ANALYSIS OF THE MM-WAVE SPECTRUM OF THE LOWEST $\Sigma$ AND $\Pi$ BENDING STATES OF ARHCN

Abstract

The lowest excited $\Sigma$ and $\Pi$ states of the van der Waals bending mode of ArHCN, including Stark-field data, were analyzed simultaneously using a linear molecular basis set and incorporating high order hyperfine and effective Coriolis coupling constants. The rotational analysis is distinctly unsatisfying, since 9 -- 11 constants are being fit to 15 energy levels (when hyperfine structure and Stark data are not included) indicating that a potential surface fit may already be more appropriate. However, as a result of the near degeneracy of these states, the perpendicular transition moment $\mu_{b}$, the component of the electric field gradient tensor $V_{xz}$, and in some instances the relative signs of the terms off-diagonal in l(or k) could be determined empirically. We estimate: $<\theta\Sigma>=102^{\circ}, <\Delta\theta^{2}\Sigma>^{1/2}=40^{\circ}, R_\Sigma=3.91 \AA, <\theta_\Pi>=85^{\circ}, <\Delta\theta^{2}\Pi>^{1/2}=40^{\circ}$, and $R{\Pi}=3.85 A$, These may be compared to the values $\theta_{eq}=0^{\circ}, <\Delta\theta^{2}{gud, state}>^{1/2}=30^{\circ}$, and $R{eq}=4.62 A$. This value of $<\theta\Sigma>$ suggests that there is still a barrier to the antilinear Ar-NCH configuration at the $v=1$ energy. The results are in good agreement with previous theoretical work on the $\Pi$ $state,^{1,2}$ but less so in the case of the $\Sigma$ $state,^{1}$ which is more sensitive to the potential surface at the antilinear configuration.