Knowledge Bank

A portion of the O-H stretch vibrational spectrum of trans-ethanol has been measured at 10 MHz resolution using the electric-resonance optothermal spectrometer at NIST. Due to the presence of atmospheric water absorptions only small continuous regions of the spectrum could be obtained. A $1.7cm-1$ region around $3678.35cm-1$ has been assigned by microwave-infrared double resonance. This region contains about 400 measured transitions of which about 240 have been rotationally assigned. These assignments show that this region is the R-branch of an a-type band. The spectra show very extensive vibrational mixing characteristic of a molecule undergoing intramolecular vibrational energy redistribution. Individual zeroth-order rovibrational lines are fractionated into about 60 components. The IVR lifetime, which measures the time scale of energy localization in the O-H stretch following coherent excitation, is about 25 ps. This lifetime appears to be anharmonically controlled since no rotational dependence is observed. The IVR rate from the O-H stretch is considerably faster than the relaxation of the methyl C-H stretch. Also the methyl C-H stretch shows evidence of strongly Ka dependent dynamics. The measured density of transitions is about 120 $transitions/cm-1$ and is independent of the rotational state. This value, which is a lowerbound to the true line density, is more than a factor of two higher than the predicted density based on state counting algorithms which properly treat the methyl and hydroxyl large amplitude motion coordinates.