Two-Dimensional Correlation Spectroscopy (2D-COS) of Gas-Phase Pyrrole Clusters in a Supersonic Jet: Treatment of Sharp Bands on a Broad Background.

Two-dimensional correlation spectroscopy (2D-COS) is a useful technique to analyze any intensity behavior of optical spectra that exhibit a complicated feature with overlapped bands. In this study, we apply 2D-COS to the infrared (IR) spectra of gas-phase pyrrole (Py) clusters. The NH stretching vibrations of the Py clusters are measured by cavity ringdown spectroscopy. The observed IR spectra of the Py clusters consist of sharp bands, full width half-maximum (FWHM) ∼1 cm-1, and a broad background (FWHM >50 cm-1). The 2D asynchronous correlation spectra reveal that the sharp bands and a broad background are assigned to small clusters of dimer to pentamer and large clusters with bulk-like structures, respectively, which support the results of our previous study. The sharp bands are also analyzed using another 2D asynchronous correlation spectrum, which is obtained by decomposing the observed IR spectra into sharp and broad components. Because the asynchronous signals are consistent with those obtained from the IR spectra without decomposition, the result would suggest that we need not to decompose the IR spectra into sharp and broad components before applying 2D-COS. However, our model simulations of 2D-COS showed a counterexample that gives an incorrect result without removing a broad background component from the IR spectra. This study strongly suggests that we need to undertake a careful treatment of the complicated IR spectrum with various widths of bands.