Topics in Current Chemistry (2018) 376:35
1 3
4 Challenges in Multidimensional VCS
A fundamental challenge of multi-VCS in the detection of vibrational coherence
in the electronically excited manifold is the small amount of excited-state population compared to the ground state or other non-actinic activated molecules (like
solvent or buffer molecules). The amount of excited molecules can be changed
by the actinic pulse energy, but in general is well below 20–30% to avoid saturation or other nonlinear effects. Usually, it is the interplay of the involved transition dipole moments (i.e., resonant or non-resonant with the ground state or with
the small excited-state population) and the spectral overlap with optical signals
originated by other states, that will determine whether a VCS signal of a given
electronic state is detectable or not. In practice, very seldom a VCS signal will
contain the vibrational signature of only one single electronic state. The only
exception for that is electronically nonresonant experiments involving only the
electronic ground state (no actinic pump). In spectrally resonant experiments, for
example, the VCS spectrum is often resonant with distinct transitions during the
course of a photoreaction, since the involved transition dipole moments and FC
overlap with higher lying states are continuously changing. This leads inevitably to a modulation of the VCS signal due to a modification of the resonance
enhancement conditions. The overlap of VCS signals originated by different
electronic states is one of several causes for the interpretation challenges in VCS
experiments and has been the motivation of new techniques to disentangle them.
One approach to assist the assignment of a Raman mode to the ground- or
excited-state manifold is directly related to the properties of the oscillatory signal
itself. Ground-state vibrational modes often show a longer-living dynamics than
transient excited states, where the dephasing may be faster, due to e.g., a photoinduced reaction or internal conversion to the ground state [63–64]. In time-domain
techniques, this leads to vibrational wavepackets with shorter dephasing times (or
broader Raman peaks in spectral domain techniques like FSRS) normally associated to excited state modes, while longer dephasing times are taken as coming from the ground state. Of course, longer and shorter are relative quantities,
which can only correctly be interpreted when taking into account the duration
of other vibrational modes and electronic population times of the involved electronic states. An additional approach often used to identify the nature of vibrational wavepackets is to rely on the phase of the oscillatory signal [13, 66–68].
For example, in spectrally dispersed time-resolved experiments, probing at
wavelengths red- and blue-detuned from the center of the absorption spectrum
often leads to oscillatory signals with a π-phase difference due to vibrational
wavepacket dynamics. This information, when combined with the ground- and
excited-state absorption bands, may assist in the assignment of a Raman mode to
the respective electronic state (see e.g. [30]).
Separation and assignment of contributions originating from different excited
states or electronic transitions can be acutely challenging in multi-VCS, especially
when the respective excited-state absorption spectra overlap. However, when they
do not overlap, the assignment can be facilitated by exploiting the tuneability of
220
Reprinted from the journal
1 3
4 Challenges in Multidimensional VCS
A fundamental challenge of multi-VCS in the detection of vibrational coherence
in the electronically excited manifold is the small amount of excited-state population compared to the ground state or other non-actinic activated molecules (like
solvent or buffer molecules). The amount of excited molecules can be changed
by the actinic pulse energy, but in general is well below 20–30% to avoid saturation or other nonlinear effects. Usually, it is the interplay of the involved transition dipole moments (i.e., resonant or non-resonant with the ground state or with
the small excited-state population) and the spectral overlap with optical signals
originated by other states, that will determine whether a VCS signal of a given
electronic state is detectable or not. In practice, very seldom a VCS signal will
contain the vibrational signature of only one single electronic state. The only
exception for that is electronically nonresonant experiments involving only the
electronic ground state (no actinic pump). In spectrally resonant experiments, for
example, the VCS spectrum is often resonant with distinct transitions during the
course of a photoreaction, since the involved transition dipole moments and FC
overlap with higher lying states are continuously changing. This leads inevitably to a modulation of the VCS signal due to a modification of the resonance
enhancement conditions. The overlap of VCS signals originated by different
electronic states is one of several causes for the interpretation challenges in VCS
experiments and has been the motivation of new techniques to disentangle them.
One approach to assist the assignment of a Raman mode to the ground- or
excited-state manifold is directly related to the properties of the oscillatory signal
itself. Ground-state vibrational modes often show a longer-living dynamics than
transient excited states, where the dephasing may be faster, due to e.g., a photoinduced reaction or internal conversion to the ground state [63–64]. In time-domain
techniques, this leads to vibrational wavepackets with shorter dephasing times (or
broader Raman peaks in spectral domain techniques like FSRS) normally associated to excited state modes, while longer dephasing times are taken as coming from the ground state. Of course, longer and shorter are relative quantities,
which can only correctly be interpreted when taking into account the duration
of other vibrational modes and electronic population times of the involved electronic states. An additional approach often used to identify the nature of vibrational wavepackets is to rely on the phase of the oscillatory signal [13, 66–68].
For example, in spectrally dispersed time-resolved experiments, probing at
wavelengths red- and blue-detuned from the center of the absorption spectrum
often leads to oscillatory signals with a π-phase difference due to vibrational
wavepacket dynamics. This information, when combined with the ground- and
excited-state absorption bands, may assist in the assignment of a Raman mode to
the respective electronic state (see e.g. [30]).
Separation and assignment of contributions originating from different excited
states or electronic transitions can be acutely challenging in multi-VCS, especially
when the respective excited-state absorption spectra overlap. However, when they
do not overlap, the assignment can be facilitated by exploiting the tuneability of
220
Reprinted from the journal