1 Introduction
Time coincidence between chemical reactions and relaxation processes such as
solvation and vibrational dephasing gives rise to interesting physical effects.
Assumptions made in traditional kinetic models based on an equilibrium version of
Fermi’s golden rule often break down in the ultrafast regime [1–3]. For example,
early femtosecond pump–probe experiments investigated the importance of
coherent vibrational motions in electron transfer [4, 5], isomerization [6], and
photodissociation [7–10] reactions. It was shown that recurrences of a photoexcited
reactant at the transition state can give rise to periodic ‘‘bursts’’ in product formation
[4, 5, 10–14]. Recent 2D photon echo studies have renewed interest in coherent
photochemical reactions and inspired deeper thought about the significance of
coherence in biological function [14–16]. To date, most studies of coherent reaction
mechanisms have been conducted with pump–probe or 2D photon echo techniques
[17, 18]. These methods possess a single delay time between laser pulses (i.e., a
population time) during which the vibrational coherences of interest evolve. With
only one population time, it is not possible to establish correlations between
separate chemical species in a reaction that is initiated by light absorption; however,
if laser pulses (and population times) are added to a traditional three-pulse
experiment, then vibrational resonances of reactants and products can be displayed
in separate dimensions of a 2D spectrum [19–21].
In this chapter, we discuss our development of two-dimensional resonance
Raman (2DRR) spectroscopy and describe how it can be used to elucidate vibronic
coherence transfer processes [22–26]. Our focus will be on ultrafast chemical
reactions where 2DRR reveals non-trivial information; however, this is not the only
way that 2DRR can be employed. For example, 2DRR can be used to uncover linebroadening mechanisms in non-reactive systems in the same manner as any other
2D vibrational spectroscopy technique. To begin, the basic sequence of events
associated with a 2DRR experiment is outlined in Fig. 1. The experiment begins
when a laser pulse initiates coherent vibrational motion on the ground state potential
of a reactant by way of a stimulated Raman transition (these motions correspond to
dimension #1). The second laser pulse promotes the system to the excited electronic
state of the reactant which then undergoes an ultrafast transition to the product state.
Most generally, the product can also exhibit coherent vibrational motion if the
transition is fast compared to the vibrational period(s) of the system (these motions
are displayed in dimension #2). The sequence outlined in Fig. 1 can be applied to a
variety of ultrafast transitions (e.g., electron transfer, energy transfer,
isomerization).
We use the term 2DRR to refer to a specific component of the fifth-order
response in which the system evolves in a purely vibrational coherence in each of
the two electronic population times. Of course, our implementation of 2DRR is
preceded by a variety of other time-resolved vibrational spectroscopy techniques
which can be adapted to provide similar information. The techniques most closely
related to 2DRR are described at fifth-order in perturbation theory. Examples
include femtosecond stimulated Raman spectroscopy (FSRS) [27, 28] and resonant
Top Curr Chem (Z) (2017) 375:87
123
248
Reprinted from the journal
Time coincidence between chemical reactions and relaxation processes such as
solvation and vibrational dephasing gives rise to interesting physical effects.
Assumptions made in traditional kinetic models based on an equilibrium version of
Fermi’s golden rule often break down in the ultrafast regime [1–3]. For example,
early femtosecond pump–probe experiments investigated the importance of
coherent vibrational motions in electron transfer [4, 5], isomerization [6], and
photodissociation [7–10] reactions. It was shown that recurrences of a photoexcited
reactant at the transition state can give rise to periodic ‘‘bursts’’ in product formation
[4, 5, 10–14]. Recent 2D photon echo studies have renewed interest in coherent
photochemical reactions and inspired deeper thought about the significance of
coherence in biological function [14–16]. To date, most studies of coherent reaction
mechanisms have been conducted with pump–probe or 2D photon echo techniques
[17, 18]. These methods possess a single delay time between laser pulses (i.e., a
population time) during which the vibrational coherences of interest evolve. With
only one population time, it is not possible to establish correlations between
separate chemical species in a reaction that is initiated by light absorption; however,
if laser pulses (and population times) are added to a traditional three-pulse
experiment, then vibrational resonances of reactants and products can be displayed
in separate dimensions of a 2D spectrum [19–21].
In this chapter, we discuss our development of two-dimensional resonance
Raman (2DRR) spectroscopy and describe how it can be used to elucidate vibronic
coherence transfer processes [22–26]. Our focus will be on ultrafast chemical
reactions where 2DRR reveals non-trivial information; however, this is not the only
way that 2DRR can be employed. For example, 2DRR can be used to uncover linebroadening mechanisms in non-reactive systems in the same manner as any other
2D vibrational spectroscopy technique. To begin, the basic sequence of events
associated with a 2DRR experiment is outlined in Fig. 1. The experiment begins
when a laser pulse initiates coherent vibrational motion on the ground state potential
of a reactant by way of a stimulated Raman transition (these motions correspond to
dimension #1). The second laser pulse promotes the system to the excited electronic
state of the reactant which then undergoes an ultrafast transition to the product state.
Most generally, the product can also exhibit coherent vibrational motion if the
transition is fast compared to the vibrational period(s) of the system (these motions
are displayed in dimension #2). The sequence outlined in Fig. 1 can be applied to a
variety of ultrafast transitions (e.g., electron transfer, energy transfer,
isomerization).
We use the term 2DRR to refer to a specific component of the fifth-order
response in which the system evolves in a purely vibrational coherence in each of
the two electronic population times. Of course, our implementation of 2DRR is
preceded by a variety of other time-resolved vibrational spectroscopy techniques
which can be adapted to provide similar information. The techniques most closely
related to 2DRR are described at fifth-order in perturbation theory. Examples
include femtosecond stimulated Raman spectroscopy (FSRS) [27, 28] and resonant
Top Curr Chem (Z) (2017) 375:87
123
248
Reprinted from the journal