New spectroscopic results on acetylene dimers and trimers

Abstract

Spectra of acetylene dimers and trimers containing one or more C₂D₂ monomer are studied in the ν₃ fundamental band region of C₂D₂ (≈2440 cm⁻¹) using a tuneable infrared diode laser to probe a pulsed supersonic slit jet expansion. Four new subbands are observed in the perpendicular band of (C₂D₂)₂, and this enables the first direct determination of the A rotational constant for an acetylene dimer. The value found for A is significantly larger than the previous indirect value based on microwave spectra. The dimer parallel band is observed and found to be highly perturbed, and observations are extended for the mixed dimer C₂D₂ – C₂H₂. The trimers (C₂D₂)₃, (C₂D₂)₂ – C₂H₂, and C₂D₂ – (C₂H₂)₂ are observed spectroscopically for the first time. Establishment of a precise band origin is difficult for (C₂D₂)₃ because of the inherent nature of the spectrum, but this is possible for two out of three of the bands of the mixed trimers.

Keywords:

• acetylene
• dimer
• trimer
• cluster
• spectroscopy
• infrared
• van der Waals

Acknowledgements

We thank L. Murdock for technical assistance and M. Rezaei for help with the experiment. We gratefully acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada.