9 On the Investigation of Excited State Dynamics with (Pump-)Degenerate
227
state, generating a fast rising signal as soon as the system is excited and starts to
relax.
The lack of a clear vibrational signature specific to a dark state in β-carotene is
in agreement with the picture obtained for other carotenoids using pump-DFWM.
The dark state observed between the S 2 and the S 1 state has a lifetime which is
between 20–80 fs, depending on the carotenoid length. In the lifetime of such a
dark state, the C–C or C=C modes will barely be able to oscillate two times before
the electronic state has completely decayed, which will lead to a contribution very
short in duration in the DFWM signal. Therefore, the signal of such a dark state will
be hardly detectable with DFWM (as well as in TA or other any other third-order
technique). This demonstrates the power of a pump DFWM: By using an initial
pump pulse to excite the molecule, the presence of such ultrafast transient states can
be clearly resolved in the deactivation.
9.4 Conclusions
Multidimensional time-resolved spectroscopy methods use additional laser-matter
interactions to open new temporal and/or spectral observation windows. This allows
observing molecular phenomena normally hidden from time-resolved techniques
with low-dimensionality. We have exploited third- and fifth-order nonlinear methods based on four wave mixing to investigate the excited state of biomolecules.
This was demonstrated to Schiff-bases in BR and solution as well as for different
carotenoids in solution. With (pump-) DFWM, it is possible to resolve the activation and deactivation of Raman modes and electronic population, leading to a
better understanding of the chemical changes, in particular in electronically excited
states.
References
1. M. Dantus, M.J. Rosker, A.H. Zewail, Real-time femtosecond probing of transition-states in
chemical-reactions. J. Chem. Phys. 87(4), 2395–2397 (1987)
2. J. Dobler et al., Excited-state reaction dynamics of bacteriorhodopsin studied by femtosecond
spectroscopy. Chem. Phys. Lett. 144(2), 215–220 (1988)
3. R.A. Mathies et al., Direct observation of the femtosecond excited-state cis-trans isomerization
in bacteriorhodopsin. Science 240(4853), 777–779 (1988)
4. P.F. Barbara, G.C. Walker, T.P. Smith, Vibrational-modes and the dynamic solvent effect in
electron and proton-transfer. Science 256(5059), 975–981 (1992)
5. J.L. Herek et al., Femtosecond real-time probing of reactions. 9. Hydrogen-atom transfer.
J. Chem. Phys. 97(12), 9046–9061 (1992)
6. K. Lenz et al., Resonance Raman and femtosecond absorption studies of vibrational-relaxation
initiated by ultrafast intramolecular proton-transfer. Chem. Phys. Lett. 229(4–5), 340–346
(1994)
7. A.H. Zewail, Femtochemistry: recent progress in studies of dynamics and control of reactions
and their transition states. J. Phys. Chem. 100(31), 12701–12724 (1996)
227
state, generating a fast rising signal as soon as the system is excited and starts to
relax.
The lack of a clear vibrational signature specific to a dark state in β-carotene is
in agreement with the picture obtained for other carotenoids using pump-DFWM.
The dark state observed between the S 2 and the S 1 state has a lifetime which is
between 20–80 fs, depending on the carotenoid length. In the lifetime of such a
dark state, the C–C or C=C modes will barely be able to oscillate two times before
the electronic state has completely decayed, which will lead to a contribution very
short in duration in the DFWM signal. Therefore, the signal of such a dark state will
be hardly detectable with DFWM (as well as in TA or other any other third-order
technique). This demonstrates the power of a pump DFWM: By using an initial
pump pulse to excite the molecule, the presence of such ultrafast transient states can
be clearly resolved in the deactivation.
9.4 Conclusions
Multidimensional time-resolved spectroscopy methods use additional laser-matter
interactions to open new temporal and/or spectral observation windows. This allows
observing molecular phenomena normally hidden from time-resolved techniques
with low-dimensionality. We have exploited third- and fifth-order nonlinear methods based on four wave mixing to investigate the excited state of biomolecules.
This was demonstrated to Schiff-bases in BR and solution as well as for different
carotenoids in solution. With (pump-) DFWM, it is possible to resolve the activation and deactivation of Raman modes and electronic population, leading to a
better understanding of the chemical changes, in particular in electronically excited
states.
References
1. M. Dantus, M.J. Rosker, A.H. Zewail, Real-time femtosecond probing of transition-states in
chemical-reactions. J. Chem. Phys. 87(4), 2395–2397 (1987)
2. J. Dobler et al., Excited-state reaction dynamics of bacteriorhodopsin studied by femtosecond
spectroscopy. Chem. Phys. Lett. 144(2), 215–220 (1988)
3. R.A. Mathies et al., Direct observation of the femtosecond excited-state cis-trans isomerization
in bacteriorhodopsin. Science 240(4853), 777–779 (1988)
4. P.F. Barbara, G.C. Walker, T.P. Smith, Vibrational-modes and the dynamic solvent effect in
electron and proton-transfer. Science 256(5059), 975–981 (1992)
5. J.L. Herek et al., Femtosecond real-time probing of reactions. 9. Hydrogen-atom transfer.
J. Chem. Phys. 97(12), 9046–9061 (1992)
6. K. Lenz et al., Resonance Raman and femtosecond absorption studies of vibrational-relaxation
initiated by ultrafast intramolecular proton-transfer. Chem. Phys. Lett. 229(4–5), 340–346
(1994)
7. A.H. Zewail, Femtochemistry: recent progress in studies of dynamics and control of reactions
and their transition states. J. Phys. Chem. 100(31), 12701–12724 (1996)