Infrared spectra of ethylene clusters: (C2D4)2 and (C2D4)3

Abstract

Spectra of ethylene dimers and trimers are studied in the ν₁₁ fundamental band region of C₂D₄ (≈2200 cm⁻¹) using a tuneable quantum cascade laser to probe a pulsed supersonic slit jet expansion. The dimer spectrum is that of a prolate symmetric top perpendicular band, with a distinctive appearance because the A rotational constant is almost exactly equal to six times the B constant. The analysis supports the previously determined cross-shaped dimer structure with D_2d symmetry. An ethylene trimer has not previously been observed with rotational resolution. The spectrum is that of an oblate symmetric top parallel band. It leads to a proposed trimer structure which is barrel shaped and has C_3h or C₃ symmetry, with the ethylene monomer C–C axes approximately aligned along the trimer symmetry axis.