1 3
Topics in Current Chemistry (2018) 376:35
experimentally observed which may overlap with the vibrational coherences of interest [73–75]. For example, interference beats originate from the interference between
vibrational modes of two different electronic states, and are an intrinsic beat of electronically resonant techniques, since vibrational coherence will be excited on both
electronic states involved. Polarization beats originate from interference between
nonlinear polarization contributions at the detector and are thus a pure optical effect.
The origin of the polarization beating can be e.g., the emission, by different kinds of
molecules, of polarizations oscillating at different vibrational frequencies and interfering at the detector. We refer the reader to references [20, 31] for more technical
details.
In general, interference between oscillatory signals resulting in additional low- or
high-frequency oscillations hampers the correct interpretation of VCS experiments.
Polarization beating, for example, is of special importance when the low-frequency
region (< 800 cm
−1
) in FFT spectra of transient dynamics is interpreted [20]. In the
spectral domain, band positions of molecular normal modes and signal contributions
from polarization beating between molecular high-frequency contributions can overlap, hampering an unambiguous interpretation and assignment. Polarization beating
is an interference effect present in all types of VCS methods, with homodyne and, to
a lesser degree, (self-)heterodyne detection. The discrimination between molecular
normal modes and polarization beating is possible in techniques where the vibrational wavepacket generation originates from two different pulses like DFWM and
CARS, where the delay τ 12 between pump and Stokes pulses can be exploited. The
separation of different response pathways has been demonstrated for several polyatomic molecules in solution using chirped, spectrally resolved DFWM (Fig. 10)
[20]. Two organic dye molecules, rhodamine B and S-9, show strong oscillatory
beating centered at τ 12 = 0 when no chirp is applied (Fig. 10a, c, respectively). When
chirp is applied, the maxima of interference contributions are shifted with a given
amount depending on the chirp applied, while beating due to vibrational wavepackets are not shifted with the delay τ 12 between pump and Stokes pulses (Fig. 10b, d).
Finally, interference beating can be also addressed by applying higher-order nonlinear techniques. Population-controlled pump-IVS is an extension of pump-IVS to
suppress undesired vibrational coherence contributions to the optical signal (Fig. 3,
bottom panel) [77–78]. It is based on the same three-beam geometry to excite
impulsively Raman vibrations, but adds an additional pulse named “dump” to interact with the sample between the pump and the probe pulses, formally resulting in
a seventh-order nonlinear time-resolved method. PC-pump-IVS follows the same
approach as pump-depletion-probe experiments in visible [80–84] and infrared [85,
86] to disentangle population relaxation pathways, and applies to the depletion of
the vibrational coherence in the excited state manifold. In PC-pump-IVS, the interaction of the narrowband dump pulse has the effect of only changing the population
of the excited state and decreasing the respective Raman signal. By measuring the
transients along the τ delay with different combination of pulses (Fig. 11a), it is possible to subtract the contribution of the ground-state and solvent vibrational coherence (Fig. 11b). A key aspect of this technique is the bandwidth of the dump pulse:
The spectrally narrow dump pulse with a pulse duration of few hundreds of femtoseconds is not able to induce any additional vibrational coherence, leading to a pure
223
Reprinted from the journal
Topics in Current Chemistry (2018) 376:35
experimentally observed which may overlap with the vibrational coherences of interest [73–75]. For example, interference beats originate from the interference between
vibrational modes of two different electronic states, and are an intrinsic beat of electronically resonant techniques, since vibrational coherence will be excited on both
electronic states involved. Polarization beats originate from interference between
nonlinear polarization contributions at the detector and are thus a pure optical effect.
The origin of the polarization beating can be e.g., the emission, by different kinds of
molecules, of polarizations oscillating at different vibrational frequencies and interfering at the detector. We refer the reader to references [20, 31] for more technical
details.
In general, interference between oscillatory signals resulting in additional low- or
high-frequency oscillations hampers the correct interpretation of VCS experiments.
Polarization beating, for example, is of special importance when the low-frequency
region (< 800 cm
−1
) in FFT spectra of transient dynamics is interpreted [20]. In the
spectral domain, band positions of molecular normal modes and signal contributions
from polarization beating between molecular high-frequency contributions can overlap, hampering an unambiguous interpretation and assignment. Polarization beating
is an interference effect present in all types of VCS methods, with homodyne and, to
a lesser degree, (self-)heterodyne detection. The discrimination between molecular
normal modes and polarization beating is possible in techniques where the vibrational wavepacket generation originates from two different pulses like DFWM and
CARS, where the delay τ 12 between pump and Stokes pulses can be exploited. The
separation of different response pathways has been demonstrated for several polyatomic molecules in solution using chirped, spectrally resolved DFWM (Fig. 10)
[20]. Two organic dye molecules, rhodamine B and S-9, show strong oscillatory
beating centered at τ 12 = 0 when no chirp is applied (Fig. 10a, c, respectively). When
chirp is applied, the maxima of interference contributions are shifted with a given
amount depending on the chirp applied, while beating due to vibrational wavepackets are not shifted with the delay τ 12 between pump and Stokes pulses (Fig. 10b, d).
Finally, interference beating can be also addressed by applying higher-order nonlinear techniques. Population-controlled pump-IVS is an extension of pump-IVS to
suppress undesired vibrational coherence contributions to the optical signal (Fig. 3,
bottom panel) [77–78]. It is based on the same three-beam geometry to excite
impulsively Raman vibrations, but adds an additional pulse named “dump” to interact with the sample between the pump and the probe pulses, formally resulting in
a seventh-order nonlinear time-resolved method. PC-pump-IVS follows the same
approach as pump-depletion-probe experiments in visible [80–84] and infrared [85,
86] to disentangle population relaxation pathways, and applies to the depletion of
the vibrational coherence in the excited state manifold. In PC-pump-IVS, the interaction of the narrowband dump pulse has the effect of only changing the population
of the excited state and decreasing the respective Raman signal. By measuring the
transients along the τ delay with different combination of pulses (Fig. 11a), it is possible to subtract the contribution of the ground-state and solvent vibrational coherence (Fig. 11b). A key aspect of this technique is the bandwidth of the dump pulse:
The spectrally narrow dump pulse with a pulse duration of few hundreds of femtoseconds is not able to induce any additional vibrational coherence, leading to a pure
223
Reprinted from the journal