Polyad unzipped

The worsening overlap between polyads, however, is not an insurmountable obstacle. We have developed an unzipping procedure whereby the fractionated patterns associated with different polyads can be disentangled from each other. 

Figure 1. Unzipped polyads (bottom) from the C2H2 A 0° DF spectrum as progressions in 04 (trans bend). Each row has constant vz (CC stretch) and each column has constant 04. Integrated intensity (deperturbed Franck-Condon factor) for each polyad in the 0° DF spectrum, arranged as progressions in 04 (top). Shading indicates the value of V2 for the progression.

The unzipping procedure reveals the diagnostically significant trends in fractionation widths and patterns illustrated by Figure 3. These trends can lead to qualitative insights into IVR mechanisms and can suggest optimal schemes for external control over intramolecular dynamics. The unzipped polyads can also yield quantitative least-squares refinements of anharmonic coupling constants, from which any dynamical quantity based on (Q, 0 may be calculated. 

Figure 3. Unzipped polyads from the 0° DF spectrum, arranged as in Fig. 1. Note that IVR, manifest as fractionation, increases as vs increases but decreases as V2 increases.

Lacking a trick to accomplish unzipping in the time domain, the dynamics uncovered in a time-domain experiment would appear far more complicated. Even if unzipping were possible, it would be necessary to record the intrapolyad dynamics of several zero-order states to obtain sufficient information to characterize a polyad Heff. Without such a characterization, it would be impossible to make even qualitative predictions of the dynamics in another region of Evib or of another class of probe-able zero-order state. 

The combination of low-resolution and spectral unzipping into noninteracting polyads enables systematic, model-free surveys of deperturbed Franck-Condon factors, deperturbed zero-order energy levels, and trends in intramolecular vibrational redistribution (IVR) rates and pathways [3]. The H[ res,/i polyad model permits extraction of the most important resonance strengths directly from fits to a few polyads [6-8]. Once these anharmonic... 

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