Direct measurements of infrared intensities of HCN and H2O + HCN ices for laboratory and observational astrochemistry

Abstract

ABSTRACT Hydrogen cyanide (HCN) is found in a wide variety of extraterrestrial environments within and beyond the Solar system, and for that reason laboratory spectroscopists have studied this compound in many spectral regions, including the infrared (IR). However, one aspect that remains to be investigated is the intrinsic IR spectral intensities of solid HCN as opposed to relative band strengths, intrinsic intensities being needed to measure HCN abundances. Here we report measurements of IR absorption coefficients and band strengths, along with supporting refractive indices and densities, of both amorphous and crystalline HCN at two temperatures, one for interstellar work and one more relevant to the outer Solar system. Spectra are presented at both temperatures, along with optical constants that can be used in numerical models. Despite widespread and longstanding interest in and investigations of solid HCN, this is the first time that the properties we are reporting have been measured in a single laboratory, avoiding the need for estimates or to combine results from various authors. We find that our measured band strength of ∼1 × 10–17 cm molecule–1 for the C≡N vibration of HCN, in both amorphous HCN and in an H2O-rich ice, is substantially higher than an earlier estimate. Unless errors of 100 per cent can be tolerated, then our new value requires a rescaling of earlier work. Our results shed light on why HCN and other nitriles have been so difficult to identify in the solid state, in contrast to their many detections in the gas phase.