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Bryan R. Henry: Research Activities

The central theme of our research concerns the experimental and theoretical study of highly vibrationally excited molecules. At high enough energies, light interacts with molecules containing XH oscillators to prepare states that are more localized than those expected on the basis of the traditionally accepted normal mode description of molecular vibrations. We have developed the local mode description of such vibrational states, and this description has now gained general acceptance. We study these states with overtone spectra. Because of the localization, these overtone spectra are extremely sensitive to the properties of XH bonds. We measure these spectra with a variety of sophisticated spectroscopic techniques including intracavity laser photoacoustic spectroscopy and use them to study molecular structure and conformation. The time scale of the overtone experiment allows us to study conformational processes that are much too fast to be studied by conventional techniques like NMR. We also use these spectra to study intramolecular vibrational energy redistribution.

We are involved in theoretical studies to determine sources of local mode overtone intensity. We have developed a very successful approach that uses our harmonically coupled anharmonic oscillator local mode model to obtain the vibrational wavefunctions and uses ab initio calculations to obtain the dipole moment functions. We have applied these calculations to relatively large molecules with different types of XH oscillators. We have compared intensities from our simple model to intensities from sophisticated variational calculations for the small molecules, H₂O and CH₂O to determine the quality of basis set needed to calculate dipole moment functions that lead to reasonable values for fundamental and overtone intensities. We have also calculated intensities at different levels of ab initio theory and have found a very surprising result. The calculations consistently show that while electron correlation in the calculation of dipole moment functions can be important for fundamental intensities, it is apparently unimportant for overtone intensities. We have calculated absolute intensities of unmeasured nitric acid and pernitric acid overtones which are important quantities for atmospheric models. We are in the process of extending this work to other important atmospheric molecules in order to quantify the effects of solar pumping of vibrational overtones with visible light in the atmosphere. Recently we have extended our intensity theory to explain our observations of the CH stretching overtone spectra of molecules undergoing methyl internal rotation. We have been able to predict the complicated methyl overtone spectral profiles in terms of torsion-stretching coupling, both in the Hamiltonian and in the dipole moment function. We have also discovered how intramolecular through space interaction can effect molecular vibrations and overtone spectra.

We have continued to work on the development of laser based photoacoustic absorption techniques. We have active research collaborations with Professors Sage (Syracuse) and Kjaergaard (Otago).

Last update to this document on 19 September, 2002, by Michael Petryk.

Email problems to Bryan Henry.