Experimental and Theoretical Studies of van der Waals Molecule Photodissociation

Citation

Hair, Sally Reid (1988) Experimental and Theoretical Studies of van der Waals Molecule Photodissociation. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/anb1-sr68. https://resolver.caltech.edu/CaltechTHESIS:01182013-160624319

Abstract

Three studies are reported on the vibrational predissociation of polyatomic van der Waals complexes. In the first, the ethylene dimer and rare gas-ethylene complexes are treated theoretically, using a local mode quantum mechanical technique. The ethylene dimer exhibits extensive mixing between the initially excited ν₇ vibration and nearby combinations of the ν₁₀ and van der Waals vibrations, while the rare gas- ethylene complexes do not. Mixing is extensive enough in the ethylene dimer to spread the oscillator strength of the ν₇ vibration over a 10 cm⁻¹ region of the spectrum, in agreement with the experimentally observed band.

The second study is a low resolution photodissociation experiment on the ethylene-methane complex. The spectra observed by exciting the ν₇ ethylene vibration of C₂H₄-CH₄, C₂H₄-CH₂D₂, and C₂H₄-CD₄ all exhibit the same width. This indicates that the shapes and widths of the observed spectra are not determined by unresolved or power-broadened rotational structure. This result underscores the importance of vibrational coupling in the dissociation process.

The final study is a laser pump-probe experiment on the Ne₂Cl₂ and Ne₃Cl₂ complexes. The Ne₂Cl₂ complex has a distorted tetrahedral geometry, as determined from high resolution, excitation spectra. Excitation shift arguments suggest a structure for Ne₃Cl₂ with the three neon atoms encircling the Cl₂ bond axis. The total van der Waals bond energy of the Ne₂Cl₂ complex is found to be between 145.6 and 148.6 cm⁻¹. When energetically possible, the Ne₂Cl₂ complex can dissociate by losing a single quantum of Cl₂ stretching energy. This indicates that the two neon atoms do not dissociate by two independent "half-collisions." The Cl₂ fragment rotational excitation is found to depend only weakly on the energy available to the fragments.

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