High-accuracy structure of cyclobutane by femtosecond rotational Raman four-wave mixing

The rotational and centrifugal constants of cyclobutanes, [C.sub.4][H.sub.8] which was measured accurately by employing femtosecond rotational Raman coherence, detected by degenerate four-wave mixing is presented. The vibrational levels of cyclobutane exhibit tunneling splitting due to the ring-puckering interconversion between the symmetry-equivalent minima via a planar barrier.