The Cologne Carbon Cluster experiment: ro-vibrational spectroscopy on C8 and other small carbon clusters.

We report on our ongoing efforts in obtaining the IR-spectra of the linear carbon cluster molecules Cn with n=8-13. So far C8, C9, C10, and C13 have been recorded at Cologne. With the exception of C8 all assignments have been secured. For C8 a tentative assignment could be derived with the bandcenter of the sigmau antisymmetric stretching mode located at nu0=2067.9779 cm(-1) and a preliminary rotational constant in the vibrational ground state of B"=0.02068 cm(-1). The measured signal to noise ratio of the ro-vibrational band is fairly weak and thus the lower J ro-vibrational transitions can not be assigned with certainty. As a consequence the band center remains uncertain by 4 J or 0.17 cm(-1). For a more reliable assignment the sensitivity of the system has to be increased by at least one order of magnitude. The envisaged sensitivity increase of our experiment will be discussed along with the intention to perform terahertz observations of the low energetic bending ro-vibrational spectra. These sub-mm wave measurements will be carried out simultaneously with the IR measurements.

High resolution infrared spectra of the linear carbon cluster C(7): the nu(4) stretching fundamental band and associated hot bands.

High resolution infrared spectra of the nu(4) fundamental antisymmetric stretching mode and associated hot bands of the linear carbon cluster C(7) were recorded using a tunable diode laser spectrometer in the frequency range of 2135-2141 cm(-1). Spectra of the nu(4) fundamental, nu(4)+nu(11)-nu(11), nu(4)+2nu(11)-2nu(11), and nu(4)+nu(8)-nu(8), bands have been analyzed and are compared to recent experimental results and high level ab initio calculations. In particular, the presented results give experimental evidence for the rigidity of C(7) and confirm theoretical predictions of a rather regular chain molecule, similar to the cases of C(4), C(5), and C(9). For the two energetically low-lying bending modes, nu(8) and nu(11), the rotational constants differ by less than 0.2%, from the ground state value, B(0)=0.030 624 4(28) cm(-1), in good agreement with the recent calculations by Botschwina [Chem. Phys. Lett. 354, 148 (2002)]. From the hot band analysis and the [script-l]-type doubling constant q, experimental values for the band origins of the nu(8) and nu(11) fundamentals have been derived.