Clément Lauzin

The FANTASIO+ set-up to investigate jet-cooled molecules: focus on overtone bands of the acetylene dimer

The experimental set-up FANTASIO, for ‘Fourier trANsform, Tunable diode and quadrupole mAss spectrometers interfaced to a Supersonic expansIOn’ (M. Herman, K. Didriche, D. Hurtmans, B. Kizil, P. Macko, A. Rizopoulos and P. Van Poucke, Mol. Phys. 105, 815 (2007)) built in Brussels has been updated. The turbomolecular pumping system of the supersonic expansion has been doubled and new mirrors, with reflectivity 99.999% instead of 99.99%, have been set in the CW-cavity ring down spectrometer probing jet-cooled molecules. The changes all together result in a signal to noise increased by up to a factor 10, around 1.5 µm. These improvements are demonstrated with various acetylene data in the 2CH excitation range, including the assignment of a new sub-band of acetylene–Ar, with K′–K′′ = 2–3. The focus is set on the acetylene dimer. Overtone sub-bands, with b- and a-type structures, are identified for the first time in the literature. They are assigned to vibrational excitation in the hat and body units of the T shaped dimer, respectively. The relevance of the overtone data on acetylene dimers for space remote sensing is highlighted.

Torsional excitation in the 2CH vibrational overtone of the C2H2?CO2 and C2H2?N2O van der Waals complexes

Infrared spectra of the weakly-bound C2H2-CO2 and C2H2-N2O complexes in the region of the 2CH acetylene overtone band (∼1.52u2009µm) were recorded using CW-cavity ring down spectroscopy in a continuous supersonic expansion. A new, c-type combination band is observed in each case. The rotational analysis of low J, K lines is performed and rotational constants are obtained. The band origins are 40.491(2) and 40.778(2) cm−1 higher in energy than the 2CH excitation bands for C2H2-CO2 and C2H2-N2O, respectively. The combination band is assigned in each case as involving intermolecular torsional excitation combined to 2CH. The values of the torsional vibrational frequency and of the xCH/torsion anharmonicity constant are briefly discussed.