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Complementary information can be found at the NASA Ames Laboratory Astrophysics Matrix Isolation / Optical Constants of Ices Laboratory pages.
The matrix isolation and optical constants (MIOC) experiment is used to measure transmission spectra of polycyclic aromatic hydrocarbons (PAHs) and PAH clusters using the matrix isolation technique. We are also measuring transmission spectra of astrophysical ices at realistic astrophysical temperatures, the first step of computing the complex index of refraction (or “optical constants”) for use in radiative transfer models of astrophysical phenomena.
An image of the MIOC vacuum chamber is shown in Figure 1. The two major instruments used for these experiments are a rotatable cryogenic stage and a Fourier transform infrared (FTIR) spectrometer for conducting IR transmission measurements.
Figure 1: The matrix isolation and optical constants (MIOC) facility. The two key instruments are the cryogenic stage (top) and the FTIR spectrometer (lower right). The cesium iodide window used as substrate for deposited ices is mounted at the tip of the cryogenic stage and is suspended within the infrared beam of the FTIR.
A top-view schematic diagram of the MIOC vacuum chamber is shown in Figure 2. The most critical aspect of the experiment is an infrared-transparent, cesium iodide (CsI) window that is mounted at the tip of the cryogenic stage and that is suspended within the infrared beam of the FTIR spectrometer. When cooled to cryogenic temperatures, ice can be condensed onto this window and the infrared transmission spectrum of the deposited ice can then be measured.
Figure 2: A top-view schematic diagram of the MIOC vacuum chamber shown in Figure 1. This view shows the rotatable CsI window in its ice deposition position. After rotating the cryogenic stage 90°, the infrared beam will pass through the window and transmission spectra can be measured. Auxiliary equipment such as a HeNe laser and photodiode for measuring the thickness of deposited ices and a microwave discharge lamp for irradiating deposited ices with vacuum-UV light are also shown here.
For matrix isolation experiments, we typically co-condense evaporated PAH molecules with argon gas in order to trap PAH molecules within a solid argon ice “matrix” coating the CsI sample window. The shape of the absorption bands of the PAH molecules will depend upon the concentration of PAH molecules within the argon deposit.
At low concentration (the rule of thumb is that this is 1000+ argon atoms per embedded PAH molecule), the solid argon “matrix” keeps the PAH molecules generally well-separated and produces a transmission spectrum very similar to the gas-phase PAH transmission spectrum. At higher concentrations, there is a higher probability that a PAH molecule will be trapped within the matrix with close proximity to one or more other PAH molecules, i.e., that clusters of PAH molecules will be present within the matrix. The spectra measured at high concentration of PAH molecules will see additional features appear due to the clustering of PAH molecules.
A key feature of this method for investigating PAH cluster spectra is that we are measuring spectra of PAH/argon ices on an ensemble basis. We measure spectra of multiple ice deposits, each deposited at the same substrate temperature and each deposited with a distinct, fixed argon-to-PAH ratio. By comparing spectra on an ensemble basis, the appearance of the PAH cluster features with increasing PAH concentration in the argon ice matrix is easily identifiable.
We have published several papers presenting spectra with PAH clusters for PAHs with sizes ranging from naphthalene (10 carbon atoms) to benzo[ghi]perylene (22 carbon atoms):
- Roser, J. E. & Allamandola, L. J. 2010, “Infrared Spectroscopy of Naphthalene Aggregation and Cluster Formation in Argon Matrices”, The Astrophysical Journal, 722, 1932
- Roser, J. E., Ricca, A., & Allamandola, L. J. 2014, “Anthracene Clusters and the Interstellar Infrared Emission Features”, The Astrophysical Journal, 783, 97
- Roser, J. E. & Ricca, A. 2015, “Polycyclic Aromatic Hydrocarbon Clusters as Sources of Interstellar Infrared Emission”, The Astrophysical Journal, 801, 108
Determining the optical constants of deposited ices requires a somewhat different process of experimental measurement than measuring matrix isolation spectra. The fundamental process of measuring the transmission spectra is the same; one simply deposits the molecular species of interest as a bulk ice film onto the window surface and measures the transmission spectrum. The complication involved arises from the extensive calculations required to extract the optical constants from the transmission spectrum.
We presented preliminary optical constants of ammonia ice at an AAS Division for Planetary Sciences conference:
- Roser, J., Dalle Ore, C., Cruikshank, D., & Ricca, A. 2018, “Laboratory study of ammonia indices of refraction with water ice.” Presented at AAS DPS Conference 50 held October 21 to 26 in Knoxville, TN, abstract ID #506.03
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