Cavity Ring Down Spectroscopy Measurements for High-Overtone Vibrational Bands of HC 3 N Ste ́ phane Douin,* ,† Marcin Gronowski, ‡ Nicolas Lamarre, † Viet-Tiep Phung, † Se ́ verine Boye ́ -Pe ́ ronne, † Claudine Cre ́ pin, † and Robert Kolos ‡ † Institut des Sciences Mole ́ culaires d’Orsay, UMR8214, CNRS, Universite ́ Paris-Sud, Bâ t. 210, F-91405 Orsay Cedex, France ‡ Institute of Physical Chemistry of the Polish Academy of Sciences, Kasprzaka 44, 01-224 Warsaw, Poland ABSTRACT: Overtone (5ν 1 and 6ν 1 ) and combination (4ν 1 + ν 3 and 4ν 1 + ν 2 ) vibrational bands of gaseous HC 3 N, located in the visible range (14 600-15 800 and 17 400-18 600 cm -1 ), were investigated by cavity ring-down absorption spectroscopy. The 5ν 1 + ν 3 and 5ν 1 + ν 2 combinations as well as the 6ν 1 + ν 5 - ν 5 hot overtone band have also been identified, on the basis of previous overtone assignments. Absolute integrated intensity values and the ensuing oscillator strengths have been measured here for the first time; f values are typically confined between 4 × 10 -12 and 7 × 10 -11 . For the even weaker 5ν 1 + ν 2 combination band, the oscillator strength was estimated as 9 × 10 -13 . The values concerning CH-stretch overtones (nν 1 ) are similar to those found in the literature for HCN and C 2 H 2 , the molecules with sp-hybridized carbon atoms. Data presented here may prove useful for studying the photochemistry triggered with visible or near-IR radiation within the atmospheres of certain Solar System bodies, including Titan. ■ INTRODUCTION HC 3 N (cyanoacetylene, cyanoethyne, propynenitrile) is a molecule of astrophysical interest. It has been detected in the interstellar medium 1-4 and cometary atmospheres, 5 in the atmosphere of the biggest Saturn’s moon Titan, 6 and also in extragalactic sources. 7 As the first member of the cyanopolyyne family, HC 3 N plays an important role in the complex network of chemical reactions taking place in these diverse environ- ments penetrated with UV and cosmic rays; cyanopolyynes are related to a whole range of unsaturated free radicals, and also to large chemical structures, like the polymeric ones present in the Titan’s atmosphere (tholins), responsible for its characteristic orange haze. Overtone frequencies strongly depend on the attractive part of the potential energy surface (PESs), whereas overtone intensities depend on its repulsive part. 8 Experimental studies of highly excited vibrational states can therefore provide information on the shape (anharmonicity) of a PES at a considerable distance from its minimum, as well as on phenomena like internal vibrational redistribution or unim- olecular reactions. Emission from highly excited vibrational states has been observed for hot (T ≥ 1500 K) astronomical objects, including some gaseous disks surrounding young stars. 9 Moreover, overtones are of importance for the Terrestrial atmospheric photochemistry, 10 and supposedly also for the atmospheres of other Solar System bodies, including Titan. Cross sections for electronic absorptions (i.e., typically, those involving UV quanta) are usually orders of magnitude higher than for vibrational overtone transitions; these latter, however, may provide the gate toward the photochemistry triggered with visible or near-IR radiation (of note, the maximum of solar emission lies in the visible). The cavity-ring down spectroscopy (CRDS) technique has already been used to measure, for some molecules, the absolute absorption cross sections of high vibrational overtones, in particular those of CH stretching modes, located in the visible range. To our knowledge, Romanini et al. 11 were the first to record, with CRDS, the nν CH (n = 5, 6, and 7) overtone transitions and related combination bands in the case of the HCN molecule (17 500-23 000 cm -1 range). Romanini et al. 12 also measured the (5ν CH + ν CC ) combination band of acetylene, near 17 500 cm -1 . Kleine et al. 13 have investigated the 6ν CH overtone of benzene, at 16 550 cm -1 , and measured the corresponding oscillator strength. DeMille et al. 14 obtained the oscillator strength for the 6ν CH overtone bands of propane, n-butane, and neopentane. The phase-shift CRDS technique has been employed by Lewis et al. 15,16 to measure the absolute absorption strength corresponding to the 5ν CH and/or 6ν CH overtone bands of ethylene, ethane, propane, n-butane, n- pentane, isobutene, and neopentane. For all above listed 5ν CH and 6ν CH bands, oscillator strengths on the order of 10 -11 to 10 -10 have been found. Received: June 19, 2015 Revised: July 24, 2015 Article pubs.acs.org/JPCA © XXXX American Chemical Society A DOI: 10.1021/acs.jpca.5b05884 J. Phys. Chem. A XXXX, XXX, XXX-XXX Downloaded by UNIV PARIS SUD on August 27, 2015 | http://pubs.acs.org Publication Date (Web): August 27, 2015 | doi: 10.1021/acs.jpca.5b05884