THz time-domain spectroscopy of interstellar ice analogs

Abstract

Although nearly 180 different mols. have been identified in the interstellar medium (ISM), mostly by radio observations, only the simplest and most abundant species have been unambiguously detected in the solid phase in the near- and mid-IR (i.e., H_2O, CO_2, CO, CH_3OH, NH_3, CH_4). Further, these observations are limited to lines-of-sight along which IR absorption spectroscopy is possible. The identification of more complex, potentially prebiotic mols. in the mid-IR is difficult because of their low expected interstellar abundances and the overlap of their absorption features with those from the more abundant species.In the THz region, however, telescopes can detect Interstellar ices in emission or absorption against dust continuum. Thus, THz searches do not require a background point source and can be attempted toward nearly arbitrary lines-of-sight. Moreover, since THz spectra are the fingerprint of inter- and intramol. forces, complex species present unique modes that do not overlap with those from simpler, more abundant mols. Spectroscopy at THz frequencies therefore has the potential to better characterize the physics and chem. of the ISM. Currently, international astronomical facilities (Herschel Space Telescope, SOFIA, and ALMA) are collecting a vast amt. of new THz spectral data that require THz lab. spectra for interpretation. The latter, however, are largely lacking. We have recently constructed a new THz time-domain spectroscopy system to investigate the spectra of interstellar relevant ice analogs in the range between 0.3 - 7.5 THz (10 - 250 cm^(-1)). The system is coupled to a FTIR spectrometer to monitor the ices in the mid-IR (4000 - 500 cm^(-1)). This work focuses on the lab. investigation of the compn. and structure of the most abundant interstellar ice analogs compared to some more complex species (e.g., Me formate).