Measuring The Difference In Collisional Interaction Of Hcn/hnc With He At Low Temperatures Using The Cpuf Technique

The T$_{2}$ time of HCN and HNC in He was measured at low temperatures using chirped pulse Fourier transform millimeter wave spectroscopy, to compare to scattering calculations and to investigate the difference of interaction between the two structural isomers. These molecules are commonly observed in the interstellar medium (ISM), where their abundance ratio can be used to elucidate conditions within an interstellar environment. Within these environments, they are expected to collide with He and H$_{2}$, which could lead to non-thermal excitation. Previous ab initio calculations of the collisions with He and H2 with HCN and HNC have shown that the rates for HNC with these colliders are much greater, despite their similarities in structure. We have performed CPUF (Chirped pulse in Uniform Flow) experiments to measure the difference in T$_{2}$ of HCN and HNC at cold temperatures in He. The J=1-0 transitions of HCN and HNC were observed in under CRESU (Reaction Kinetics in Uniform Supersonic Flow) conditions using cold flows of He. Pulsed laser photolysis of vinyl cyanide was used to produce HCN and HNC under comparable conditions. The T$_{2}$ time was fit directly from the free induction decays of these experiments at different temperatures, which is directly comparable to the linewidth in the frequency domain. This was then compared to pressure broadening cross sections produced from close coupling scattering calculations performed on ab initio potential energy surfaces. We find that there is a difference in interaction of HCN and HNC with He at low temperatures and will discuss the impacts this may have for observing these species in the ISM