When a series of spectra are acquired as a function of controlled perturbation variable, such as temperature, pressure, electric field strength, concentration, and any arbitrary variables, a conventional spectral analysis is performed on the basis of intensity changes at a specific frequency. However, a highly overlapped spectral region is challenging for the single-frequency-based analysis. Two-dimensional correlation spectroscopy (2DCOS), which was originally proposed by Noda in 1986, compares spectral intensity variations at a frequency pair with respect to a perturbation variable. 2DCOS provides positive and negative correlations between two spectral components. It also provides specific sequences of certain spectral intensity changes and is used to enhance the spectral resolution of highly overlapped spectra. 2DCOS has been applied extensively to analyze various types of spectral data from Raman and IR to X-ray, UV-Vis, and fluorescence.
We have applied 2DCOS analysis methods to unravel spectral signatures of phase transitions occurring in polymers and other complex materials [1,2,3]. We also have developed theories to advance moving-window approaches to 2DCOS [4,5].
 Y. Jin, A. P. Kotula, A. R. Hight Walker, K. B. Migler, and Y. J. Lee, Phase-Specific Raman Analysis of n-Alkane Melting by Moving-Window Two-Dimensional Correlation Spectroscopy, J. Raman Spectrosc. 47, 1375 (2016).
 Y. Jin, A. P. Kotula, A. R. Hight Walker, K. B. Migler, and Y. J. Lee, Raman Identification of Multiple Melting Peaks of Polyethylene, Macromolecules 50, 6174 (2017).
 I. S. Ryu, X. Liu, Y. Jin, J. Sun, and Y. J. Lee, Stoichiometric analysis of competing intermolecular hydrogen bonds using infrared spectroscopy, RSC Advances 8, 23481 (2018).
 Y. J. Lee, Analytical and Numerical Characterization of Autocorrelation and Perturbation-Correlation Moving-Window Methods, Appl. Spectrosc. 71, 1321 (2017).
 Y. J. Lee, Least Squares Moving-Window Spectral Analysis, Appl. Spectrosc. 71, 1894 (2017).