Characterization of Highly Excited Vibrational States of Hydrogen-Cyanide how Bright are They?.

This thesis presents a systematic high-resolution determination of the absolute overtone intensities of a molecule of fundamental interest. Through a combination of intracavity laser photoacoustic and long pathlength absorption spectroscopy, we have determined the absolute intensities of virtually all of the detectable overtone and combination bands of HCN from 5400-18400 cm ^{-1}, as well as the average dipole moment of four highly excited vibrational states in the visible. This study provides a critical test of theory, as well as an extensive accurate data set that can be used to empirically determine the potential and electric dipole moment functions for HCN. We have determined a potential energy surface for HCN by fitting to a form of an expansion in the Morse oscillator coordinate. The observations used included 40 vibrational energy levels of three isotopomers and 12 vibration-rotation interaction constants. The vibrational energy levels were calculated variationally and the vibration -rotation constants via perturbation theory expressions. The rms error in the fit of the vibrational energy levels was less than 3 cm^{-1} for energy levels up to 18400 cm^{-1} . Botschwina has calculated a vibrational potential using the CEPA-1 method. His surface is in excellent agreement with the empirical one, and his predictions for the observed vibrational energy levels have an rms error of 10.5 cm ^{-1}. He also produced a dipole moment surface from his ab initio points. The question remains, which of the two surfaces is in fact closer to reality, at least for the experimentally studied region. We have used the theoretical dipole moment surface and predicted the overtone intensities for our empirical potential energy surface and compared with both the experimental values and those reported earlier by Botschwina. We find much to our surprise that the intensities predicted for the two surfaces are very different, a factor of four for the (0,0,6) band at 18377 cm^{ -1}. Furthermore, the theoretical potential agrees significantly better with the experiment than the empirical potential. While there are certainly errors in the theoretical dipole moment surface, the natural explanation of these results is that the ab initio potential is closer to reality than the best empirical surface obtainable from exhaustive spectroscopic data.