Measuring the differences in collisional interactions of isomers at low temperatures: HCN and HNC with He

HCN and HNC represent the most abundant isomeric system observed in the interstellar medium, even though HCN is much more thermodynamically stable than HNC. For many years, observations of cold interstellar environments have found an apparent overabundance of HNC, a non-intuitive result considering that most mechanisms invoked to create these two molecules will produce them in similar ratios. Theory predicts that HNC is more easily collisionally excited than HCN by H2 and He, which would explain the observational results. We validate these calculations through measuring the elastic and inelastic scattering of HCN and HNC with He at low temperatures in a new experiment. Continuous supersonic uniform flows of He are used to cool photolytically produced HCN and HNC to 10 K. Using a new chirped pulse Fourier transform millimeter wave spectrometer, we measure the absolute pressure broadening cross sections of the astrophysically important j=1-0 transition, and compare these to close coupling calculations on ab initio surfaces. The cross sections for HNC with He are dramatically different from those of HCN with He, largely driven by the difference in inelastic rates. The underlying reasons for this strong difference will be explored and the astrophysical implications discussed.