Knowledge Bank

The $\nu 2$ and $\nu 4$ bending vibrations of the methyl cation $CH3+$ are strongly coupled by Coriolis interaction because of the near degeneracy of the two vibrational modes. Ab initio theoretical $studies1,2,3$ have predicted them to be within $1-20cm-1$. This is somewhat similar to the case of SiH3+ reported by Davies and $Smith4$. The patchy coverage of the diode lasers and the strong Coriolis coupling make the analysis difficult. From $1503-1297cm-1,95$ lines have been observed in a liquid cooled $He/H2/CH4$ plasma discharge using 5 diode lasers and velocity modulation. Six transitions in the $\nu 4$ band were readily assigned since the $\nu 4$ levels involved in these transitions are free of the strong Coriolis interaction because of the absence of odd J, K = 0 levels in the $\nu 2$ vibrational state due to the Pauli exclusion principle. PP(J = K) transitions in the $\nu 4$ band are also free of the interaction and can be analyzed by using only diagonal matrix elements. A complete analysis of the bands is in progress and will be reported at the meeting. la. Feodor Lynen Fellow (Alexander von Humbolt Foundation).