Infrared absorption of small molecular clusters in molecular beams

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Small molecular clusters are formed in pulsed molecular beams of rare gas atoms seeded with a few percent of the molecule of interest. The molecular beam is interrogated by a beam from either a color center or diode IR laser. The laser beam undergoes multiple reflections and intersects the molecular beam typically 25 times before being detected with an InSb photodiode. The combination of short duration molecular beam pulses and boxcar integration of the photodiode current avoids low-frequency laser noise and permits absorptions as small as 10 μm to be detected. Color center laser radiation at 3 μm is used to excite C-H stretching vibrations in complexes containing acetylene, including argon-acetylene, acetylene dimer, and acetylene trimer. The observed vibration-rotation spectra provide geometries for the clusters as well as changes in vibrational frequencies on complex formation. It is notable that the structure of acetylene trimer is that of a planar triangle. Five-micron diode laser radiation is used to study the carbonyl stretching vibrations in clusters containing carbonyl sulfide.