displacement consistent with triiodide (d = 7.0). Reasons for this were discussed in
Ref. [22]. The dependence on d is important to consider because Franck–Condon
active modes in many larger molecules possess displacements that are much less
than 1.0. Fortunately, the ratio, |E cas (x 1 , x 2 )|/|E
(5) (x 1 , x 2 )|, also decreases linearly
with the signal emission frequency.
The calculation in Fig. 7 employs a fairly large 1-mM concentration and deep
ultraviolet detection wavelength of 267 nm. It is not necessary to carefully tune
experimental parameters in triiodide because of its large displacement, d (Fig. 7c
shows that the direct response dominates for displacements [ 2). Larger polyatomic
molecules generally possess much smaller displacements (d \ 1). Fortunately, it is
usually possible to detect signal emission at longer wavelengths for larger
molecules, particularly for conjugated systems with delocalized excitations. For
example, control experiments and model calculations show contributions from
cascades to be negligible in 2DRR experiments conducted on myoglobin with a 0.2mM concentration and 400-nm detection [24]. Harel and co-workers similarly found
negligible contributions from cascades in a related 2D Raman experiment in which
one of the pulses is pre-resonant with an electronic transition [34]. Our model
calculations suggest that the cascaded signal intensity can reach and potentially
exceed 10% of the total signal strength in a three-beam geometry with small
crossing angles. For this reason, we recommend conducting control experiments
based on the signal phase and sample concentration to rule out cascades when new
systems are studied with all fifth-order Raman techniques (e.g., FSRS, RP-D4WM).
Specialized beam geometries can be employed for challenging cases [53].
3 Experimental Methods
Approaches that we have used to conduct 2DRR spectroscopy differ in the
bandwidths, frequencies, and geometries of the laser beams [22, 24, 56]. In this
section, we describe how these aspects of a 2DRR pulse sequence can be used to
selectively detect the three signal components defined in Sect. 2.1.
3.1 Pulse Sequences
In Fig. 8, we present pulse sequences that have been used to isolate each of the three
signal components discussed in Sect. 2.1. The nonlinearity associated with terms
1–4 can be probed with a degenerate six-wave mixing configuration in which all
pulses are resonant with the triiodide reactant. A resonant pump degenerate fourwave mixing approach is used to enhance signal components corresponding to terms
5–8. Finally, we find that terms 9–12 are most conveniently detected with a
traditional pump–repump–probe setup. It should be noted that the fifth-order 2DRR
nonlinearity is detected in each case even though the approaches differ in the
number of laser pulses (i.e., more than one field-matter interaction can occur with
each pulse).
Suppression of third-order cascades in electronically off-resonant 2D Raman
experiments was achieved using carefully designed laser beam geometries in which
Top Curr Chem (Z) (2017) 375:87
123
259
Reprinted from the journal
Ref. [22]. The dependence on d is important to consider because Franck–Condon
active modes in many larger molecules possess displacements that are much less
than 1.0. Fortunately, the ratio, |E cas (x 1 , x 2 )|/|E
(5) (x 1 , x 2 )|, also decreases linearly
with the signal emission frequency.
The calculation in Fig. 7 employs a fairly large 1-mM concentration and deep
ultraviolet detection wavelength of 267 nm. It is not necessary to carefully tune
experimental parameters in triiodide because of its large displacement, d (Fig. 7c
shows that the direct response dominates for displacements [ 2). Larger polyatomic
molecules generally possess much smaller displacements (d \ 1). Fortunately, it is
usually possible to detect signal emission at longer wavelengths for larger
molecules, particularly for conjugated systems with delocalized excitations. For
example, control experiments and model calculations show contributions from
cascades to be negligible in 2DRR experiments conducted on myoglobin with a 0.2mM concentration and 400-nm detection [24]. Harel and co-workers similarly found
negligible contributions from cascades in a related 2D Raman experiment in which
one of the pulses is pre-resonant with an electronic transition [34]. Our model
calculations suggest that the cascaded signal intensity can reach and potentially
exceed 10% of the total signal strength in a three-beam geometry with small
crossing angles. For this reason, we recommend conducting control experiments
based on the signal phase and sample concentration to rule out cascades when new
systems are studied with all fifth-order Raman techniques (e.g., FSRS, RP-D4WM).
Specialized beam geometries can be employed for challenging cases [53].
3 Experimental Methods
Approaches that we have used to conduct 2DRR spectroscopy differ in the
bandwidths, frequencies, and geometries of the laser beams [22, 24, 56]. In this
section, we describe how these aspects of a 2DRR pulse sequence can be used to
selectively detect the three signal components defined in Sect. 2.1.
3.1 Pulse Sequences
In Fig. 8, we present pulse sequences that have been used to isolate each of the three
signal components discussed in Sect. 2.1. The nonlinearity associated with terms
1–4 can be probed with a degenerate six-wave mixing configuration in which all
pulses are resonant with the triiodide reactant. A resonant pump degenerate fourwave mixing approach is used to enhance signal components corresponding to terms
5–8. Finally, we find that terms 9–12 are most conveniently detected with a
traditional pump–repump–probe setup. It should be noted that the fifth-order 2DRR
nonlinearity is detected in each case even though the approaches differ in the
number of laser pulses (i.e., more than one field-matter interaction can occur with
each pulse).
Suppression of third-order cascades in electronically off-resonant 2D Raman
experiments was achieved using carefully designed laser beam geometries in which
Top Curr Chem (Z) (2017) 375:87
123
259
Reprinted from the journal