Infrared laser absorption spectroscopy of the v 4 ( a , ) fundamenUal and associated vi , ( G , ) hot band of CT : Evidence fo . r alternating rigidity in linear carbon clusters

The first characterization of the bending potential of the C!, cluster is reported via the observation of the u = 1’ and u = 2’levels of the Y,~ (n-, ) bend as hot bands associated with the v4 (0; ) antisymmetric stretch fundamental. The lower state hot band rotational constants aremeasured to be 1004.4(1.3) and 1123.6(9.0) MHz,constitutinga9.3% and22% increase over the ground state rotational constant [ 9 18.89 (41) MHz]. These large increases are strong evidence for extremely large amplitude, anharmonic bending modes in this cluster. In addition, quartic and sextic‘centrifugal distortion constants determined for the ground and Ye = 1 states are found to be anomalously large and negative, evidencing strong perturbations between stretching and bending modes.