Top Curr Chem (Z) (2018) 376:10
1 3
contrast to the box-car geometry [31–33]. Several groups have shown the potential
and the capability to perform “two-color” kind of experiments by exploiting the collinear geometry [34]. In these experiments, the excitation axis is given by a first
identical pulse pair resonant with the electronic transition of the system under study,
while the detection axis can be spectrally tuned and depends solely on the bandwidth
of the third pulse [35–37]. The generation of the first pulse pair is usually obtained
by pulse-shaping techniques [38, 39], however, recently a new compact TranslatingWedge-Based Identical Pulses eNcoding System TWINS [40, 41] device has been
shown to have capability of creating intrinsically phase-locked pulses exploiting the
birefringence of non-linear crystals. The main advantages of the collinear geometry
are the phase stability and the possibility to carry two-colors experiments.
The fully collinear geometry uses only one beam, which is pulse shaped into
phase coherent pulse trains [31, 42]. It is based on a homodyne detection. The main
advantage of this technique is to provide a simplified experimental design.
A summary of the various implementations of 2DES, together with benefits and
drawbacks, bandwidth limitations, and typical bandwidths used has been nicely
reviewed by Ogilvie et al. in a recent review [43]. A detailed discussion of the several technical aspects on how to perform 2DES experiments goes beyond the scope
of this book, however review papers and books are available describing various
2DES implementations, as can be seen in references [11, 38, 44].
3 Heterogeneity
One of the main challenges in condensed-phase experiments is the heterogeneity
of the samples, which present very broad absorption spectra at room temperature.
2DES allows unraveling the presence of multi-chromophoric systems and sometimes disentangles their dynamics. Furthermore, the temporal evolution of 2D line
shape allows distinguishing a dynamic to a static contribution of a molecular dipole
transition. The inhomogeneous broadening of a system is interpretable from the
shape of diagonal peaks on a 2DES map (Fig. 4).
Firstly, at the waiting time T = 0, any particular transition excited will not yet
have undergone dynamical processes that could change its resonant frequency; thus,
its resonant frequency will be detected at its excitation frequency. In the homogeneous limit, the absorptive signal along the diagonal is expected to have a 2D-Lorentzian shape, because the rephasing and non-rephasing signals are identical [45].
As the inhomogeneous broadening increases, the 2D line shape broadens along the
diagonal axis while the antidiagonal remains unchanged, i.e., the rephasing signal
is larger than the non-rephasing signal at early time. Thus, it is possible to quantify
the linewidth broadening along the diagonal to determine how heterogeneous the
system is [3, 46, 47]. The lineshape along the diagonal will have an elliptical shape
whose profile along the diagonal frequency axis is proportional to the traditional 1D
absorption spectrum [13]. The width perpendicular to the diagonal represents the
homogeneous linewidth. The ellipticity of 2D line shape quantifies how much the
system is inhomogeneous.
32
Reprinted from the journal
1 3
contrast to the box-car geometry [31–33]. Several groups have shown the potential
and the capability to perform “two-color” kind of experiments by exploiting the collinear geometry [34]. In these experiments, the excitation axis is given by a first
identical pulse pair resonant with the electronic transition of the system under study,
while the detection axis can be spectrally tuned and depends solely on the bandwidth
of the third pulse [35–37]. The generation of the first pulse pair is usually obtained
by pulse-shaping techniques [38, 39], however, recently a new compact TranslatingWedge-Based Identical Pulses eNcoding System TWINS [40, 41] device has been
shown to have capability of creating intrinsically phase-locked pulses exploiting the
birefringence of non-linear crystals. The main advantages of the collinear geometry
are the phase stability and the possibility to carry two-colors experiments.
The fully collinear geometry uses only one beam, which is pulse shaped into
phase coherent pulse trains [31, 42]. It is based on a homodyne detection. The main
advantage of this technique is to provide a simplified experimental design.
A summary of the various implementations of 2DES, together with benefits and
drawbacks, bandwidth limitations, and typical bandwidths used has been nicely
reviewed by Ogilvie et al. in a recent review [43]. A detailed discussion of the several technical aspects on how to perform 2DES experiments goes beyond the scope
of this book, however review papers and books are available describing various
2DES implementations, as can be seen in references [11, 38, 44].
3 Heterogeneity
One of the main challenges in condensed-phase experiments is the heterogeneity
of the samples, which present very broad absorption spectra at room temperature.
2DES allows unraveling the presence of multi-chromophoric systems and sometimes disentangles their dynamics. Furthermore, the temporal evolution of 2D line
shape allows distinguishing a dynamic to a static contribution of a molecular dipole
transition. The inhomogeneous broadening of a system is interpretable from the
shape of diagonal peaks on a 2DES map (Fig. 4).
Firstly, at the waiting time T = 0, any particular transition excited will not yet
have undergone dynamical processes that could change its resonant frequency; thus,
its resonant frequency will be detected at its excitation frequency. In the homogeneous limit, the absorptive signal along the diagonal is expected to have a 2D-Lorentzian shape, because the rephasing and non-rephasing signals are identical [45].
As the inhomogeneous broadening increases, the 2D line shape broadens along the
diagonal axis while the antidiagonal remains unchanged, i.e., the rephasing signal
is larger than the non-rephasing signal at early time. Thus, it is possible to quantify
the linewidth broadening along the diagonal to determine how heterogeneous the
system is [3, 46, 47]. The lineshape along the diagonal will have an elliptical shape
whose profile along the diagonal frequency axis is proportional to the traditional 1D
absorption spectrum [13]. The width perpendicular to the diagonal represents the
homogeneous linewidth. The ellipticity of 2D line shape quantifies how much the
system is inhomogeneous.
32
Reprinted from the journal