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The experimental setups for ultrafast multiple laser pulse experiments are not trivial to construct. Ordinary spectroscopic instrumentations found in conventional laboratories, such as a commercial FT IR instrument equipped with a
Nonlinear Optical 2D Spectroscopy
Michelson interferometer, are not suitable for such measurements. This requirement tends to limit the activities in this field to a few specialized academic laboratories with access to ultrafast laser pulses. No routine commercial instrumentation is currently available for nonlinear optical 2D spectroscopy measurements. New experimental developments, including the incorporation of the heterodyne detection method,5 are obviously helpful in the advancement of the field, while other major difficulties in this class of measurements such as the effect of third order cascades persist. The definitive demonstration of the utility of nonlinear optical 2D spectroscopy techniques, not only in scientifically significant research work but also in practical applications of general interest, will undoubtedly accelerate the broader activities in this field. The field of nonlinear optical 2D spectroscopy is still very rapidly evolving. The discussion on the direction of the development, as well as the potential impact on broad optical spectroscopy in general, is well beyond the scope or intent of this book. Readers are directed to some of the review articles describing the recent progress in this field.9-14
7.1.2 COMPARISON WITH GENERALIZED 2D CORRELATION SPECTROSCOPY
Conceptually, ultrafast laser pulse based nonlinear optical 2D spectroscopy techniques are, in a sense, true optical analogues of earlier two-dimensional experimental methods developed in the field of NMR.8 Plurality of time scales is incorporated directly into the optical measurement by the use of multiple light frequencies or intervals of laser pulses. The evolution of population changes of a given vibrational state and the relaxation of the coherence state or phase are probed and analyzed in a manner parallel to the procedures used in 2D NMR.
The generalized 2D correlation spectroscopy discussed throughout this book, on the other hand, employs a very different approach. The multidimensional nature of generalized 2D correlation analysis arises from the use of an external perturbation from an independent source and time scale, which usually does not have the same magnitude of the ultrafast time scale of the frequency of light or pulse intervals. The time scales of the external perturbations are usually restricted to those of much slower processes of chemical interactions involving multiple atoms and nuclei and even larger submolecular functional groups. In other words, the experimental scope of nonlinear optical 2D spectroscopy and that of generalized 2D correlation spectroscopy are substantially different from the start.
The apparent similarity of plotting spectral intensities along two independent spectral variable axes suggests the possible commonality of the two techniques. Such fortuitous visual similarity, however, may be quite misleading, as the origins of 2D peaks are based on fundamentally different treatments of physical observations in those experiments. Ironically, research activities in both nonlinear optical 2D spectroscopy and generalized 2D correlation spectroscopy started around the same time,115 and consequently the terminologies, such as 2D IR
Other Types of Two-dimensional Spectroscopy
or 2D Raman spectroscopy, are used extensively among research groups from both sides, but referring to very different techniques. In this book, 2D correlation spectroscopy refers to our basic method of analyzing a set of dynamic spectra obtained under an external perturbation.
7.1.3 OVERLAP BETWEEN GENERALIZED 2D CORRELATION AND NONLINEAR SPECTROSCOPY
Some fundamental differences clearly exist between nonlinear optical 2D spectroscopy and generalized 2D correlation spectroscopy, as pointed out above. The former followed the fruitful path already defined by the success of 2D NMR spectroscopy, while the latter had to venture the uncharted path of coupling conventional spectroscopic measurements with an external physical perturbation to yield 2D correlation spectra. The two fields seem so far apart in the experimental design that one may prematurely conclude that no overlap should exist between the two except for the superficial similarity of the 2D display schemes used for the final construction of spectral maps. Actually, there are some possibilities where the two fields can substantially overlap and even merge to create useful experimental methods.
It was pointed out earlier in the book that the generalized 2D correlation scheme is truly a universally applicable technique without any restriction to the type, waveform, and most importantly time scale of the experiment. Thus, it is possible to apply the basic concept of generalized 2D correlation directly to the analysis of data obtained in an ultrafast laser pulse experiment. Pulsed experiments usually consist of a train of multiple pulses to excite the system. The combination of different pulse sequences yields a set of signals encoded with multiple time scales defined by the intervals between pulses and the duration after the (usually the last) probe pulse. If one considers the entire train of exciting pulses as a set of external perturbations, the set of spectral signals thus collected can be treated as a dynamic spectrum in a manner familiar to generalized 2D correlation spectroscopy. Thus, any sets of nonlinear optical data obtained under multiple pulse conditions may potentially be analyzed by the same formal treatment used in generalized 2D correlation spectroscopy.