest lying excited singlet state. The first excited singlet state S 1
is indeed a very well characterized dark state, whose energy, electronic lifetime and
vibrational spectra [12] have been determined in many spectroscopic investigations.
However, several experimental findings such as the deviation of the S 2 lifetime dependence on the conjugation length of the carotenoids from the energy gap law raise
the question whether additional dark states between the S 2 and S 1 states exist and
potentially participate in the relaxation pathway [45, 46]. Such states have already
been predicted by theory [47] but are extremely difficult to detect spectroscopically,
since they cannot even be excited via two-photon absorption, as is the case for the
S 1 state [48], and have very short lifetimes.
Applying pump-DFWM to lycopene, a carotenoid with 11 conjugated double
bonds, we find direct evidence for the contribution of an additional electronic state
T. Buckup et al.
has been demonstrated previously for RPSB [36]. An alternative activation mechanism of the low-frequency modes compared to a direct excitation through lightmatter interaction can be given as coherent internal vibrational energy redistribution
starting from directly excited high-frequency vibrational levels in the S 1 state [40]
(Fig. 9.7). Such an activation mechanism has been proposed for RPSB previously
based on the need for a huge amount of excess vibrational energy (∼ 5000 cm −1 ) in
order to observe the low-frequency modes [40]. However, our DFWM experiments
reveal that such a threshold can only be on the order of a few hundreds of wavenumbers (200–500 cm −1 ). In the context of the activation of the low-frequency modes, it
is important to note that very similar observations have been determined for the outof-plane modes for BR (Fig. 9.6). This observation is indicative for a conclusion that
both types of modes are acceptor modes for access vibrational energy deposition in
the excited state and are therefore responsible for ultrafast relaxation possibly along
the reaction coordinate. Their specific observation only for BR can be indicative for
the conclusion that these modes are responsible for the much faster isomerization in
the protein environment compared to the chromophore in solution and also for the
much higher selectivity and quantum yield for photoproduct formation [42].
9.3.2 Detection of Dark States
9.3.2.1 Introduction
The identification and characterization of dark electronic states is of the greatest
challenges in spectroscopy of excited dynamics. Since these states cannot be excited
directly, as the denotation ‘dark state’ already implies, identification of dark states
on photochemical processes in biological systems is often difficult. Nonetheless, the
processes that cannot be explained without the presence of dark states are numerous
and the examples range from photo-damage of DNA [43] to light-harvesting [44]. In
the latter case, a long standing and still ongoing debate is held on the possible participation of dark electronic states in the energy dissipation pathway in carotenoids.
It is well known that the first bright electronic singlet state (S 2 ) of these natural pigments that is excited via absorption of light in the green-blue region of the visible
spectrum is not the lowest lying excited singlet state. The first excited singlet state S 1
is indeed a very well characterized dark state, whose energy, electronic lifetime and
vibrational spectra [12] have been determined in many spectroscopic investigations.
However, several experimental findings such as the deviation of the S 2 lifetime dependence on the conjugation length of the carotenoids from the energy gap law raise
the question whether additional dark states between the S 2 and S 1 states exist and
potentially participate in the relaxation pathway [45, 46]. Such states have already
been predicted by theory [47] but are extremely difficult to detect spectroscopically,
since they cannot even be excited via two-photon absorption, as is the case for the
S 1 state [48], and have very short lifetimes.
Applying pump-DFWM to lycopene, a carotenoid with 11 conjugated double
bonds, we find direct evidence for the contribution of an additional electronic state