level diagrams are used to visualize these contributions (Fig. 3). Detailed
descriptions of such diagrams can be found in Refs. [10, 62–64].
In brief, the four arrows represent four light-matter interactions in a hypothetical
vibrational three-level system of an anharmonic mode (q vib ) in an electronic ground
state potential. The blue/red color-coding of the arrows is intended to visualize the
interactions between different vibrational levels. Some of the elements of the
response function involve only transitions between the ground state |0i and the first
excited state |1i and are termed ground state bleach (Fig. 3a) and stimulated
emission (Fig. 3b). In addition, elements exist that involve transitions from the first
to the second excited state |2i. These elements are therefore referred to excited state
absorption (Fig. 3c). Note that the exemplarily depicted energy level diagrams in
Fig. 3 only represent parts of the total amount of signal contributions. Generally, the
total response contains contributions, for which either a vibrational echo is emitted
from the sample (rephasing pathways) or not (non-rephasing pathways). The latter
group is also sometimes denoted as ‘‘virtual echo’’ contributions [65]. A final 2D IR
spectrum, generally referred to as a ‘‘fully absorptive’’ spectrum, will contain all
different contributions. Several reports on 2D IR spectroscopy exist, where
rephasing and non-rephasing contributions have been plotted separately; however,
these representations are not widely used anymore in the recent literature
[10, 66, 67]. An important aspect for 2D IR spectroscopy is the involvement of
four different light-matter interactions (Eq. 2), each of which is associated with a
transition-dipole moment (the fourth and last interaction is the emission of the
signal light). As a result, the signal scales with the absorption coefficient squared as
opposed to linear (e.g. FT IR) spectroscopy, which contains only two light matter
interactions [10, 68]. This property of nonlinear spectroscopy can be used to
enhance the contrast of 2D IR spectroscopy over linear methods and to help
interpreting congested spectra (see Sects. 3.1.2 and 4.2).
2.1 Molecular Information from Diagonal and Off-Diagonal Peaks in 2D IR
Spectra
2.1.1 Diagonal Peaks
2D IR spectra contain significantly more information about the sample compared to
linear FT IR spectroscopy. Besides the aspect of ultrafast temporal resolution,
valuable information is already contained in the observation of pairs of signals for
each vibrational transition that is excited. These pairs are associated with ground
state bleach/stimulated emission (blue, Fig. 3a) and excited state absorption (red,
Fig. 3a), respectively. The origin of the two contributions is the anharmonicity of
the electronic ground state potential, which gives rise to frequency-shifted, higherlying transitions (Fig. 4a). Thus, the frequency separation of the blue and red peaks
in a 2D IR spectrum can be used to retrieve the value of the anharmonicity.
However, the frequency separation of the two bands does not automatically give the
anharmonicity value. If the width of the band is larger than the anharmonic shift,
positive and negative contributions add in the overlapping region and the
anharmonicity has to be obtained by fitting lineshape functions to the bands.
Top Curr Chem (Z) (2017) 375:86
123
122
Reprinted from the journal
descriptions of such diagrams can be found in Refs. [10, 62–64].
In brief, the four arrows represent four light-matter interactions in a hypothetical
vibrational three-level system of an anharmonic mode (q vib ) in an electronic ground
state potential. The blue/red color-coding of the arrows is intended to visualize the
interactions between different vibrational levels. Some of the elements of the
response function involve only transitions between the ground state |0i and the first
excited state |1i and are termed ground state bleach (Fig. 3a) and stimulated
emission (Fig. 3b). In addition, elements exist that involve transitions from the first
to the second excited state |2i. These elements are therefore referred to excited state
absorption (Fig. 3c). Note that the exemplarily depicted energy level diagrams in
Fig. 3 only represent parts of the total amount of signal contributions. Generally, the
total response contains contributions, for which either a vibrational echo is emitted
from the sample (rephasing pathways) or not (non-rephasing pathways). The latter
group is also sometimes denoted as ‘‘virtual echo’’ contributions [65]. A final 2D IR
spectrum, generally referred to as a ‘‘fully absorptive’’ spectrum, will contain all
different contributions. Several reports on 2D IR spectroscopy exist, where
rephasing and non-rephasing contributions have been plotted separately; however,
these representations are not widely used anymore in the recent literature
[10, 66, 67]. An important aspect for 2D IR spectroscopy is the involvement of
four different light-matter interactions (Eq. 2), each of which is associated with a
transition-dipole moment (the fourth and last interaction is the emission of the
signal light). As a result, the signal scales with the absorption coefficient squared as
opposed to linear (e.g. FT IR) spectroscopy, which contains only two light matter
interactions [10, 68]. This property of nonlinear spectroscopy can be used to
enhance the contrast of 2D IR spectroscopy over linear methods and to help
interpreting congested spectra (see Sects. 3.1.2 and 4.2).
2.1 Molecular Information from Diagonal and Off-Diagonal Peaks in 2D IR
Spectra
2.1.1 Diagonal Peaks
2D IR spectra contain significantly more information about the sample compared to
linear FT IR spectroscopy. Besides the aspect of ultrafast temporal resolution,
valuable information is already contained in the observation of pairs of signals for
each vibrational transition that is excited. These pairs are associated with ground
state bleach/stimulated emission (blue, Fig. 3a) and excited state absorption (red,
Fig. 3a), respectively. The origin of the two contributions is the anharmonicity of
the electronic ground state potential, which gives rise to frequency-shifted, higherlying transitions (Fig. 4a). Thus, the frequency separation of the blue and red peaks
in a 2D IR spectrum can be used to retrieve the value of the anharmonicity.
However, the frequency separation of the two bands does not automatically give the
anharmonicity value. If the width of the band is larger than the anharmonic shift,
positive and negative contributions add in the overlapping region and the
anharmonicity has to be obtained by fitting lineshape functions to the bands.
Top Curr Chem (Z) (2017) 375:86
123
122
Reprinted from the journal