Experimental Confirmation of Quantum Monodromy: The Millimeter Wave Spectrum of Cyanogen Isothiocyanate NCNCS

We have made energy-momentum maps for the experimental end-over-end rotational energy and the two-dimensional bending vibrational energy, both of which confirm the dominating effects of nontrivial quantum monodromy in cyanogen isothiocyanate. Accidental resonances in the rotational spectra yield accurate intervals between bending states.

Figure 1

Ab initio predicted equilibrium structure of NCNCS.

Figure 2

Experimentally determined $B_{\mathrm{eff}}$ values for NCNCS plotted versus $K_a$.

Figure 3

Quantum monodromy plot of calculated $v_b$ bending vibrational levels. The color scheme follows Fig. 2. A local Coriolis resonance is indicated between the two bending states $v_b=2$, $K_a=4^e$ and $v_b=3$, $K_a=1^e$.

Figure 4

Cut through the effective two-dimensional bending potential energy function $E_{\mathrm{pot}}(\rho ,\mathrm{\text{Azimuth}})/hc$ of the mode ${\nu }_b\equiv {\nu }_7$ for NCNCS. The bending coordinate $\rho$(CNC) is defined as the supplement of the CNC bond angle. The vibrational term values $E(K_a,v_b)/hc$ are also entered.