the x 1 axis, which is in this case the degree for electronic excitation. The second
spectral axis (x 3 ) is, as almost always, derived by spectrally dispersing the probe
pulse. Also in this variant, the signal is heterodyne-detected using a local oscillator,
which in most cases is the probe pulse itself, as the reported studies have been
performed in the pump-probe geometry. To map out the full molecular dynamics of
the sample, the population delay (t 2 ) is finally scanned to derive information about
electronic and vibrational relaxation and the correlations between electronic
vibrational frequencies, dynamics of solvent-solute interactions, or the dynamics of
electronic and vibrational correlations. Note that 2D EV spectroscopy depends on
the electronic relaxation times and may, therefore, provide significantly longer
observation windows as compared to 2D VE spectroscopy, what might constitute
and important advantage of the method.
The processes that underlie the signal generation can again be visualized using
energy level diagrams (Fig. 27b and c). Depending on the average photon energy,
the spectral width of the VIS excitation pulse and the vibrational frequency of the
modes of the molecule under study, different vibrational levels in the excited
electronic state (|ei) can contribute to the signal generation processes. Therefore,
multiple diagrams for excited state absorption, (a), have to be considered. In
contrast, generally only a single ground state bleach diagram is active (Fig. 27c),
which is justified as long as the spectral width of the VIS pump pulses does not
allow the excitation multiple vibrational levels in the electronic ground state, i.e. the
generation of vibrational coherences. Moreover, stimulated emission diagrams
between vibrational levels in the excited electronic state may also be considered, but
such contributions have not been observed experimentally so far. Note that only
exemplary pathways for non-rephasing diagrams are shown and complementary
rephasing pathways contribute as well, as demonstrated in more detailed theoretical
descriptions of 2D EV spectroscopy [282, 283].
Also, for 2D EV spectroscopy, the signals depend on the frequency shift of
vibrational transitions in ground and excited electronic states, similar to the 2D VE
methods. However, even in the case of negligible frequency shifts in the two
electronic states, a signal could still be measured if the electronic excitation
promotes the sample to higher-lying (v = 0) vibrational states in |ei, or if the IR
transition dipole moment is different in the two electronic states. Depending on the
sign of the frequency-shift between ground and excited electronic states, excited
state absorption transitions can show up on the high or the low-frequency side of the
ground state bleach signal. Regarding the shape of the signals in 2D EV
spectroscopy, the 2D spectra report on the correlation between electronic and
vibrational degrees of freedom. This correlation has been investigated in detail with
response function approaches for modelling 2D EV signals of simple model systems
[282, 285]. In case of mixed electronic and vibrational spectroscopies, these types of
correlations can be positive, zero, as well as negative. This is due to the details of
the shape of the potential energy surfaces in ground and excited electronic states
[285]. In general, correlations are determined by spectral elongation of the signal
along the pump and the probe axis, similar to the determination of spectral diffusion
in 2D IR methods (Sect. 3.1.3). The dynamical changes in shapes of 2D EV spectra
then report on the loss of that correlation with increasing population delay.
Top Curr Chem (Z) (2017) 375:86
123
177
Reprinted from the journal
spectral axis (x 3 ) is, as almost always, derived by spectrally dispersing the probe
pulse. Also in this variant, the signal is heterodyne-detected using a local oscillator,
which in most cases is the probe pulse itself, as the reported studies have been
performed in the pump-probe geometry. To map out the full molecular dynamics of
the sample, the population delay (t 2 ) is finally scanned to derive information about
electronic and vibrational relaxation and the correlations between electronic
vibrational frequencies, dynamics of solvent-solute interactions, or the dynamics of
electronic and vibrational correlations. Note that 2D EV spectroscopy depends on
the electronic relaxation times and may, therefore, provide significantly longer
observation windows as compared to 2D VE spectroscopy, what might constitute
and important advantage of the method.
The processes that underlie the signal generation can again be visualized using
energy level diagrams (Fig. 27b and c). Depending on the average photon energy,
the spectral width of the VIS excitation pulse and the vibrational frequency of the
modes of the molecule under study, different vibrational levels in the excited
electronic state (|ei) can contribute to the signal generation processes. Therefore,
multiple diagrams for excited state absorption, (a), have to be considered. In
contrast, generally only a single ground state bleach diagram is active (Fig. 27c),
which is justified as long as the spectral width of the VIS pump pulses does not
allow the excitation multiple vibrational levels in the electronic ground state, i.e. the
generation of vibrational coherences. Moreover, stimulated emission diagrams
between vibrational levels in the excited electronic state may also be considered, but
such contributions have not been observed experimentally so far. Note that only
exemplary pathways for non-rephasing diagrams are shown and complementary
rephasing pathways contribute as well, as demonstrated in more detailed theoretical
descriptions of 2D EV spectroscopy [282, 283].
Also, for 2D EV spectroscopy, the signals depend on the frequency shift of
vibrational transitions in ground and excited electronic states, similar to the 2D VE
methods. However, even in the case of negligible frequency shifts in the two
electronic states, a signal could still be measured if the electronic excitation
promotes the sample to higher-lying (v = 0) vibrational states in |ei, or if the IR
transition dipole moment is different in the two electronic states. Depending on the
sign of the frequency-shift between ground and excited electronic states, excited
state absorption transitions can show up on the high or the low-frequency side of the
ground state bleach signal. Regarding the shape of the signals in 2D EV
spectroscopy, the 2D spectra report on the correlation between electronic and
vibrational degrees of freedom. This correlation has been investigated in detail with
response function approaches for modelling 2D EV signals of simple model systems
[282, 285]. In case of mixed electronic and vibrational spectroscopies, these types of
correlations can be positive, zero, as well as negative. This is due to the details of
the shape of the potential energy surfaces in ground and excited electronic states
[285]. In general, correlations are determined by spectral elongation of the signal
along the pump and the probe axis, similar to the determination of spectral diffusion
in 2D IR methods (Sect. 3.1.3). The dynamical changes in shapes of 2D EV spectra
then report on the loss of that correlation with increasing population delay.
Top Curr Chem (Z) (2017) 375:86
123
177
Reprinted from the journal