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Topics in Current Chemistry (2018) 376:28
so-called multidimensional methods. There have been several implementations
of multidimensional spectroscopy methods, and the label “multidimensional”
has been loosely applied to describe different types of techniques in which two
axes (frequency–frequency or frequency–time) are used to display the data in
a “2D plot”. Multidimensional spectroscopy has been carried out, for example,
by exploiting the dynamic hole-burning effect in the spectral domain, i.e., by
actively controlling the (fairly narrowband) central wavelength of the pump spectrum and correlating it with the corresponding changes in a (broadband) probe
spectrum. This method works well from the infrared [34] spectral range that
addresses vibrational transitions up to the UV region where electronic transitions
with high photon energy are considered [35]. The most wide-spread implementation of multidimensional laser techniques nowadays is, however, based on pulse
sequences of laser light with temporal durations of only a few femtoseconds. In
this case, the signal is at least partly acquired in the time domain using variations
of various time delays, as we will explain further below. The spectral information
is then obtained by performing Fourier transformations of the time-domain contributions [36–38].
In a very general way, the basic principle of multidimensional spectroscopy relies
on the possibility to manipulate the formation of a so-called photon echo signal from
the sample [39]. This optical photon echo has a close relation to the Hahn spinecho [40], which has been known to chemists from the field of NMR spectroscopy
for decades. To generate such an echo response, multiple light–matter interactions
are exploited to generate a signal. In NMR spectroscopy, the signal stems from the
sample’s macroscopic magnetization after radio-frequency excitation, and the sample is held in an external magnetic field that is inevitable to only make the otherwise
energetically degenerate spin states spectroscopically “visible”. In contrast to that,
in optical spectroscopy, the signal source is an electric polarization that originates
from light–matter interactions between several laser pulses and the sample’s transition dipoles. Note that in this case no other external fields are necessary due to
the naturally occurring energetic splitting of optical transitions based on electronic,
vibrational, or rotational quantum numbers. Despite these very fundamental differences in the investigated systems, the experimental concept is transferable and the
analogy between NMR and optical spectroscopy has often been drawn, for instance
by using Bloch vector representations to visualize the interaction sequences [41].
In such a description, which will be repeatedly used in this collection in the context
of various methods, a first pulse in a general photon echo experiment induces an
oscillating electric polarization in the sample. This polarization dephases over time
due to time-dependent transition frequencies in the sample. The time dependence of
the transition frequencies stem from interactions of the molecules with their environment. Dephasing causes a so-called free-induction decay of the polarization, the
evolution of which is mapped out in time by a second laser pulse. That second interaction induces a new polarization in the sample, which is also subjected to relaxation processes, i.e., energy relaxation or again dephasing, dependent on the underlying interactions. Finally, a third light–matter interaction is used to generate another
polarization, which radiates again an oscillatory signal field off the sample. Dependent on the exact choice of experimental parameters, i.e., combinations of frequencies
3
Reprinted from the journal
Topics in Current Chemistry (2018) 376:28
so-called multidimensional methods. There have been several implementations
of multidimensional spectroscopy methods, and the label “multidimensional”
has been loosely applied to describe different types of techniques in which two
axes (frequency–frequency or frequency–time) are used to display the data in
a “2D plot”. Multidimensional spectroscopy has been carried out, for example,
by exploiting the dynamic hole-burning effect in the spectral domain, i.e., by
actively controlling the (fairly narrowband) central wavelength of the pump spectrum and correlating it with the corresponding changes in a (broadband) probe
spectrum. This method works well from the infrared [34] spectral range that
addresses vibrational transitions up to the UV region where electronic transitions
with high photon energy are considered [35]. The most wide-spread implementation of multidimensional laser techniques nowadays is, however, based on pulse
sequences of laser light with temporal durations of only a few femtoseconds. In
this case, the signal is at least partly acquired in the time domain using variations
of various time delays, as we will explain further below. The spectral information
is then obtained by performing Fourier transformations of the time-domain contributions [36–38].
In a very general way, the basic principle of multidimensional spectroscopy relies
on the possibility to manipulate the formation of a so-called photon echo signal from
the sample [39]. This optical photon echo has a close relation to the Hahn spinecho [40], which has been known to chemists from the field of NMR spectroscopy
for decades. To generate such an echo response, multiple light–matter interactions
are exploited to generate a signal. In NMR spectroscopy, the signal stems from the
sample’s macroscopic magnetization after radio-frequency excitation, and the sample is held in an external magnetic field that is inevitable to only make the otherwise
energetically degenerate spin states spectroscopically “visible”. In contrast to that,
in optical spectroscopy, the signal source is an electric polarization that originates
from light–matter interactions between several laser pulses and the sample’s transition dipoles. Note that in this case no other external fields are necessary due to
the naturally occurring energetic splitting of optical transitions based on electronic,
vibrational, or rotational quantum numbers. Despite these very fundamental differences in the investigated systems, the experimental concept is transferable and the
analogy between NMR and optical spectroscopy has often been drawn, for instance
by using Bloch vector representations to visualize the interaction sequences [41].
In such a description, which will be repeatedly used in this collection in the context
of various methods, a first pulse in a general photon echo experiment induces an
oscillating electric polarization in the sample. This polarization dephases over time
due to time-dependent transition frequencies in the sample. The time dependence of
the transition frequencies stem from interactions of the molecules with their environment. Dephasing causes a so-called free-induction decay of the polarization, the
evolution of which is mapped out in time by a second laser pulse. That second interaction induces a new polarization in the sample, which is also subjected to relaxation processes, i.e., energy relaxation or again dephasing, dependent on the underlying interactions. Finally, a third light–matter interaction is used to generate another
polarization, which radiates again an oscillatory signal field off the sample. Dependent on the exact choice of experimental parameters, i.e., combinations of frequencies
3
Reprinted from the journal