Extracting Molecular Dynamics in Acetylene From Unzipped Dispersed Fluorescence Spectra

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Dispersed Fluorescence (DF) spectra of the $\tilde{A} \leftrightarrow \tilde{X}$ system of Acetlene have been recorded utilizing $\tilde{A}$ State intermediates that have 0, 2, and 3 quanta of excitation in the trans-bend Intramolecular Vibrational Redistribution is manifested in the spectra by the degree of fractionation of the zero-order state among the molecular eigenstates. Recently a method for unzipping complex fractionation patterns in the dispersed fluorescence spectra have been $employed^{12}$. Once the DF Spectrum is unzipped, two trends in IVR become apparent. For the particular chromostates selected in these experiments, IVR increases with increasing excitation in the trans-bend, $\nu_{4}$ and, surprisingly IVR decreases with increasing excitation in the CC stretch, $\nu_{2}$. Since the density of states increases with increasing $\nu_{2}$ and $\nu_{4}$ one would expect for statistical reasons to observe an increase in IVR with increasing $\nu_{4}$ consistent with the spectrum and increasing $\nu_{2}$. contrary to the spectrum. As opposed to statistical expectations, both IVR trends turn out to be local effects. The extent of IVR, for the initial states prepared in these expriments, is controlled by the strength of the off diagonal matrix elements, $H_{ij}$ and the zero-order energy differences. $\Delta E$, of the first few initial resonance steps, or tiers, and is independent of the number or density of states.


1 S.A.B. Solina, J.P. O'Brien, R.W. Field, W.F. Polik, Ber, Bunsen. Phys. Chem. accepted. 2. S.A.B. Solina, J.P. O'Brien, R.W. Field, W.F. Polik. Symposium on Molecular Spectroscopy, 1995 ""Unzipped Acetylene Dispersed Fluorescence: An unconventional form of Franck-Condon Analysis''
Author Institution: Massachusetts Institute of Technology, Cambridge, MA 02139; Hope College, Holland, MI 49423