Infrared laser absorption spectroscopy of the ν4(σu) fundamental and associated ν11(πu) 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 v=11 and v=20 levels of the ν11(πu) bend as hot bands associated with the ν4(σu) 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 ν4=1 states are found to be anomalously large and negative, evidencing strong perturbations between stretching and bending modes.