Knowledge Bank

Stimulated emission pumping spectra of HCN in its ground electronic state have been measured using a pulsed tunable Argon fluoride laser with a frequency doubled pulsed dye laser. Sixty seven vibrational states between 8900 and $18900cm-1$ have been observed. Eighty percent of the States can be described within a traditional normal mode picture. A full set of anharmonic vibrational constants was derived unifying the SEP data reported here with previous infrared data. Twenty percent of the states could not be explained by the normal mode picture and a systematic analysis was performed to show that only a few of the unexplained states might be a linear superposition of $zeroth$ order normal mode HCN states. Since most of the unexplained states cannot be constructed out of normal mode HCN states, it is suggested that ``isomerizing'' delocalized vibrational states are playing a role in the observed vibrational structure. Direct comparison is possible with ab initio vibrational structure $calculations1$ on the only available three dimensional potential energy $surface2$. The experimental results show clearly that the true potential has a much higher barrier to isomerization. The present state of experimental characterization of the HCN/HNC systems should be good enough to derive a quantitatively accurate potential energy surface for this prototypical isomerization reaction.