MEASUREMENT OF THE BINDING ENERGY OF IODINE-RARE GAS VAN DER WAALS MOLECULES
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Date
1979
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Ohio State University
Abstract
A supersonic free jet expansion has been used to produce van der Waals complexes of iodine and various rare gases. Single excited state vibronic levels of the $B ^{3}\Pi_{0^{+}_{u}}$ electronic state have been populated with a tunable cw dye laser. The excitation process not only electronically excites the complex but also vibrationally excites the iodine-iodine stretching mode. Vibrational predissociation of the complex occurs when energy originally localized in vibrational motion of the chemical bond is transferred to the van der Waals bond causing dissociation of the complex. Vibrational anharmonicity in the B state of $I_{2}$ causes the vibrational quanta to become smaller as one proceeds up the vibrational ladder, and therefore the minimum number of vibrational quanta required for dissociation is dependent on the initial vibrational level of the excited complex. As one-proceeds up the vibrational ladder one changes from a point where n quanta are required for dissociation to a level (unique for each complex) where $n+ 1$ quanta are required. Location of the level where this change occurs allows one to bracket the van der Waals binding energy within a few $cm^{-1}$. We have dispersed the fluorescence from the $I_{2}{^{*}}$ fragment that is produced when iodine-rare gas complexes dissociate, and have thereby measured the minimum number of vibrational quanta used in the dissociation process. From these measurements we have inferred the following van der Waals binding energies (In $cm^{-1}$): \begin{eqnarray*} && l_{2} He \quad D_{0} \leq 19\\ && l_{2} Ne \quad 65\leq D_{0} \leq 67\\ && l_{2} Ar \quad 220\leq D_{0} \leq 226 \end{eqnarray*}
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