Knowledge Bank

We have studied the fluorescence spectrum of the product $I2\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}$ produced in the photodissociation of van der Waals complexes of $I2$ and helium. These complexes were formed in a supersonic free jet. Product state distribution of the fragment $I2\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}$ were inferred from bond intensities in the fluorescence spectrum. The major dissociation channel for all observed species $I2Hen$, (n = 1-3) involved loss of n quanta from the $I2$ stretch although minor $n÷1$ quantum channels were observed. The one quantum channel for $I2He2$ and $I2He3$ is energetically allowed but was not observed. This Implies that dissociation of the larger complexes is sequential and that two rare gas atoms cannot dissociate simultaneously by sharing a single vibrational quantum. Fluorescence excitation spectra have shown that the absorption wavelength of each complex is shifted by a constant amount to shorter wavelengths by the amount of each rare gas atom. This band shift rule has been observed in many other van der Waals complexes. Vibrational relaxation cross sections of $I2$ by He were measured at various positions in the jet. These cross sections were found to increase with decreasing transnational temperature, and were much lower then $measured1$ room temperature cross sections. Similar behavior has been observed at temperatures intermediate between room temperature and the temperature used in this $experiment.2$