Coherent Raman and infrared spectroscopy of HCN complexes in free jet expansions and in equilibrium samples

Small hydrogen bonded complexes of HCN and DCN have been examined in the gas phase using FTIR and photoacoustic Raman spectroscopy (PARS). Four distinct bands are seen and assigned to CN stretching modes of the HCN dimer (D) and trimer (T). Previous microwave studies have shown the HCN dimer to be linear and analysis of the IR trimer band shape unambiguously demonstrates that the HCN trimer is also linear. The pressure dependent intensity of the observed transitions yields IR and Raman cross sections relative to the monomer of: (σD/σM)IR =30±10, (σT‘/σM)IR =500±200, (σD/σM)Ram =0.5±0.1, (σT/σM)Ram =3.3±0.7. The dramatic enhancements seen in the IR cross sections are due to small but significant (∼2×) changes in the CH bond dipole derivative. CARS spectroscopy has been used to examine the complexes formed in supersonic expansions of pure HCN (DCN) and HCN diluted in Ar and He carriers. Due to the cooling and collision-free conditions in the jet, this method yields higher effective resolution than is possible with equilibrium samples. The added ability to ‘‘tune’’ the size of complexes by varying expansion conditions has allowed us to observe a number of other modes in HCN and DCN dimer and trimer as well as in higher polymers. These data have been used to partially refine a harmonic valence force field for the stretching modes of the dimer and trimer complexes.