Knowledge Bank

In the high resolution spectrum of nitromethane near 1584 \wn each of the zeroth order transitions of the lowest internal rotor state is split into a clump of 4 or 5 rovibrational transitions. Individual perturbing (dark) states are followed as a function of the {\it J }, {\it K}, and {\it K} rotational quantum numbers. Four stages of vibration-torsion-rotation coupling are identified: (i) sub-picosecond {\it c}-axis Coriolis coupling to a torsionally excited state dark vibration (\nub{7} + \nub{10} or \nub{5}), (ii) {\it a}-axis Coriolis coupling between A and A torsional states of the dark vibration, (iii) {\it c}-axis Coriolis coupling of the dark vibration back to dark rotational states of the asymmetric N-O stretch, and (iv) at {\it J }=8 and higher a strong coupling to a second torsionally excited dark vibration .