ACETALDEHYDE SPECTROSCOPY AND GENERIC METHYL-ROTOR IVR QUESTIONS

Author Institution: Molecular Physics Division, National Institute of Standards and TechnologyThe present status will be described of a large ongoing effort to apply high-resolution experiment and theory to the $CH_{3}$-CHO molecule in order to obtain answers to prototypical questions in intramolecular energy transfer processes. The discussion will be divided into three parts, one part focussing on the ladder of methyl-top internal rotation energy levels, which are in some sense the main contributors to the bath states of the vibrational quasi-continuum, one part focussing on the small-amplitude vibrational fundamentals, and one part focussing on interactions with low-energy precursors of bath states in the quasi-continuum. Essentially all microwave and infrared transitions in acetaldehyde involving torsional levels below the top of the internal rotation barrier have now been fit with a single model to experimental accuracy. Work in progress involves (i) trying to extend assignments and numerical fits to torsion-rotation levels above the barrier, and (ii) trying to understand the qualitative distribution of energy levels using classical mechanics and rotational energy surfaces. High-resolution Fourier-transform or jet-cooled laser spectra of several vibrational fundamentals listed in the 1972 Shimanouchi Tables are under study. Among other developments, $CO_{2}$-side-band spectra from NIST unequivocally confirm Hollenstein and G\""{u}ndthard's reassignment of the $920 cm^{-1}$ rocking fundamental (1972 Tables) to a combination band, which in turn supports Hollenstein and G\""{u}ndthard's reassignment of the $867 cm^{-1}$ rocking fundamental (1972 Tables) to the C-C stretch. These reassignments are also supported by the ab initio calculations of Wiberg, Walters and Colson. In an attempt to find evidence for ``fragmented spectra'' (characteristic of interactions of a bright state with dark bath states) at vibrational energies as low as possible, the laser-diode C=O stretching fundamental at $1734 cm^{-1}$ has been recorded at NIST. One important question (unanswered at the time of this abstract) is whether extensive fragmentation will occur for the C---O fundamental band ($\nu_{4}$) itself, or whether (as suggested by cold-beam time-domain IVR studies) significant excitation of the internal rotor ($v_{15}$) is required, so that extensive fragmentation will appear only in hot bands of the type $\nu_{4}+n\nu_{15} - n\nu_{15}$. A related question, of course, is whether signficant overall rotational excitation in the absence of internal rotor excitation would be sufficient to induce extensive fragmentation (high-J states are also absent in the NIST laser diode spectrum)