CARS spectroscopy in supersonic jets of ammonia monomers and clusters

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Abstract
Supersonic molecular beams of ammonia seeded with various rare gases are investigated in the expansion region using one-dimensional coherent anti-Stokes Raman scattering (CARS). A Raman spectrum of the ν3 vibration of the NH3 monomer with resolved rotational structure and inversion splitting is reported. Spectroscopy of this vibration is ideally suited to determine the rotational temperature in the beam. In the spectral region between 3100 and 3500 cm−1, broad structures are observed which can be attributed to clusters. These structures arise at strong stagnation conditions and increase when going from pure NH3 expansions to NH3/He and NH3/Ne mixtures. Analysis of the employed beams by mass spectrometry suggests that large clusters containing ∼5–50 NH3 molecules are responsible for the observed features. A narrow-peaked structure, red shifted by 3.5 cm−1 from the ν1 Q branch, is attributed to small clusters or NH3 molecules residing on the outside of the cluster whose H atoms are free and not bound to the complex.