The chemical and physical processes which are taking place in the icy mantles in many astronomical environments are of key importance to unravel the molecular complexity observed in space. The dust grains properties are significantly influenced by the composition of their ice layers, therefore it is necessary to develop a methodology to investigate how different ices influence observational properties of dust grains. There are still many open questions about the role of the ice mantles in space, how they contribute to the formation of complex organic molecules (COMs) up to a level of complexity which can allow the formation of pre-biotic molecular species in very early stages of star formation, and how they influence the physical processes which ultimately brings to the formation of planetary systems.Laboratory spectroscopic techniques offer an established route to analyze the chemical and physical processes on ices in a controlled environment. The experimental data have supported and guided the interpretation of the observational data through the last thirty years, and they continue to serve as a unique tool to untangle the astrophysical processes. In order to provide a solid experimental ground to the observational data and to assist the development of chemical modeling, a laboratory facility specialized in the molecular spectroscopic investigation of astrophysical ice analogs has been designed and developed at CAS.
THz Time-Domain Spectrometer (TDS)
Experimentally determined optical constants and in particular absorption coefficients of astrophysical ice analogs in the terahertz region are missing. These data are very important to determine how the dust opacity changes when the grains are covered with ice mantles. Thus, more accurate mass determinations can be carried out using the dust continuum emission, especially in cold and dense regions where CO mainly resides in solid form on top of dust grains surfaces, forming thick icy mantles. Time-domain pulsed THz spectroscopic technique (TDS) has the unique advantage of being able to measure both the amplitude and phase of sub-picosecond THz pulses in a wide spectral range in a single measurement and, thus, to reconstruct directly the optical properties without the use of the Kramers-Kronig relations. The TDS uses an ultrafast laser (λ=785 nm, Δtpulse=100 fs) which provides, in combination with high-performance photo-conductive antennas, a large spectral bandwidth (0.05–5 THz) and a high dynamic range (4 orders of magnitude) in signal. It is capable of pulse pump-probe delays up to 650 ps, and is thus compatible with a wide range of optically-active samples. The instrument has built-in mounts for small samples, and it can also be interfaced to a closed cycle, ultra-low vibration cryostat.
Fourier Transform Infraed Spectrometer (FTIR)
Since its first identification, water ice has been observed in the insterstellar medium along several lines of sight and in different sources, mainly in dense molecular clouds. Water is the most abundant component of ice observed in space. However, other molecular species have been identified as minor ice components by their spectroscopic features in the infrared region. The characterization of ice mixtures with different chemical composition and structure is necessary to explore the observing capabilities of new infrared space telescopes, like JWST.
The laboratory operates a FTIR spectrometer capable of resolution of 0.003 cm-1. It has a number of sources, detectors and optics providing access to the wavelength range 5 - 15000 cm-1 (0.7-2000 μm). The instrument is routinely coupled to a closed cycle, ultra-low vibration cryostat to characterise the ice analogs in the infrared domain.
A Raman microscope coupled with a customized design cryostat is installed in the CASICE laboratory. The aim of the set-up is to investigate the diffusion properties in ice mixtures, and between the ice layers and the substrate. Microspectroscopy is the primary technique to analyse spectroscopic properties in 2D and 3D space enabling the study of composite materials. With this technique is thus possible to examine samples like meteorites and interstellar dust, and to assess the role of different substrates in the interaction with the ice mantles. The designed set-up is configured with a tunable gas inlet capable to deposit spatially resolved ice sample to address their surface diffusion properties. The microscope is equipped with two lasers at 488 and 785 nm, operating in Confocal Fluorescence Microscopy mode, within a μm scale resolution.
A Xe-lamp from Asahi Spectra with an UV, visible and IR mirror modules, is available for sample processing with a bandwidth of 250 – 385, 385 – 740 and 750 – 1050 nm, respectively. A collimator lens ensures an homogeneous irradiation of the sample. The lens is fixed on a stage, which enabled us to adjust the distance between the lens and the sample with a precision of 0.5 mm. The set-up is calibrated with an UV broadband surface sensor at 150 mW/cm2.