1 3
Topics in Current Chemistry (2018) 376:35
contributions in FSRS (Fig. 5b) [54]. The intensity modulation of the spectrally
narrow Raman pulse has been identified as a major artifact in FSRS, often requiring ad hoc scaling of baselines (see e.g., Fig. 5a FSRS-baseline fit). This artifact
can be corrected by a factor numerically calculated by including transient absorption changes measured under similar experimental intensities [55]. More recently,
the use of carefully crafted Raman spectra in form of watermarks has been used to
easily identify Raman resonances in the raw signal [56]. The last word in the correction of the baseline distortions in FSRS has not been spoken yet and it is an active
research focus in the field of multidimensional VCS.
3.2 Pump‑IVS
Pump-IVS is based on the combination of impulsive stimulated Raman scattering
(ISRS) with an actinic excitation. As FSRS, it can be easily implemented since the
signal is generated in the direction of the probe beam. However, it requires scanning
two time delays, namely the photoreaction time T between the actinic pulse and the
“repump” pulse (or Raman pump, or even impulsive pump, simply called “pump”
hereafter), and the waiting time τ between the pump and probe pulse, during which
the wavepacket dynamics evolve. Typical acquisition times for pump-IVS are of
several tens of minutes for laser systems with kHz repetition rates. The acquired
data must then be post-processed by Fourier transformation along τ to reveal the
T-dependent impulsive Raman spectrum. The spectral range of the Raman spectrum depends exclusively on the bandwidth of the pump spectrum and the length of
the transients measured along the τ delay. The pump bandwidth limits the highest
upper Raman frequency and the length of the τ scanning interval defines the lowest
detectable Raman frequency. Raman spectra from as low as few tens of wavenumbers up to 3000 cm
−1
have been demonstrated with pump-IVS [19, 57, 58]. Detection of very low frequency Raman modes (< 200 cm
−1
) is another central advantage
of time-domain methods in comparison with spectral domain methods like FSRS,
which are constrained by optical filters to spectrally cut the Raman pump from the
Raman spectrum. Moreover, since the probe spectrum in pump-IVS is normally
spectrally resolved via a grating spectrometer, Raman spectra can be obtained at several detection wavelengths, depending only on the bandwidth of the probe spectrum.
Experimentally, pump-IVS setups are based on non-collinear optical parametric
amplifiers (nc-OPA) [59, 60] to generate the broadband spectra of the actinic and
pump pulses. Pulse durations below 10 fs have been successfully used [19, 57, 58].
The probe pulse is usually a white light supercontinuum with a very broad spectrum
spanning from about 300 nm up to 750 nm or more (depending on the crystal used
for white light generation). The pump spectrum is usually spectrally resonant with
excited-state absorption bands. Typically, the transients along the τ delay are measured for different T delays with respect to actinic pulse with and without the pump
pulse by using a chopper (Fig. 6). This assists in the separation of the transients
originated in the excited-state manifold from the ground-state or solvent contributions (induced by the actinic pulse alone). The extraction of the excited-state signal
induced by the VCS pump pulse can be easily obtained if the pump spectrum is
217
Reprinted from the journal
Topics in Current Chemistry (2018) 376:35
contributions in FSRS (Fig. 5b) [54]. The intensity modulation of the spectrally
narrow Raman pulse has been identified as a major artifact in FSRS, often requiring ad hoc scaling of baselines (see e.g., Fig. 5a FSRS-baseline fit). This artifact
can be corrected by a factor numerically calculated by including transient absorption changes measured under similar experimental intensities [55]. More recently,
the use of carefully crafted Raman spectra in form of watermarks has been used to
easily identify Raman resonances in the raw signal [56]. The last word in the correction of the baseline distortions in FSRS has not been spoken yet and it is an active
research focus in the field of multidimensional VCS.
3.2 Pump‑IVS
Pump-IVS is based on the combination of impulsive stimulated Raman scattering
(ISRS) with an actinic excitation. As FSRS, it can be easily implemented since the
signal is generated in the direction of the probe beam. However, it requires scanning
two time delays, namely the photoreaction time T between the actinic pulse and the
“repump” pulse (or Raman pump, or even impulsive pump, simply called “pump”
hereafter), and the waiting time τ between the pump and probe pulse, during which
the wavepacket dynamics evolve. Typical acquisition times for pump-IVS are of
several tens of minutes for laser systems with kHz repetition rates. The acquired
data must then be post-processed by Fourier transformation along τ to reveal the
T-dependent impulsive Raman spectrum. The spectral range of the Raman spectrum depends exclusively on the bandwidth of the pump spectrum and the length of
the transients measured along the τ delay. The pump bandwidth limits the highest
upper Raman frequency and the length of the τ scanning interval defines the lowest
detectable Raman frequency. Raman spectra from as low as few tens of wavenumbers up to 3000 cm
−1
have been demonstrated with pump-IVS [19, 57, 58]. Detection of very low frequency Raman modes (< 200 cm
−1
) is another central advantage
of time-domain methods in comparison with spectral domain methods like FSRS,
which are constrained by optical filters to spectrally cut the Raman pump from the
Raman spectrum. Moreover, since the probe spectrum in pump-IVS is normally
spectrally resolved via a grating spectrometer, Raman spectra can be obtained at several detection wavelengths, depending only on the bandwidth of the probe spectrum.
Experimentally, pump-IVS setups are based on non-collinear optical parametric
amplifiers (nc-OPA) [59, 60] to generate the broadband spectra of the actinic and
pump pulses. Pulse durations below 10 fs have been successfully used [19, 57, 58].
The probe pulse is usually a white light supercontinuum with a very broad spectrum
spanning from about 300 nm up to 750 nm or more (depending on the crystal used
for white light generation). The pump spectrum is usually spectrally resonant with
excited-state absorption bands. Typically, the transients along the τ delay are measured for different T delays with respect to actinic pulse with and without the pump
pulse by using a chopper (Fig. 6). This assists in the separation of the transients
originated in the excited-state manifold from the ground-state or solvent contributions (induced by the actinic pulse alone). The extraction of the excited-state signal
induced by the VCS pump pulse can be easily obtained if the pump spectrum is
217
Reprinted from the journal