The information provided by 2DRR spectroscopy differs from that obtained in a
traditional pump–probe measurement in that the reaction is initiated from a nonequilibrium state. That is, a system is usually at equilibrium before the (single)
pump pulse is absorbed in a traditional pump–probe experiment. In the 2DRR
technique, the chemical reaction is initiated by a second pump pulse, which arrives
while the reactant is undergoing coherent vibrational motions. Such wave packet
motions correspond to Franck–Condon active modes for the optical transition,
whereas the vibrational motions in dimension #2 are Franck–Condon active for the
non-radiative transition (i.e., the chemical reaction). Notably, the most prominent
vibrational modes must also be Franck–Condon active for light absorption of the
product in order for the final laser pulse to induce signal emission. For this reason,
2DRR spectroscopy can be used to distinguish modes that project onto the reaction
coordinate from so-called spectator modes which are Franck–Condon active only
for light absorption. Physically interesting effects are revealed when wave packet
motions along the reaction coordinate in dimension #1 affect properties of the
product in dimension #2. For example, the application to the photodissociation
reaction of triiodide that we focus on in this chapter will show that coherent motion
of the reactant in dimension #1 directly influences the distribution of vibrational
quanta in the product [22, 23].
The extraordinary photodissociation mechanism of triiodide has motivated
numerous ultrafast spectroscopic studies [7, 8, 36–41]. Light absorption in the
ultraviolet spectral range induces photodissociation on a timescale that is shorter
than [7, 37] or comparable to [40, 42] the * 300-fs vibrational period of the
symmetric stretching mode. Therefore, the photodissociation process acts as an
impulse that initiates vibrational coherence in the bond stretching coordinate of the
diiodide product. Earlier work has shown that the oscillatory transient absorption
response of this system reflects symmetry breaking in the excited state [38], whereas
the ‘‘chirp’’ in the waveform of the vibrational coherence represents time evolution
of the bond strength during the reaction [40]. It should be noted that the reaction is
more complex than originally thought; relatively recent work shows that distinct
populations of free solvated diiodide and a contact fragment pair (diiodide and
iodine) are produced by photodissociation [43]. 2DRR spectroscopy is insensitive to
the contact radical pair because its vibrational motion is known to be overdamped
[42].
This chapter is organized as follows. In Sect. 2, we discuss theoretical aspects of
the 2DRR response. Experimental approaches are detailed in Sect. 3. Our
application to the photodissociation reaction of triiodide is then reviewed in Sect.
4. Finally, we conclude by summarizing key findings and discussing future
directions in Sect. 5.
2 2DRR Signal Generation Mechanism for a Photoinduced Reaction
The 2DRR response function possesses a large number of components that can be
selectively enhanced by tuning laser pulses into the electronic resonances of the
reactant and/or product. Reaction mechanisms, line-broadening dynamics, and/or
Top Curr Chem (Z) (2017) 375:87
123
250
Reprinted from the journal
traditional pump–probe measurement in that the reaction is initiated from a nonequilibrium state. That is, a system is usually at equilibrium before the (single)
pump pulse is absorbed in a traditional pump–probe experiment. In the 2DRR
technique, the chemical reaction is initiated by a second pump pulse, which arrives
while the reactant is undergoing coherent vibrational motions. Such wave packet
motions correspond to Franck–Condon active modes for the optical transition,
whereas the vibrational motions in dimension #2 are Franck–Condon active for the
non-radiative transition (i.e., the chemical reaction). Notably, the most prominent
vibrational modes must also be Franck–Condon active for light absorption of the
product in order for the final laser pulse to induce signal emission. For this reason,
2DRR spectroscopy can be used to distinguish modes that project onto the reaction
coordinate from so-called spectator modes which are Franck–Condon active only
for light absorption. Physically interesting effects are revealed when wave packet
motions along the reaction coordinate in dimension #1 affect properties of the
product in dimension #2. For example, the application to the photodissociation
reaction of triiodide that we focus on in this chapter will show that coherent motion
of the reactant in dimension #1 directly influences the distribution of vibrational
quanta in the product [22, 23].
The extraordinary photodissociation mechanism of triiodide has motivated
numerous ultrafast spectroscopic studies [7, 8, 36–41]. Light absorption in the
ultraviolet spectral range induces photodissociation on a timescale that is shorter
than [7, 37] or comparable to [40, 42] the * 300-fs vibrational period of the
symmetric stretching mode. Therefore, the photodissociation process acts as an
impulse that initiates vibrational coherence in the bond stretching coordinate of the
diiodide product. Earlier work has shown that the oscillatory transient absorption
response of this system reflects symmetry breaking in the excited state [38], whereas
the ‘‘chirp’’ in the waveform of the vibrational coherence represents time evolution
of the bond strength during the reaction [40]. It should be noted that the reaction is
more complex than originally thought; relatively recent work shows that distinct
populations of free solvated diiodide and a contact fragment pair (diiodide and
iodine) are produced by photodissociation [43]. 2DRR spectroscopy is insensitive to
the contact radical pair because its vibrational motion is known to be overdamped
[42].
This chapter is organized as follows. In Sect. 2, we discuss theoretical aspects of
the 2DRR response. Experimental approaches are detailed in Sect. 3. Our
application to the photodissociation reaction of triiodide is then reviewed in Sect.
4. Finally, we conclude by summarizing key findings and discussing future
directions in Sect. 5.
2 2DRR Signal Generation Mechanism for a Photoinduced Reaction
The 2DRR response function possesses a large number of components that can be
selectively enhanced by tuning laser pulses into the electronic resonances of the
reactant and/or product. Reaction mechanisms, line-broadening dynamics, and/or
Top Curr Chem (Z) (2017) 375:87
123
250
Reprinted from the journal