Top Curr Chem (Z) (2018) 376:10
1 3
quantify electronic couplings and correlations between different excited states
[3, 4, 11].
(b) 2DES distinguishes inhomogeneous and homogeneous broadening by the analysis of the lineshapes of the spectroscopic signals, enabling the individual levels
to be singled out in strongly congested spectra [3, 15–19].
(c) 2DES can follow the parallel pathways by which the coupled electronic dynamics evolve after photoexcitation in real time. This makes the 2DES technique a
particularly powerful tool for tracking energy and electron transfer processes [4,
13, 20].
Our aim here is to provide examples highlighting the real advantages of using
2DES. As physical chemists, we know that to understand a photo-initiated reaction,
one should initially assign spectral signatures to reactants, products—eventually
intermediates—and then propose a kinetic model to explain the evolution of the species, based on the observed spectral changes. However, in complex condensed-phase
systems, spectral signatures are often overlapped and difficult to untangle. For example, steady-state measurements (e.g., linear absorption, fluorescence, anisotropy,
and circular dichroism) supply information on the electronic structure. However,
homogeneous line broadening can significantly obscure physical insights such as the
distinction between homogeneous and inhomogeneous broadening and sometimes
cryogenic temperature experiments are not able to reach the desirable spectral resolution. Further insights can be gained by time-resolved techniques, such as transient
absorption and pump–probe spectroscopies, which can also track photo-induced
dynamics, as energy and electron transfers. However, these techniques have intrinsic
resolution limitations on either high temporal or high-frequency resolved-capability.
In this context, 2DES has emerged as an optical technique that can accomplish many
of the goals of conventional spectroscopies, also overcoming all the limitations mentioned above.
2 Implementation
2DES implementation faces two main technical challenges: (1) it requires a careful
control of the interferometric stability between pulse pairs [21], which need to be
phase-locked within a small fraction of their carrier wavelengths (i.e., a precision
of few nanometers is required for ~ λ/100 at 550 nm) [3, 22, 23]; (2) to fully exploit
its benefits, 2DES calls for ultra-broadband pulses, with the duration of just a few
optical cycles, which are challenging to generate and control. Thus, the phase stability in a pair of broadband pulses has been the limiting factor that explains why
2DES was developed after 2D-IR spectroscopy [24]. However, thanks to the technological advancement of ultrafast optics, 2DES techniques are rapidly evolving and
have today various applications, providing access to systems that contain electronic
transitions spanning from the ultraviolet to the near-infrared and beyond. Technically, one can build a 2DES apparatus following three different types of geometries:
(1) the non-collinear so-called “box-car” geometry (Fig. 3a), and (2) the collinear,
30
Reprinted from the journal
1 3
quantify electronic couplings and correlations between different excited states
[3, 4, 11].
(b) 2DES distinguishes inhomogeneous and homogeneous broadening by the analysis of the lineshapes of the spectroscopic signals, enabling the individual levels
to be singled out in strongly congested spectra [3, 15–19].
(c) 2DES can follow the parallel pathways by which the coupled electronic dynamics evolve after photoexcitation in real time. This makes the 2DES technique a
particularly powerful tool for tracking energy and electron transfer processes [4,
13, 20].
Our aim here is to provide examples highlighting the real advantages of using
2DES. As physical chemists, we know that to understand a photo-initiated reaction,
one should initially assign spectral signatures to reactants, products—eventually
intermediates—and then propose a kinetic model to explain the evolution of the species, based on the observed spectral changes. However, in complex condensed-phase
systems, spectral signatures are often overlapped and difficult to untangle. For example, steady-state measurements (e.g., linear absorption, fluorescence, anisotropy,
and circular dichroism) supply information on the electronic structure. However,
homogeneous line broadening can significantly obscure physical insights such as the
distinction between homogeneous and inhomogeneous broadening and sometimes
cryogenic temperature experiments are not able to reach the desirable spectral resolution. Further insights can be gained by time-resolved techniques, such as transient
absorption and pump–probe spectroscopies, which can also track photo-induced
dynamics, as energy and electron transfers. However, these techniques have intrinsic
resolution limitations on either high temporal or high-frequency resolved-capability.
In this context, 2DES has emerged as an optical technique that can accomplish many
of the goals of conventional spectroscopies, also overcoming all the limitations mentioned above.
2 Implementation
2DES implementation faces two main technical challenges: (1) it requires a careful
control of the interferometric stability between pulse pairs [21], which need to be
phase-locked within a small fraction of their carrier wavelengths (i.e., a precision
of few nanometers is required for ~ λ/100 at 550 nm) [3, 22, 23]; (2) to fully exploit
its benefits, 2DES calls for ultra-broadband pulses, with the duration of just a few
optical cycles, which are challenging to generate and control. Thus, the phase stability in a pair of broadband pulses has been the limiting factor that explains why
2DES was developed after 2D-IR spectroscopy [24]. However, thanks to the technological advancement of ultrafast optics, 2DES techniques are rapidly evolving and
have today various applications, providing access to systems that contain electronic
transitions spanning from the ultraviolet to the near-infrared and beyond. Technically, one can build a 2DES apparatus following three different types of geometries:
(1) the non-collinear so-called “box-car” geometry (Fig. 3a), and (2) the collinear,
30
Reprinted from the journal