INFRARED-LASER ABSORPTION-SPECTROSCOPY OF THE NU-4(SIGMA-U) FUNDAMENTAL AND ASSOCIATED NU-11(PI-U) HOT BAND OF C7 - EVIDENCE FOR ALTERNATING RIGIDITY IN LINEAR CARBON CLUSTERS

The first characterization of the bending potential of the C7 cluster is reported via the observation of the upsilon = 1(1) and upsilon = 2(0) levels of the nu-11 (tau-u) bend as hot bands associated with the nu-4 (sigma-u) antisymmetric stretch fundamental. The lower state hot band rotational constants are measured to be 1004.4(1.3) and 1123.6(9.0) MHz, constituting a 9.3% and 22% increase over the ground state rotational constant [918.89(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 nu-4 = 1 states are found to be anomalously large and negative, evidencing strong perturbations between stretching and bending modes.