1 3
Topics in Current Chemistry (2018) 376:35
difference between VCS and multi-VCS. Indeed, with multi-VCS, vibrational activity may be triggered and probed in a transient state by using laser spectra on resonance with one specific electronic transition of this very state, which may not be
resonant with the ground state absorption. This allows taking advantage of the socalled resonant enhancement for generating a vibrational wavepacket specifically in
this transient state.
For completeness, we note that another implementation of multi-VCS uses what
we call here the actinic pump to specifically trigger a vibrational wavepacket in the
ground state (by ISRS), followed by the subsequent three-pulse VCS scheme to generate and monitor an excited-state vibrational wavepacket from this non-stationary
ground state population. This enables correlating the vibrational activity on the
excited state to that of the ground state in a so-called two-dimensional resonance
Raman (2DRR) spectroscopy. Another chapter in this collection is dedicated to this
technique. Therefore here, we limit the discussion to a scheme where the effect of
the actinic pulse can be simply understood as initiating a photoreaction by populating an excited state.
3.1 Femtosecond Stimulated Raman Scattering (FSRS)
FSRS is a frequency-domain spectroscopy based on stimulated Raman scattering
generated after an actinic pulse (Fig. 3, top panel). The signal is generated in the
same direction as the probe beam, leading to a self-heterodyne detection geometry.
The delay T between the probe pulse and the actinic pulse is scanned and Raman
spectra are detected in dependence of this delay. The first implementation of FSRS
in its three-beam configuration used a 10-Hz laser and had a Raman resolution of
only 76 cm
−1
[45]. After almost 25 years of experimental development, the stateof-the-art FSRS setup nowadays offers a Raman resolution of about 10 cm
−1
while
using actinic pulses with durations of less than 100 fs, and covering the whole ultraviolet and visible spectral range [47–50].
The probe pulse is spectrally broad and usually red-shifted with respect to the
narrow spectrum of the Raman pump. The advantage of a broadband implementation is that a full Raman spectrum, i.e., typically from 200 to 3000 cm
−1
, can be
directly recorded and the high peak intensity of femtosecond pulses enhances the
efficiency of the stimulated Raman process generating vibrational wavepackets [40].
While the actinic pulse is electronic resonant with the ground-state absorption, the
Raman pump-probe pair is usually off-resonant in most experiments. The ability of
tuning the spectrum of the Raman pump-probe pair allows to probe specific electronic transitions, other than those involving the ground state [51]. Thus, all Raman
modes in the excited state or subsequent transient states can be measured, not only
those Franck–Condon active. Being a spectral domain technique where only the
scan of the actinic pulse T delay is required, the acquisition of several spectra is
intrinsically faster than time domain techniques where an additional probe τ delay
must be scanned (Fig. 3). While FSRS has been already demonstrated with a single
laser shot acquisition [40], typical acquisition times of FSRS transients are only a
few minutes [52].
215
Reprinted from the journal
Topics in Current Chemistry (2018) 376:35
difference between VCS and multi-VCS. Indeed, with multi-VCS, vibrational activity may be triggered and probed in a transient state by using laser spectra on resonance with one specific electronic transition of this very state, which may not be
resonant with the ground state absorption. This allows taking advantage of the socalled resonant enhancement for generating a vibrational wavepacket specifically in
this transient state.
For completeness, we note that another implementation of multi-VCS uses what
we call here the actinic pump to specifically trigger a vibrational wavepacket in the
ground state (by ISRS), followed by the subsequent three-pulse VCS scheme to generate and monitor an excited-state vibrational wavepacket from this non-stationary
ground state population. This enables correlating the vibrational activity on the
excited state to that of the ground state in a so-called two-dimensional resonance
Raman (2DRR) spectroscopy. Another chapter in this collection is dedicated to this
technique. Therefore here, we limit the discussion to a scheme where the effect of
the actinic pulse can be simply understood as initiating a photoreaction by populating an excited state.
3.1 Femtosecond Stimulated Raman Scattering (FSRS)
FSRS is a frequency-domain spectroscopy based on stimulated Raman scattering
generated after an actinic pulse (Fig. 3, top panel). The signal is generated in the
same direction as the probe beam, leading to a self-heterodyne detection geometry.
The delay T between the probe pulse and the actinic pulse is scanned and Raman
spectra are detected in dependence of this delay. The first implementation of FSRS
in its three-beam configuration used a 10-Hz laser and had a Raman resolution of
only 76 cm
−1
[45]. After almost 25 years of experimental development, the stateof-the-art FSRS setup nowadays offers a Raman resolution of about 10 cm
−1
while
using actinic pulses with durations of less than 100 fs, and covering the whole ultraviolet and visible spectral range [47–50].
The probe pulse is spectrally broad and usually red-shifted with respect to the
narrow spectrum of the Raman pump. The advantage of a broadband implementation is that a full Raman spectrum, i.e., typically from 200 to 3000 cm
−1
, can be
directly recorded and the high peak intensity of femtosecond pulses enhances the
efficiency of the stimulated Raman process generating vibrational wavepackets [40].
While the actinic pulse is electronic resonant with the ground-state absorption, the
Raman pump-probe pair is usually off-resonant in most experiments. The ability of
tuning the spectrum of the Raman pump-probe pair allows to probe specific electronic transitions, other than those involving the ground state [51]. Thus, all Raman
modes in the excited state or subsequent transient states can be measured, not only
those Franck–Condon active. Being a spectral domain technique where only the
scan of the actinic pulse T delay is required, the acquisition of several spectra is
intrinsically faster than time domain techniques where an additional probe τ delay
must be scanned (Fig. 3). While FSRS has been already demonstrated with a single
laser shot acquisition [40], typical acquisition times of FSRS transients are only a
few minutes [52].
215
Reprinted from the journal