redox-active samples in bulk solution under electrochemical conditions [225, 226].
Although not yet demonstrated, this method should also be applicable to
immobilized molecules at the electrode-electrolyte interface [227]. Furthermore,
surface-specific SFG spectroscopy can also be used under electrochemical
conditions [228–230], however, has not yet been implemented as a multidimensional variant for measuring ultrafast dynamics.
4.2 2D IR Microscopy
Conventional 2D IR spectroscopy in transmission or reflection mode uses laser
beams, which are focused onto a sample in spatial dimensions of several tens to
hundreds of micrometers. The signal thus contains spatially averaged contributions
over extended regions of the sample. While this is generally not problematic for
isotropic samples such as molecules in liquids, spatial averaging can be challenging
for samples that are immobilized on surfaces, or confined in three-dimensional
structures. This is because local structures of the molecules can be considerably
different since orientations with respect to surfaces or local solvent environments
can be drastically different. 2D IR is an ideal method for resolving such spatial
differences in ultrafast vibrational dynamics and to obtain chemically specific
temporally and spatially resolved information of samples. Consequently, recent
experimental developments have focused on the combination of the method with
microscopy [59, 60]. In an initial variant of 2D IR microscopy a tight focusing
method with reflective optics in combination with a single-beam implementation of
pump-probe-type 2D IR spectroscopy was employed to achieve a nearly diffractionlimited spatial resolution [59]. Chemical contrast between molecules dissolved in a
bulk and strongly interacting liquid against adsorbed molecules on small, noninteracting polystyrene beads could then be achieved by measuring different
vibrational lifetimes.
The essential challenge in the combination of 2D IR spectroscopy and
microscopy is to acquire data for a sufficiently large number of different spots
with similar data quality compared to a spatially integrated signal on a reasonable
timescale. Depending on the required spatial resolution and the area of interest, this
can easily result in a few thousand-fold increase of measurement time. To overcome
this problem, Zanni et al. have recently developed a 2D IR wide-field microscope,
using a dual acousto-optic modulator (AOM) pulse shaper and a focal-plane array
detector (FPA). This way the authors were able to simultaneously acquire
16,000 2D IR spectra, thus avoiding scanning the sample ‘‘step-by-step’’.
Figure 17a shows a schematic overview of the experimental setup, in which the
output of a single OPA is split into two portions that are fed into a dual AOM to
generate two pairs of polarization-orthogonal pulses for double time-domain data
acquisition [231]. The two beams are afterwards recombined to irradiate a spot of
about 100 lm in diameter of the sample. Behind the sample the pump and probe
pulses are separated by a polarizer and the probe light is imaged onto the FPA.
Under the chosen conditions, the microscope records for each pixel about 1.1 lm
separated spots from the sample. The performance of the microscope was
characterized by measuring 2D IR spectra of samples containing polystyrene beads
Top Curr Chem (Z) (2017) 375:86
123
157
Reprinted from the journal
Although not yet demonstrated, this method should also be applicable to
immobilized molecules at the electrode-electrolyte interface [227]. Furthermore,
surface-specific SFG spectroscopy can also be used under electrochemical
conditions [228–230], however, has not yet been implemented as a multidimensional variant for measuring ultrafast dynamics.
4.2 2D IR Microscopy
Conventional 2D IR spectroscopy in transmission or reflection mode uses laser
beams, which are focused onto a sample in spatial dimensions of several tens to
hundreds of micrometers. The signal thus contains spatially averaged contributions
over extended regions of the sample. While this is generally not problematic for
isotropic samples such as molecules in liquids, spatial averaging can be challenging
for samples that are immobilized on surfaces, or confined in three-dimensional
structures. This is because local structures of the molecules can be considerably
different since orientations with respect to surfaces or local solvent environments
can be drastically different. 2D IR is an ideal method for resolving such spatial
differences in ultrafast vibrational dynamics and to obtain chemically specific
temporally and spatially resolved information of samples. Consequently, recent
experimental developments have focused on the combination of the method with
microscopy [59, 60]. In an initial variant of 2D IR microscopy a tight focusing
method with reflective optics in combination with a single-beam implementation of
pump-probe-type 2D IR spectroscopy was employed to achieve a nearly diffractionlimited spatial resolution [59]. Chemical contrast between molecules dissolved in a
bulk and strongly interacting liquid against adsorbed molecules on small, noninteracting polystyrene beads could then be achieved by measuring different
vibrational lifetimes.
The essential challenge in the combination of 2D IR spectroscopy and
microscopy is to acquire data for a sufficiently large number of different spots
with similar data quality compared to a spatially integrated signal on a reasonable
timescale. Depending on the required spatial resolution and the area of interest, this
can easily result in a few thousand-fold increase of measurement time. To overcome
this problem, Zanni et al. have recently developed a 2D IR wide-field microscope,
using a dual acousto-optic modulator (AOM) pulse shaper and a focal-plane array
detector (FPA). This way the authors were able to simultaneously acquire
16,000 2D IR spectra, thus avoiding scanning the sample ‘‘step-by-step’’.
Figure 17a shows a schematic overview of the experimental setup, in which the
output of a single OPA is split into two portions that are fed into a dual AOM to
generate two pairs of polarization-orthogonal pulses for double time-domain data
acquisition [231]. The two beams are afterwards recombined to irradiate a spot of
about 100 lm in diameter of the sample. Behind the sample the pump and probe
pulses are separated by a polarizer and the probe light is imaged onto the FPA.
Under the chosen conditions, the microscope records for each pixel about 1.1 lm
separated spots from the sample. The performance of the microscope was
characterized by measuring 2D IR spectra of samples containing polystyrene beads
Top Curr Chem (Z) (2017) 375:86
123
157
Reprinted from the journal