1 3
Topics in Current Chemistry (2018) 376:35
a result, the pump-induced vibrational coherence is a vibrational wavepacket in
the S 1 electronic state in the first case, or in S 0 in the latter case. Since these two
cases correspond to two terms of the perturbative expansion at the same order, a
short-enough laser pump pulse will in general produce vibrational wavepackets
both in the ground and excited states [13–15]. Particular chirp (i.e., time-ordering of the spectral components within the short pump pulse) may be engineered
to favor S 1 or S 0 wavepacket formation [16–21]. The impulsive formation of a
ground-state wavepacket upon interaction with an ultrashort resonant laser pulse
is a Raman process, named “resonant impulsive stimulated Raman scattering”
RISRS [22, 23]. It also operates with a non-resonant laser pulse (ISRS) [24].
In the sequential pump-probe scheme discussed here (i.e., the probe pulse does
not overlap temporally with the pump pulse), the first two interactions with the
pump field are followed, after a “waiting” time τ, by a third interaction with the
probe field, as illustrated in Fig. 1c. The latter couples (i.e., creates a “coherence”
between) the pump-induced population and another electronic state. The resulting
(third-order) polarization of the molecular system radiates a field that builds up constructively in a fourth wave (to be detected), which propagates in a specific direction k 1 − k 2 + k 3 imposed by the phase matching condition [25]. In the present case,
where the first two interactions occur with the same pump pulse k 1 = k 2 , the fourth
wave propagates collinearly with the probe pulse k 3 . Hence, one detects the co-propagating third and fourth waves, which means detecting the fourth wave heterodyned
by the incident probe field. Finally, this experiment may be described as the measurement of the linear absorbance of the probe beam (i.e., first order in the probe
field), by the non-stationary population induced (at the second order of perturbation)
by the pump [26].
The vibrational wavepackets produced by the quantum superposition of vibrational states results in the classical oscillation of the vibrational degrees of freedom (bonds elongations, torsions, etc.). These oscillatory molecular motions induce
an oscillatory modulation of all the linear optical properties of the system such as
absorbance, dichroism, birefringence, etc. [15, 22]. Hence, the pump-induced change
in the spectrum or polarization state of the transmitted probe oscillates accordingly
as a function of the waiting time τ. The sequential pump-probe experiment described
here and performed with ultrashort laser pulses, so as to generate and probe vibrational wavepackets both in the ground and excited states, is referred to as impulsive vibrational spectroscopy (IVS). Fourier transformation of the oscillatory signals
reveals the Raman activity of the system. For a ground-state wavepacket—RISRS or
ISRS—the resulting vibrational spectrum coincides with the molecular Raman spectrum as measured directly in the frequency domain by conventional Raman spectroscopy [27–29]. The limited bandwidth of the laser pulses may, however, result in
the attenuation of the relative intensity of the highest-frequency vibrational modes
or even prevent from detecting them. In practice, 10–12 fs pulses are short enough
to trigger and detect vibrational activities up to the 3000 cm
−1
range. Hence, VCS of
ground-state wavepackets is a time-domain equivalent of frequency-domain Raman
spectroscopy. For excited-state wavepackets, time–frequency representations of
mode-specific coherent oscillations was recently proposed as a spectroscopic tool
for detecting CInt’s [30].
211
Reprinted from the journal
Topics in Current Chemistry (2018) 376:35
a result, the pump-induced vibrational coherence is a vibrational wavepacket in
the S 1 electronic state in the first case, or in S 0 in the latter case. Since these two
cases correspond to two terms of the perturbative expansion at the same order, a
short-enough laser pump pulse will in general produce vibrational wavepackets
both in the ground and excited states [13–15]. Particular chirp (i.e., time-ordering of the spectral components within the short pump pulse) may be engineered
to favor S 1 or S 0 wavepacket formation [16–21]. The impulsive formation of a
ground-state wavepacket upon interaction with an ultrashort resonant laser pulse
is a Raman process, named “resonant impulsive stimulated Raman scattering”
RISRS [22, 23]. It also operates with a non-resonant laser pulse (ISRS) [24].
In the sequential pump-probe scheme discussed here (i.e., the probe pulse does
not overlap temporally with the pump pulse), the first two interactions with the
pump field are followed, after a “waiting” time τ, by a third interaction with the
probe field, as illustrated in Fig. 1c. The latter couples (i.e., creates a “coherence”
between) the pump-induced population and another electronic state. The resulting
(third-order) polarization of the molecular system radiates a field that builds up constructively in a fourth wave (to be detected), which propagates in a specific direction k 1 − k 2 + k 3 imposed by the phase matching condition [25]. In the present case,
where the first two interactions occur with the same pump pulse k 1 = k 2 , the fourth
wave propagates collinearly with the probe pulse k 3 . Hence, one detects the co-propagating third and fourth waves, which means detecting the fourth wave heterodyned
by the incident probe field. Finally, this experiment may be described as the measurement of the linear absorbance of the probe beam (i.e., first order in the probe
field), by the non-stationary population induced (at the second order of perturbation)
by the pump [26].
The vibrational wavepackets produced by the quantum superposition of vibrational states results in the classical oscillation of the vibrational degrees of freedom (bonds elongations, torsions, etc.). These oscillatory molecular motions induce
an oscillatory modulation of all the linear optical properties of the system such as
absorbance, dichroism, birefringence, etc. [15, 22]. Hence, the pump-induced change
in the spectrum or polarization state of the transmitted probe oscillates accordingly
as a function of the waiting time τ. The sequential pump-probe experiment described
here and performed with ultrashort laser pulses, so as to generate and probe vibrational wavepackets both in the ground and excited states, is referred to as impulsive vibrational spectroscopy (IVS). Fourier transformation of the oscillatory signals
reveals the Raman activity of the system. For a ground-state wavepacket—RISRS or
ISRS—the resulting vibrational spectrum coincides with the molecular Raman spectrum as measured directly in the frequency domain by conventional Raman spectroscopy [27–29]. The limited bandwidth of the laser pulses may, however, result in
the attenuation of the relative intensity of the highest-frequency vibrational modes
or even prevent from detecting them. In practice, 10–12 fs pulses are short enough
to trigger and detect vibrational activities up to the 3000 cm
−1
range. Hence, VCS of
ground-state wavepackets is a time-domain equivalent of frequency-domain Raman
spectroscopy. For excited-state wavepackets, time–frequency representations of
mode-specific coherent oscillations was recently proposed as a spectroscopic tool
for detecting CInt’s [30].
211
Reprinted from the journal