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As we have already noted earlier, there have been many attempts with varying degrees of success to develop different types of 2D correlation spectroscopy schemes utilizing various forms of optical spectroscopic probes, such as IR, NIR and Raman. These efforts may be currently classified into two extensive but distinct streams of major scientific research activities. One such stream is concerned with the development of nonlinear optical 2D spectroscopy techniques based on some pulsed laser excitations.1-7 The advancement in the field is creating an exciting possible link between optical spectroscopy and well-established multiple pulse approaches adopted by 2D NMR.8 There are many reviews written on this type of optical 2D spectroscopy,9-14 so we will only briefly discuss this subject in this chapter.
The second stream, which is more relevant to the theme of this book, has its origin in a simple correlation scheme introduced in late 1980s to the analysis of dynamic IR spectra obtained under a sinusoidal perturbation.15-18 Chapter 8 discusses numerous examples of such dynamic 2D IR spectroscopy and related topics. Another form of 2D correlation technique, utilizing well-known statistical tools, soon appeared for the analysis of different types of spectral data.19-21 Such statistical and chemometrical techniques are still active parts of the 2D correlation spectroscopy field.22-36 The early development of statistical 2D correlation,20 21 which demonstrated an important departure from the original dynamic 2D IR method bound by sinusoidal waveforms, led to the introduction of the formal generalized 2D correlation concept in 1993.37 We will examine some of the ideas not fully explored in Chapter 6 behind the development of the statistical 2D correlation methods.21-24 32-36
Many attempts have also been made to greatly refine the original 2D correlation method. We will discuss some of the interesting variant form of 2D correlation methods. Moving window 2D correlation introduced by Richardson et al.,38 based on the idea of segmenting data matrix into small subsets, was further utilized by Sasic et al.39,40 Model-based 2D correlation methods, such as those introduced by Dluhy et al.43-45 or that of Eads and Noda,46 are also emerging as a new and powerful alternative to the conventional 2D correlation analysis.
Two-Dimensional Correlation Spectroscopy-Applications in Vibrational and Optical Spectroscopy
I. Noda and Y. Ozaki © 2004 John Wiley & Sons, Ltd ISBN: 0-471-62391-1
Other Types of Two-dimensional Spectroscopy
7.1 NONLINEAR OPTICAL 2D SPECTROSCOPY
Only a brief mention is made here on the newly emerging field of nonlinear optical 2D spectroscopy techniques based on ultrafast femtosecond laser pulses.1-7 These techniques, including various nonlinear vibrational spectroscopic methods such as nonlinear 2D Raman and 2D IR measurements, utilize a combination of multiple short optical pulses, often with prescribed delay times among them defining additional time scales. Multiple time-domain data thus collected may be converted to 2D frequency domain plots. As such, they are conceptually much closer to original 2D experiments developed in NMR field based on the use of radio frequency pulses.8
7.1.1 ULTRAFAST LASER PULSES
Modern progress in laser technology has made it possible to produce experimentally accessible ultrafast optical pulses in the time scale of femtosecond ranges. The ultrafast impulsive excitations of such pulses are actually shorter than the vibrational periods or dephasing time scales of molecules. Measurements on such a short time scale have also enabled researchers to probe real-time dynamics of molecules in a complex and mutually interacting system such as a liquid solution. Third order nonresonant Raman spectroscopy based on the threefold interactions of visible laser pulses, for example, is now routinely employed in the study of the vibrational mode of solutions. Higher order nonlinear experiments, such as fifth order nonresonant Raman measurements, may even be extended to multidimensional optical spectroscopy experiments using the delay time between the optical pulses as the second independent time variable in the study. Such multiple time experiments will lead to measurements physically analogous to those in the conventional 2D NMR spectroscopy.
The theoretical foundation for the possibility of carrying out a multidimensional nonlinear optical experiment was laid out by the classic paper published by Tanimura and Mukamel.1 They showed, among other things, the possibility of differentiating between homogeneous and inhomogeneous vibrational line broadening if a fifth order, instead of a third order, nonresonant spectroscopy experiment is carried out. Following the publication of this direction-setting theoretical paper, Tominaga and Yoshihara reported the first measurement of fifth order signals detected from the intermolecular low frequency vibrational mode of a CS2 solution using ultrafast nonresonant six-wave mixing with five different pulses.2 This development was followed by the other experimental work of Steffen and Duppen.3 Tokmakoff and Flemming soon reported the first 2D Raman spectral map of a nonlinear optical measurement.4 Thus, the era of nonlinear optical 2D spectroscopy based on ultrafast laser pulses started.