1 3
Topics in Current Chemistry (2018) 376:35
domain the spectroscopic signatures of vibrational wavepackets in both the ground
and excited state, [34] thus providing, in principle, the same spectroscopic information as IVS does in the time domain. In this frequency-domain VCS approach,
the Raman pump-pulse duration is the observation window which limits the achievable spectral resolution, in the same way as the time window considered for Fourier
transformation with respect to the waiting time τ limits the spectral resolution of
Raman spectra recorded in the time domain.
A fundamental difference between time-domain and frequency-domain VCS is
that in the latter case, the three light–molecule interactions are no longer engineered
sequentially as in IVS or DFWM, but the interaction with the probe wave may occur
at any moment before, between or after both interactions with the pump pulse. In
fact, these various time-ordering options correspond to distinct terms in the thirdorder perturbative expansion, and all of them contribute to the third-order polarization [35] while in the sequential scheme by experimental design, only those where
the probe interaction is the last one contribute. Hence, a drawback of the frequencydomain VCS is the background signal generated by contributions other than the
stimulated Raman signal of interest. The features and challenges of each technique
will be presented in more detail in the next section.
3 Multidimensional Vibrational Coherence Spectroscopy (Multi‑VCS)
All the VCS approaches introduced above engineer a third-order light–molecule
interaction which simultaneously reveals the vibrational activity of both electronic
states coupled by a resonant pump laser field, i.e., the ground and the Franck–Condon excited state. This may pose a challenge to distinguish between vibrational
signatures specific of each electronic state. Above all, this does not generally give
access to the vibrational signatures of other possible transient states produced along
the course of a photoreaction. In this regard, VCS has been further developed to be
sensitive specifically to the excited states and to successive transient states by adding
an “actinic” pulse. The role of this actinic pulse is to trigger a photoreaction prior to
generating and probing a vibrational wavepacket by a subsequent third-order VCS
scheme applied after a given time delay T. The vibrational activity can thus be monitored along the successive structures and electronic states achieved by the molecular system during the course of its photoreaction. This leads to the multidimensional
character of the VCS, where usually one axis displays Raman frequencies while the
other axis shows the photoreaction time delay T.
Several implementations of multidimensional VCS (multi-VCS) have been proposed, called “transient CARS” [36], “pump-IVS” [37], “pump-DFWM” [38, 39]
and, in the spectral domain, femtosecond stimulated Raman spectroscopy (FSRS)
[40], ultrafast Raman loss spectroscopy (URLS) [42–43]. Some of these are schematically introduced in Fig. 3. Here, it is important to mention a potential problem
in the semantics of the word “pump”: The actinic pulse has received different names
by different groups and techniques along all years, e.g., “pump”, “initial pump”,
or simply “excitation pulse”. The word “pump” in front of the techniques names,
e.g., “pump-IVS” or “pump-DFWM” denotes the actinic pulse which triggers the
213
Reprinted from the journal
Topics in Current Chemistry (2018) 376:35
domain the spectroscopic signatures of vibrational wavepackets in both the ground
and excited state, [34] thus providing, in principle, the same spectroscopic information as IVS does in the time domain. In this frequency-domain VCS approach,
the Raman pump-pulse duration is the observation window which limits the achievable spectral resolution, in the same way as the time window considered for Fourier
transformation with respect to the waiting time τ limits the spectral resolution of
Raman spectra recorded in the time domain.
A fundamental difference between time-domain and frequency-domain VCS is
that in the latter case, the three light–molecule interactions are no longer engineered
sequentially as in IVS or DFWM, but the interaction with the probe wave may occur
at any moment before, between or after both interactions with the pump pulse. In
fact, these various time-ordering options correspond to distinct terms in the thirdorder perturbative expansion, and all of them contribute to the third-order polarization [35] while in the sequential scheme by experimental design, only those where
the probe interaction is the last one contribute. Hence, a drawback of the frequencydomain VCS is the background signal generated by contributions other than the
stimulated Raman signal of interest. The features and challenges of each technique
will be presented in more detail in the next section.
3 Multidimensional Vibrational Coherence Spectroscopy (Multi‑VCS)
All the VCS approaches introduced above engineer a third-order light–molecule
interaction which simultaneously reveals the vibrational activity of both electronic
states coupled by a resonant pump laser field, i.e., the ground and the Franck–Condon excited state. This may pose a challenge to distinguish between vibrational
signatures specific of each electronic state. Above all, this does not generally give
access to the vibrational signatures of other possible transient states produced along
the course of a photoreaction. In this regard, VCS has been further developed to be
sensitive specifically to the excited states and to successive transient states by adding
an “actinic” pulse. The role of this actinic pulse is to trigger a photoreaction prior to
generating and probing a vibrational wavepacket by a subsequent third-order VCS
scheme applied after a given time delay T. The vibrational activity can thus be monitored along the successive structures and electronic states achieved by the molecular system during the course of its photoreaction. This leads to the multidimensional
character of the VCS, where usually one axis displays Raman frequencies while the
other axis shows the photoreaction time delay T.
Several implementations of multidimensional VCS (multi-VCS) have been proposed, called “transient CARS” [36], “pump-IVS” [37], “pump-DFWM” [38, 39]
and, in the spectral domain, femtosecond stimulated Raman spectroscopy (FSRS)
[40], ultrafast Raman loss spectroscopy (URLS) [42–43]. Some of these are schematically introduced in Fig. 3. Here, it is important to mention a potential problem
in the semantics of the word “pump”: The actinic pulse has received different names
by different groups and techniques along all years, e.g., “pump”, “initial pump”,
or simply “excitation pulse”. The word “pump” in front of the techniques names,
e.g., “pump-IVS” or “pump-DFWM” denotes the actinic pulse which triggers the
213
Reprinted from the journal