Rovibrational Spectroscopy on Linear C7 and C8 in a Supersonic Jet Expansion (English shop)

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In the scope of this work, high resolution rovibrational measurements have been
performed on the linear carbon chains C7 and C8. The spectra have been recorded
with the Cologne Carbon Cluster Experiment, which combines a high resolution
tunable IR diode laser spectrometer with a UV laser ablation source. It was a
main task of this work to modify the existing experimental setup aiming for a sig-
nificant increase of the signal-to-noise ratio. The accomplished rearrangements
enabled the investigation of weaker absorption signals like hot band transitions.
More than 70 rovibrational transitions occuring in the spectral region from 2066.8
to 2069.0 cm−1 have been observed and subsequently assigned to the ν5 anti-
symmetric stretching fundamental of linear C8. From a least squares fit, a band
origin close to 2067.89 cm−1 and a rotational constant for the vibrational ground-
state of approximately 0.0208 cm−1 have been derived. The analysis of the mea-
sured triplet splitting yields evidence for an uncoupling of the spin resulting in a
transition between Hund’s coupling case (a) and case (b) scheme.
For C7, infrared measurements of the ν4 fundamental antisymmetric stretching
mode and associated hot bands have been recorded in the frequency range
from 2135.0 to 2141.0 cm−1. Spectra of the ν4 fundamental, as well as the
ν4 + ν11 − ν11, ν4 + ν8 − ν8, and ν4 + 2ν11 − 2ν11,  = 2 bands have been analysed
and are compared to hitherto existing experimental results [35, 36] and high-level
ab initio calculations ⁹. For the two energetically low-lying bending modes, ν8
and ν11, the obtained rotational constants differ only by 0.2 % from the ground
state value, B0 = 0.0306259(29) cm−1, in good agreement with recent calcula-
tions. From the hot band analysis, the -type doubling constants q and exper-
imental values for the band origins of the ν8 and ν11 fundamentals have been
derived for the first time, namely ν11 = 54(11) cm−1 and ν8 = 81(23) cm−1. The
results presented in this work give experimental evidence for the non-floppiness
of C7 and confirm the theoretical predictions of a rather regular chain molecule,
similar to the cases of C4, C5, and C9