very powerful method for rigorously testing basic assumptions about chemical
dynamics.
The concept of (non-)Markovian dynamics is in fact much broader as in the
described comparatively simple case of hydrogen bond formation/dissociation
between two small molecules in a heterogeneous solvent. Markov-state models are
very relevant for protein dynamics and the timescales for structural changes in such
complex systems cover many orders of magnitude [247–249]. However, the
intrinsic dependence of the 3D IR signal on the lifetime of the vibrationally excited
levels is likely to limit the application of the method for comparatively slow
dynamics of vibrational relaxationn. Only in certain cases vibrational labels are
long-lived enough to sample dynamic processes on the timescale of several
hundreds of picoseconds or even nanoseconds [15, 111, 112].
3D IR spectroscopy has also been successfully applied to other sample systems.
Also Hamm et al. have applied the concept to study the dynamics of highly
vibrationally excited states of isotope-diluted ice (HOD in H 2 O) [239]. It was found
that the higher lying vibrational states in the ground state potential decay extremely
fast, that is, on the order of 200 fs. By theoretical considerations, it was concluded
that this ultrashort lifetime is based on mode mixing of the OD-stretch coordinate
with lattice degrees of freedom of the ice crystal. For this particular system, the
authors, therefore, identified a crossover behavior between adiabatic and nonadiabatic approximations, which occurs in comparatively low-lying energetic
regions of an electronic potential due to the strong anharmonicity of the OD-stretch
vibration of water.
Unfortunately, 3D IR spectroscopy is a complex experimental method and the
contributing fifth-order response functions scale in a cubic manner with the
absorption coefficients and so does the signal. Contrary to initial hopes and
expectations [10], it will thus be very challenging to apply 3D IR to a broad range of
sample systems and particularly cases such as surfaces and interfaces, where the
method bears a great potential to investigate heterogeneous dynamics.
4.4 Transient 2D IR Spectroscopy
While 3D IR spectroscopy is able to resolve non-equilibrium dynamics in the
electronic ground state of a sample, considerably more information can be obtained
by the help of a couple of additional variants of transient 2D IR spectroscopy. In
transient 2D IR methods, the sample is brought to a non-equilibrium state, and
subsequently 2D IR spectra are measured either in dependence of the delay of initial
perturbation, or in dependence of the conventional IR population delay. The amount
of information that can be obtained is manifold, including dynamics from excited
electronic states [39, 250–257], photo-induced chemical reactions [38, 258, 259], or
different types of non-equilibrium sample conditions such as temperature variations
[260–264]. Most importantly, this concept allows an extension of experimentally
available timescales far beyond vibrational relaxation. This is because the only thing
that is needed is a spectral shift, intensity change or broadening of a vibrational
transition upon perturbation, which, depending on the system under investigation,
may principally last forever. The discussion here is focused on essentially the above
Top Curr Chem (Z) (2017) 375:86
123
163
Reprinted from the journal
dynamics.
The concept of (non-)Markovian dynamics is in fact much broader as in the
described comparatively simple case of hydrogen bond formation/dissociation
between two small molecules in a heterogeneous solvent. Markov-state models are
very relevant for protein dynamics and the timescales for structural changes in such
complex systems cover many orders of magnitude [247–249]. However, the
intrinsic dependence of the 3D IR signal on the lifetime of the vibrationally excited
levels is likely to limit the application of the method for comparatively slow
dynamics of vibrational relaxationn. Only in certain cases vibrational labels are
long-lived enough to sample dynamic processes on the timescale of several
hundreds of picoseconds or even nanoseconds [15, 111, 112].
3D IR spectroscopy has also been successfully applied to other sample systems.
Also Hamm et al. have applied the concept to study the dynamics of highly
vibrationally excited states of isotope-diluted ice (HOD in H 2 O) [239]. It was found
that the higher lying vibrational states in the ground state potential decay extremely
fast, that is, on the order of 200 fs. By theoretical considerations, it was concluded
that this ultrashort lifetime is based on mode mixing of the OD-stretch coordinate
with lattice degrees of freedom of the ice crystal. For this particular system, the
authors, therefore, identified a crossover behavior between adiabatic and nonadiabatic approximations, which occurs in comparatively low-lying energetic
regions of an electronic potential due to the strong anharmonicity of the OD-stretch
vibration of water.
Unfortunately, 3D IR spectroscopy is a complex experimental method and the
contributing fifth-order response functions scale in a cubic manner with the
absorption coefficients and so does the signal. Contrary to initial hopes and
expectations [10], it will thus be very challenging to apply 3D IR to a broad range of
sample systems and particularly cases such as surfaces and interfaces, where the
method bears a great potential to investigate heterogeneous dynamics.
4.4 Transient 2D IR Spectroscopy
While 3D IR spectroscopy is able to resolve non-equilibrium dynamics in the
electronic ground state of a sample, considerably more information can be obtained
by the help of a couple of additional variants of transient 2D IR spectroscopy. In
transient 2D IR methods, the sample is brought to a non-equilibrium state, and
subsequently 2D IR spectra are measured either in dependence of the delay of initial
perturbation, or in dependence of the conventional IR population delay. The amount
of information that can be obtained is manifold, including dynamics from excited
electronic states [39, 250–257], photo-induced chemical reactions [38, 258, 259], or
different types of non-equilibrium sample conditions such as temperature variations
[260–264]. Most importantly, this concept allows an extension of experimentally
available timescales far beyond vibrational relaxation. This is because the only thing
that is needed is a spectral shift, intensity change or broadening of a vibrational
transition upon perturbation, which, depending on the system under investigation,
may principally last forever. The discussion here is focused on essentially the above
Top Curr Chem (Z) (2017) 375:86
123
163
Reprinted from the journal