triiodide does not absorb light in the visible spectral range at equilibrium, whereas
diiodide possesses a significant extinction coefficient (see Fig. 2). Therefore, the
2DRR signal strength is at least an order of magnitude larger than the four-wave
mixing background produced by the ethanol solvent when the visible pulses are
tuned to wavelengths longer than 460 nm [23]. The experiment can therefore be
conducted without a time-consuming differencing scheme.
Finally, we find that the traditional pump–repump–probe geometry shown in
Fig. 9c is most convenient when the first four field-matter interactions are resonant
with triiodide and the final two are resonant with diiodide (terms 9–12 in Fig. 3)
[65]. The change in transmission of the continuum probe pulse induced by
absorption of the pump and repump pulses is detected in this scheme. Both the
pump and repump pulses must be chopped on a shot-to-shot basis to subtract a large
background in this geometry, which makes this a fairly time consuming approach.
Fortunately, the vibrational resonances of triiodide and diiodide are near 112 cm
-1 ,
so a high density of points is not required. The 2D spectra in our published works
were obtained by sampling a 40 by 40 grid of points with step sizes ranging from 60
to 75 fs [22, 23].
4 Application to the Photodissociation Reaction of Triiodide
Experiments conducted on the photodissociation reaction of triiodide show that it is
indeed possible to isolate the three classes of nonlinearities defined in Sect. 2
[22, 56]. Triiodide is in many ways an ideal system with which to demonstrate such
a decomposition of the response function. As discussed above, the electronic
resonances of the reactant and product are well-separated, and the photodissociation
reaction is faster than or comparable to the vibrational periods of the systems. In
addition, the Franck–Condon active modes in the reactant and product possess
extremely large displacements between ground and excited state potential energy
minima. As a result, the 2DRR response dominates over the portion of the fifthorder nonlinearity associated with vibrational populations.
In Fig. 10, we present 2DRR spectra acquired using each of the three pulse
sequences defined in Fig. 8. The all-UV pulse configuration yields vibrational
resonances of triiodide in both dimensions, whereas the response of only diiodide is
observed with the second pulse sequence. The 2DRR spectra in Fig. 10a, b differ
slightly in both the resonance frequencies and line widths. At equilibrium, the
vibrational mode frequencies of triiodide and diiodide differ by only a few wave
numbers [7]; however, the resonance frequencies of diiodide detected by 2DRR
reflect a highly non-equilibrium distribution of vibrational quanta. For this reason,
the vibrational coherence frequency also depends on the detection wavelength due
to anharmonicity [37]. A 2DRR spectrum in which resonances of triiodide and
diiodide are displayed in separate dimensions is shown in Fig. 10c. The resonances
are slightly off-diagonal at the 500-nm detection wavelength where we obtain the
best signal-to-noise ratio. In agreement with earlier literature [37], we have also
confirmed that the vibrational coherence frequency of diiodide decreases in x 2 as
Top Curr Chem (Z) (2017) 375:87
123
262
Reprinted from the journal
diiodide possesses a significant extinction coefficient (see Fig. 2). Therefore, the
2DRR signal strength is at least an order of magnitude larger than the four-wave
mixing background produced by the ethanol solvent when the visible pulses are
tuned to wavelengths longer than 460 nm [23]. The experiment can therefore be
conducted without a time-consuming differencing scheme.
Finally, we find that the traditional pump–repump–probe geometry shown in
Fig. 9c is most convenient when the first four field-matter interactions are resonant
with triiodide and the final two are resonant with diiodide (terms 9–12 in Fig. 3)
[65]. The change in transmission of the continuum probe pulse induced by
absorption of the pump and repump pulses is detected in this scheme. Both the
pump and repump pulses must be chopped on a shot-to-shot basis to subtract a large
background in this geometry, which makes this a fairly time consuming approach.
Fortunately, the vibrational resonances of triiodide and diiodide are near 112 cm
-1 ,
so a high density of points is not required. The 2D spectra in our published works
were obtained by sampling a 40 by 40 grid of points with step sizes ranging from 60
to 75 fs [22, 23].
4 Application to the Photodissociation Reaction of Triiodide
Experiments conducted on the photodissociation reaction of triiodide show that it is
indeed possible to isolate the three classes of nonlinearities defined in Sect. 2
[22, 56]. Triiodide is in many ways an ideal system with which to demonstrate such
a decomposition of the response function. As discussed above, the electronic
resonances of the reactant and product are well-separated, and the photodissociation
reaction is faster than or comparable to the vibrational periods of the systems. In
addition, the Franck–Condon active modes in the reactant and product possess
extremely large displacements between ground and excited state potential energy
minima. As a result, the 2DRR response dominates over the portion of the fifthorder nonlinearity associated with vibrational populations.
In Fig. 10, we present 2DRR spectra acquired using each of the three pulse
sequences defined in Fig. 8. The all-UV pulse configuration yields vibrational
resonances of triiodide in both dimensions, whereas the response of only diiodide is
observed with the second pulse sequence. The 2DRR spectra in Fig. 10a, b differ
slightly in both the resonance frequencies and line widths. At equilibrium, the
vibrational mode frequencies of triiodide and diiodide differ by only a few wave
numbers [7]; however, the resonance frequencies of diiodide detected by 2DRR
reflect a highly non-equilibrium distribution of vibrational quanta. For this reason,
the vibrational coherence frequency also depends on the detection wavelength due
to anharmonicity [37]. A 2DRR spectrum in which resonances of triiodide and
diiodide are displayed in separate dimensions is shown in Fig. 10c. The resonances
are slightly off-diagonal at the 500-nm detection wavelength where we obtain the
best signal-to-noise ratio. In agreement with earlier literature [37], we have also
confirmed that the vibrational coherence frequency of diiodide decreases in x 2 as
Top Curr Chem (Z) (2017) 375:87
123
262
Reprinted from the journal