VIBRATIONAL STRUCTURE OF HYDROGEN-CYANIDE UP TO 18900 CM-1

Stimulated-emission pumping (SEP) spectra of HCN have been measured by using a pulsed, tunable argon fluoride laser with a frequency-doubled, pulsed dye laser. Sixty-seven vibrational states of the ground electronic state between 8 900 and 18 900 cm have been observed. Eighty percent of the states can be described within a traditional normal mode context. A full set of anharmonic vibrational constants was derived, unifying the SEPdata reported here with previous infrared and overtone data. This set of molecular constants is expected to be able to predict the position of normal mode states below 19 000 cm with an accuracy within 3 cm-'. Twenty percent of the states could not be assigned to unperturbed normal mode states, and a systematic analysis was performed in an attempt to find a simple explanation for them based on possible perturbations. Except for the lowest energies, no simple explanation was found, suggesting that delocalized isomerizing vibrational states are playing a role in the observed vibrational structure at higher energy. Direct comparison with assigned normal mode states derived from quantum-mechanical vibrational-structure calculations on the only available three-dimensional potential energy surface were made possible by these experiments. The deviation between experiment and theory asa function of the number of bending quanta, the vibrational motion thatn couples strongly to the isomerization reaction coordinate, makes clear that the isomerization barrier height is too low on this surface. The present state of experimental characterization of the HCN system should be good enough to permit a high-quality potential energy surface to be derived for highly vibrationally excited HCN. © 1990 Optical Society of America.