42
D. Ghosh et al.
Fig. 2 Typical pulse sequence used in 2D IR experiment
is denoted by t (detection period). In practice, 2D IR experiments are realized in two
different optical geometries, BOXCAR geometry and pump-probe geometry. It is also
important to remember that both of these methods can be used in either transmission
or reflection mode. In this article, we will be mostly focusing on transmission mode
spectra.
The BOXCAR geometry involves four-wave mixing where the IR pulse generated
using difference frequency generation is further divided into four pulses (Fig. 3).
Three of these pulses with wave vector k 1 , k 2 , and k 3 are arranged in a manner such
that they are focused into the sample coming from three corners of the square. The
signal is emitted from the fourth corner of the square with a new, unique wave vector
k s . The BOXCAR geometry provides control over the polarization of each pulse
which can be utilized to explore the polarization dependent excitation pathways in a
chemical system [9]. One benefit of the BOXCAR geometry is that the signal, emitted
in a different direction as compared to the input IR pulses, is background free. The
fourth beam, which does not pass through the sample, acts as a local oscillator (LO)
to enable heterodyne detection. This allows phase information to be retained in the
experiment. The LO also aids in signal detection as the LO amplifies the signal.
2D IR spectra produced with a pump-probe geometry use pulse shaping approach
which involves modulating the phase and amplitude of individual frequencies to
produce the pulse pair required for 2D IR spectroscopy. A typical geometry for
the mid-IR pulse shaper is shown in Fig. 4. In this approach, the mid-IR beam is
separated in a strong pump pulse and a weak probe pulse. The pump pulse is then
passed through the acousto-optics modulator (AOM). AOM creates two temporally
Fig. 3 Schematic representation of BOXCAR geometry
D. Ghosh et al.
Fig. 2 Typical pulse sequence used in 2D IR experiment
is denoted by t (detection period). In practice, 2D IR experiments are realized in two
different optical geometries, BOXCAR geometry and pump-probe geometry. It is also
important to remember that both of these methods can be used in either transmission
or reflection mode. In this article, we will be mostly focusing on transmission mode
spectra.
The BOXCAR geometry involves four-wave mixing where the IR pulse generated
using difference frequency generation is further divided into four pulses (Fig. 3).
Three of these pulses with wave vector k 1 , k 2 , and k 3 are arranged in a manner such
that they are focused into the sample coming from three corners of the square. The
signal is emitted from the fourth corner of the square with a new, unique wave vector
k s . The BOXCAR geometry provides control over the polarization of each pulse
which can be utilized to explore the polarization dependent excitation pathways in a
chemical system [9]. One benefit of the BOXCAR geometry is that the signal, emitted
in a different direction as compared to the input IR pulses, is background free. The
fourth beam, which does not pass through the sample, acts as a local oscillator (LO)
to enable heterodyne detection. This allows phase information to be retained in the
experiment. The LO also aids in signal detection as the LO amplifies the signal.
2D IR spectra produced with a pump-probe geometry use pulse shaping approach
which involves modulating the phase and amplitude of individual frequencies to
produce the pulse pair required for 2D IR spectroscopy. A typical geometry for
the mid-IR pulse shaper is shown in Fig. 4. In this approach, the mid-IR beam is
separated in a strong pump pulse and a weak probe pulse. The pump pulse is then
passed through the acousto-optics modulator (AOM). AOM creates two temporally
Fig. 3 Schematic representation of BOXCAR geometry