1 3
Topics in Current Chemistry (2018) 376:35
depletion of electronic population. PC-pump-IVS has been demonstrated with dump
pulses with a duration of 200 fs (i.e. about 75 cm
−1
FWHM spectral width) [77].
5 Application of Multidimensional VCS
5.1 How can Multidimensional VCS Help a Chemist?
The most natural way of describing how multi-VCS can assist a chemist is by comparing it to Raman spectroscopy. In its essence, multi-VCS delivers a sequence of
Raman spectra obtained after an actinic pulse interacts with the sample. In Raman
spectroscopy, structural and interaction information comes from the energy, width,
and amplitude of Raman bands, which assist in the identification of specific molecular species and conformations. Multi-VCS provides the same information and goes
beyond by measuring how Raman frequencies shift and amplitudes evolve in time
(for example, Refs. [88–89]). Hence, it reveals time-resolved structural information
on transient molecular species along the course of a photoreaction. In this regard,
multi-VCS follows the same conceptual approach as transient spontaneous Raman
measurements with picosecond pulses [91–93] pioneered by Lauberau et al. more
than four decades ago [94]. Multi-VCS differs from transient picosecond Raman
spectroscopy, however, in many aspects. The first one is the signal intensity of
the spontaneous Raman signal detected in the transient picosecond Raman spectroscopy, which is intrinsically much weaker than the coherent/stimulated signal
detected in multi-VCS methods. The second one is related to the Fourier relation
between the spectral resolution and pulse duration. Transient picosecond Raman
spectroscopy is limited to molecular processes much slower than picoseconds due to
the intrinsic time duration of the narrow-band Raman pulse, if a spectral resolution
of few wavenumbers is desired. For example, a 1-ps Raman probe pulse leads to an
intrinsic band broadening of more than 10 cm
−1
. It is also interesting to note that the
vibrational content in multi-VCS can be very similar to the one detected by transient
infrared absorption spectroscopies, in particular for complex systems where symmetry rules are relaxed and vibrations can be probed by Raman as well as infrared
interactions.
Generally, following how the frequency of Raman bands change in dependence
of the actinic pulse time delay T gives information on how bond strengths are modified during a photochemical reaction [37, 89, 95]. Frequency blue-shifts indicate
stiffening of the vibrational motion or relaxation within an anharmonic potential,
while red shifts may hint at elongation of a given chemical bond. On the other hand,
the lack of any shift or amplitude changes is usually taken as the signature of nonreactive coordinates. The changes in the amplitude of Raman bands in multi-VCS
report on the formation and breaking of chemical bonds after interaction with the
actinic pulse. When correlated with frequencies changes, they become the central
feature in the mapping of structural changes during photochemical reactions. Moreover, the evolution of the amplitude of a band does not need to follow a first-order
kinetics and decay or grow exponentially, it can also show very complex dynamics. For example, modulation of the amplitude of a Raman band with the frequency
225
Reprinted from the journal
Topics in Current Chemistry (2018) 376:35
depletion of electronic population. PC-pump-IVS has been demonstrated with dump
pulses with a duration of 200 fs (i.e. about 75 cm
−1
FWHM spectral width) [77].
5 Application of Multidimensional VCS
5.1 How can Multidimensional VCS Help a Chemist?
The most natural way of describing how multi-VCS can assist a chemist is by comparing it to Raman spectroscopy. In its essence, multi-VCS delivers a sequence of
Raman spectra obtained after an actinic pulse interacts with the sample. In Raman
spectroscopy, structural and interaction information comes from the energy, width,
and amplitude of Raman bands, which assist in the identification of specific molecular species and conformations. Multi-VCS provides the same information and goes
beyond by measuring how Raman frequencies shift and amplitudes evolve in time
(for example, Refs. [88–89]). Hence, it reveals time-resolved structural information
on transient molecular species along the course of a photoreaction. In this regard,
multi-VCS follows the same conceptual approach as transient spontaneous Raman
measurements with picosecond pulses [91–93] pioneered by Lauberau et al. more
than four decades ago [94]. Multi-VCS differs from transient picosecond Raman
spectroscopy, however, in many aspects. The first one is the signal intensity of
the spontaneous Raman signal detected in the transient picosecond Raman spectroscopy, which is intrinsically much weaker than the coherent/stimulated signal
detected in multi-VCS methods. The second one is related to the Fourier relation
between the spectral resolution and pulse duration. Transient picosecond Raman
spectroscopy is limited to molecular processes much slower than picoseconds due to
the intrinsic time duration of the narrow-band Raman pulse, if a spectral resolution
of few wavenumbers is desired. For example, a 1-ps Raman probe pulse leads to an
intrinsic band broadening of more than 10 cm
−1
. It is also interesting to note that the
vibrational content in multi-VCS can be very similar to the one detected by transient
infrared absorption spectroscopies, in particular for complex systems where symmetry rules are relaxed and vibrations can be probed by Raman as well as infrared
interactions.
Generally, following how the frequency of Raman bands change in dependence
of the actinic pulse time delay T gives information on how bond strengths are modified during a photochemical reaction [37, 89, 95]. Frequency blue-shifts indicate
stiffening of the vibrational motion or relaxation within an anharmonic potential,
while red shifts may hint at elongation of a given chemical bond. On the other hand,
the lack of any shift or amplitude changes is usually taken as the signature of nonreactive coordinates. The changes in the amplitude of Raman bands in multi-VCS
report on the formation and breaking of chemical bonds after interaction with the
actinic pulse. When correlated with frequencies changes, they become the central
feature in the mapping of structural changes during photochemical reactions. Moreover, the evolution of the amplitude of a band does not need to follow a first-order
kinetics and decay or grow exponentially, it can also show very complex dynamics. For example, modulation of the amplitude of a Raman band with the frequency
225
Reprinted from the journal