Time-domain THz spectroscopy of interstellar ice analogs: New instrumentation and optical constant extraction

Abstract

In the interstellar medium, atoms and simple mols. can react on the surface of icy dust grains to form more complex org. mols. (COMs), including prebiotic mols. As such, an increased understanding of the chem. evolution on icy grains, including the unambiguous identification of COMs in these ices, is fundamentally important. Modern observational facilities in the THz (far-IR) region of the electromagnetic spectrum offer the promise of detecting COMs and prebiotic mols. in the solid phase. However, these observations necessarily rely upon exptl. lab. spectra to interpret their results. In particular, high-quality optical consts. are esp. important for use in analyzing astronomical data and in radiative transfer models.This work reports improvements made to the Caltech astrochem. ice spectroscopy setup. We discuss the implementation of an optical parametric amplifier (OPA) to generate ultrashort pulses centered at 1867 nm to improve the efficiency, relative to 800 nm, of a two-color, plasma-based, pulsed THz source for the time-domain THz spectrometer. An optically contacted, two-layer GaP crystal is implemented on the detection side to improve the resoln. of the THz spectrometer to 10 GHz, while maintaining spectral coverage from 0.3 - 7.5 THz (9 - 250 cm^(-1)). We employ the improved spectrometer to study pure, mixed, and layered astrochem. ice analogs. We report high resoln., high signal-to-noise spectra of these ices - the foundation of an astrochem. ice database for the interpretation of astronomical data. In addn., we report optical consts. corresponding to the real and imaginary parts of the complex index of refraction. Given that THz spectra are collected in the time domain, both optical consts. can be found from a straightforward data-processing algorithm without the need to perform a Kramers-Kronig anal.