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D. Ghosh et al.
6 Recent Developments in 2D IR
Several recent developments of 2D IR spectroscopy will be discussed in this section.
As discussed above, 2D IR was mainly developed to study samples in the condensed
phase, where multiple IR pulses are focused into the sample. This limits the
exploitable overlapped path length through samples to a few millimeters. To circumvent this limitation, 2D IR experiments have been performed with a hollow waveguide
which enable acquisition of 2D IR spectra of low concentration gas-phase sample
[105]. Exploring the dynamics of a chemical system is limited by the vibrational
lifetime of the probe. The short lifetimes of the common vibrational probes do not
allow 2D IR to interrogate chemical/biological processes which are much slower than
the lifetime. To overcome this shortcoming, vibrational probes with long lifetimes
have been explored [83]. Alternatively, a new spectroscopic approach is to excite the
molecules into the first electronic excited state (S 1 ) which allows access to longer lifetimes. Vibrationally promoted electronic resonance spectroscopy (VIPERS) has been
reported which utilizes a UV/Vis pulse to excite the molecules to S 1 [106]. A combination of AOM based pulse shaping, rotating wave frame and phase cycling has been
utilized to select specific Liouville pathways [107]. Full spectrum 2D IR has been
developed covering the entire mid-IR spectral range which helps to decouple energy
transfer or vibrational coupling and allows three dimensional molecular conformations to be directly determined [108]. Broadband IR light sources, e.g. a laser plasma
source for femtosecond mid-IR pulses with bandwidth spanning the entire vibrational IR spectrum, are being developed that can be used for ultrafast spectroscopy
across the entire mid-IR spectrum [109]. Several surface sensitive 2D IR techniques
have been reported which employ surface enhancement, methods for studying electrochemical interfaces (2D ATR IR), and extensions for resolving nonequilibrium
processes (transient 2D IR) [110]. 2D ATR IR, combining spectro-electrochemistry
and 2D IR spectroscopy, is based on ultrathin conductive layers of noble metals and
indium–tin oxide (ITO) as working electrodes on a single-reflection attenuated total
reflectance (ATR) element in conjunction with ultrafast, multidimensional ATR spectroscopy [111]. Transient 2D IR is a UV pump narrowband-IR-pump broadband-IRprobe experiment of fifth order in the laser field to measure 2D IR spectra of transient
species [112]. Obtaining 2D IR spectra of heterogeneous samples like perovskites
and metal organic frameworks is generally hindered by severe light scattering. Use
of choppers and shutters at strategic positions in the BOXCAR geometry has been
reported to eliminate light scattering from heterogenous sample [112]. Additionally,
polarization selective experiments have been reported to separate out solvent reorientation dynamics and solvent induced spectral diffusion [113]. Data analysis has
recently incorporated neural network modelling to extract the dynamical timescales
from 2D IR spectrum [114].
D. Ghosh et al.
6 Recent Developments in 2D IR
Several recent developments of 2D IR spectroscopy will be discussed in this section.
As discussed above, 2D IR was mainly developed to study samples in the condensed
phase, where multiple IR pulses are focused into the sample. This limits the
exploitable overlapped path length through samples to a few millimeters. To circumvent this limitation, 2D IR experiments have been performed with a hollow waveguide
which enable acquisition of 2D IR spectra of low concentration gas-phase sample
[105]. Exploring the dynamics of a chemical system is limited by the vibrational
lifetime of the probe. The short lifetimes of the common vibrational probes do not
allow 2D IR to interrogate chemical/biological processes which are much slower than
the lifetime. To overcome this shortcoming, vibrational probes with long lifetimes
have been explored [83]. Alternatively, a new spectroscopic approach is to excite the
molecules into the first electronic excited state (S 1 ) which allows access to longer lifetimes. Vibrationally promoted electronic resonance spectroscopy (VIPERS) has been
reported which utilizes a UV/Vis pulse to excite the molecules to S 1 [106]. A combination of AOM based pulse shaping, rotating wave frame and phase cycling has been
utilized to select specific Liouville pathways [107]. Full spectrum 2D IR has been
developed covering the entire mid-IR spectral range which helps to decouple energy
transfer or vibrational coupling and allows three dimensional molecular conformations to be directly determined [108]. Broadband IR light sources, e.g. a laser plasma
source for femtosecond mid-IR pulses with bandwidth spanning the entire vibrational IR spectrum, are being developed that can be used for ultrafast spectroscopy
across the entire mid-IR spectrum [109]. Several surface sensitive 2D IR techniques
have been reported which employ surface enhancement, methods for studying electrochemical interfaces (2D ATR IR), and extensions for resolving nonequilibrium
processes (transient 2D IR) [110]. 2D ATR IR, combining spectro-electrochemistry
and 2D IR spectroscopy, is based on ultrathin conductive layers of noble metals and
indium–tin oxide (ITO) as working electrodes on a single-reflection attenuated total
reflectance (ATR) element in conjunction with ultrafast, multidimensional ATR spectroscopy [111]. Transient 2D IR is a UV pump narrowband-IR-pump broadband-IRprobe experiment of fifth order in the laser field to measure 2D IR spectra of transient
species [112]. Obtaining 2D IR spectra of heterogeneous samples like perovskites
and metal organic frameworks is generally hindered by severe light scattering. Use
of choppers and shutters at strategic positions in the BOXCAR geometry has been
reported to eliminate light scattering from heterogenous sample [112]. Additionally,
polarization selective experiments have been reported to separate out solvent reorientation dynamics and solvent induced spectral diffusion [113]. Data analysis has
recently incorporated neural network modelling to extract the dynamical timescales
from 2D IR spectrum [114].