Speaker
Dr
Hans-Martin Frey
(University of Bern)
Description
The gas phase rotational motion of the all trans alkanes butane to octane has been measured in real time using femtosecond (fs) time resolved rotational Raman rotational coherence spectroscopy (RR-RCS) at two temperatures T=160 and 295 K. Four-wave mixing transients are recorded over times up to 5 ns. The method probes the rotation of non-polar (and polar) gas phase molecules with fs time resolution. In addition to the dominant RCS signals of the lowest-energy all-trans form, for n-pentane to n-octane the rotational recurrences of several low energy gauche rotamers are also observed in the room temperature T=295 K gas-cell experiments, Due to large differences in structure, the contributions of the different gauche and trans rotamers to the RCS signal are clearly separated. At room temperature the low-energy thermally excited vibrational bending levels contribute up to 50% of the population for each given rotamer. Upon cooling the alkanes to 80 K in a supersonic jet, almost all the gauche rotamers and the thermally excited low frequency bending vibrations relax to the all trans v'=0 levels.
The supersonic jet measurements yield accurate B0 rotational constants. Combining these B0 with the results of all-electron coupled-cluster CCSD(T) calculations, the intermolecular distances can be predicted.
Primary author
Dr
Hans-Martin Frey
(University of Bern)
Co-authors
Mr
Philipp Kowalewski
(University of Bern)
Prof.
Samuel Leutwyler
(University of Bern)
Mr
Takuya Den
(University of Bern)
