Topics in Current Chemistry (2018) 376:28
1 3
generating shorter and even shorter pulses with a currently supposed record of about
40 attoseconds (10
−18
s) [50].
Another way of classifying multidimensional time-resolved techniques is related
to the ability to disentangle rephasing (optical echo formation) or non-rephasing (no
echo) processes [51]. Most currently existing multidimensional spectroscopy methods are so-called “four-wave-mixing” techniques. In such methods, three light–matter interactions with external optical fields (input) take place and the fourth interaction (output) stems from the emission of the signal light (see Fig. 1). As discussed
above, the term “rephasing” originates from the formation of an echo signal due to
the different interactions with the incoming pulses, and its existence and dynamics
are inherent properties of the sample under study. It is important to stress once again
that whether a given optical signal is based on a rephasing or non-rephasing process can be experimentally controlled by the chosen geometry of the incoming and
detected beam as well as the temporal ordering of the involved pulses [42]. The use
of these experimental parameters is carefully discussed in the following contributions for each experimental technique.
3 2D Plots
As briefly mentioned in the initial section, there are many ways of displaying the
time-resolved data that are acquired in multidimensional spectroscopy methods. In
most cases, the representation involves the spectral domain using optical frequency
representations. Very rare is the direct depiction of the time delay itself between two
laser interactions as an axis to display the data. Only few examples exist in the literature, for which the optical period of the light used to generate a signal is so long
that it is comparable to dephasing times [52]. The 2D plots associated with multidimensional techniques often rather exhibit data that is processed via a Fourier transformation of time-resolved signals. The reason to use the spectral representation is
simply that the signals are intuitively much easier to interpret as compared to the
time domain counterparts (just like one would normally analyze a linear absorption
signal in the spectral domain rather than in the time domain). The way the transformation is performed depends not only on the experimental details of each technique
but also on the nature of the sought molecular property, or dynamics of interest. In
essence, most 2D plots share the core idea of correlating an excitation with a detection frequency (Fig. 3a, b, d). However, there exist important special cases in which
one axis is represented by a delay (e.g., TR-IVS, Fig. 3c).
Two spectral axes (with frequency (THz) or energy units (eV, cm
−1
) are often
employed in 2D electronic, infrared, and terahertz spectroscopy methods (Fig. 3a,
b). The two axes in these cases are obtained in a very similar way. The detection
frequency axis is generally obtained by recording the signal spectrum directly with
a spectrometer. This can be done either by measuring the spectral intensity, or by
measuring the electric signal field by an experimental trick, i.e., through interference with a known local oscillator (heterodyne detection). The second axis is named
the excitation axis and is often either obtained from direct spectral scanning of the
frequency of the excitation pulse (for example, with a tunable spectral filter), or by a
6
Reprinted from the journal
1 3
generating shorter and even shorter pulses with a currently supposed record of about
40 attoseconds (10
−18
s) [50].
Another way of classifying multidimensional time-resolved techniques is related
to the ability to disentangle rephasing (optical echo formation) or non-rephasing (no
echo) processes [51]. Most currently existing multidimensional spectroscopy methods are so-called “four-wave-mixing” techniques. In such methods, three light–matter interactions with external optical fields (input) take place and the fourth interaction (output) stems from the emission of the signal light (see Fig. 1). As discussed
above, the term “rephasing” originates from the formation of an echo signal due to
the different interactions with the incoming pulses, and its existence and dynamics
are inherent properties of the sample under study. It is important to stress once again
that whether a given optical signal is based on a rephasing or non-rephasing process can be experimentally controlled by the chosen geometry of the incoming and
detected beam as well as the temporal ordering of the involved pulses [42]. The use
of these experimental parameters is carefully discussed in the following contributions for each experimental technique.
3 2D Plots
As briefly mentioned in the initial section, there are many ways of displaying the
time-resolved data that are acquired in multidimensional spectroscopy methods. In
most cases, the representation involves the spectral domain using optical frequency
representations. Very rare is the direct depiction of the time delay itself between two
laser interactions as an axis to display the data. Only few examples exist in the literature, for which the optical period of the light used to generate a signal is so long
that it is comparable to dephasing times [52]. The 2D plots associated with multidimensional techniques often rather exhibit data that is processed via a Fourier transformation of time-resolved signals. The reason to use the spectral representation is
simply that the signals are intuitively much easier to interpret as compared to the
time domain counterparts (just like one would normally analyze a linear absorption
signal in the spectral domain rather than in the time domain). The way the transformation is performed depends not only on the experimental details of each technique
but also on the nature of the sought molecular property, or dynamics of interest. In
essence, most 2D plots share the core idea of correlating an excitation with a detection frequency (Fig. 3a, b, d). However, there exist important special cases in which
one axis is represented by a delay (e.g., TR-IVS, Fig. 3c).
Two spectral axes (with frequency (THz) or energy units (eV, cm
−1
) are often
employed in 2D electronic, infrared, and terahertz spectroscopy methods (Fig. 3a,
b). The two axes in these cases are obtained in a very similar way. The detection
frequency axis is generally obtained by recording the signal spectrum directly with
a spectrometer. This can be done either by measuring the spectral intensity, or by
measuring the electric signal field by an experimental trick, i.e., through interference with a known local oscillator (heterodyne detection). The second axis is named
the excitation axis and is often either obtained from direct spectral scanning of the
frequency of the excitation pulse (for example, with a tunable spectral filter), or by a
6
Reprinted from the journal