Samenvatting

This thesis describes how the abundance of CO2 a ects the porosity of H2O:CO2 ice mixtures deposited at di erent growth temperatures ranging from 30 - 70 K. The mixing ratios of the used ice samples H2O:CO2 = 10:1, 4:1 and 2:1 are of astrophysical relevance. Laser optical interference is used to monitor the changing ice thickness during deposition and to determine the temperature dependent optical constants.
The porosity of the H2O ice sampls is predicted using the Maxwell-Garnett and Bruggeman e ective medium approximations (EMAs) - which treat the ice samples as heterogeneous materials - and the determined e ective (average) refractive index. The predicted porosities are compared to the experimental value calculated with the Lorentz-Lorenz equation - which treats the ice samples as homogeneous materials - using the determined e ective(average) refractive index. Using the Lorentz-Lorenz equation the porosity of these pure H2O ice samples ranges from p = (0:27 0:02) to p = (0:22 0:02). The the mean value
of the experimentally obtained porosity corresponds to the predicted porosity within 0.3%; when the errors are taken into account the results of the three methods are similar. From this it is concluded that the ice samples can be treated as homogeneous material in order to derive the e ective (average) refractive index from the experimental data. The porosity of the H2O:CO2 ice mixtures is predicted with the extended Maxwell-Garnett and Bruggeman
EMAs, and the determined e ective (average) refractive index of the ice samples. Using these models the porosity of the porous H2O:CO2 = 10:1 ice samples ranges from p = (0:32 0:03) to p = (0:25 0:03), the predicted porosity of the porous H2O:CO2 = 4:1 ice samples ranges
from p = (0:27 0:03) to p = (0:19 0:03) and the predicted porosity of the porous H2O:CO2 = 2:1 ice samples ranges from p = (0:24 0:03) to p = (0:11 0:03). From these results it is concluded that the predicted porosity of the porous H2O:CO2 = 10:1 and 4:1 can not be
distinguished from the porosity of the pure H2O ice samples when the errors are taken into account. The porosity of the porous H2O:CO2= 2:1 ice samples is signi cant lower than the porosity of the pure H2O ice samples. A lower limit for the porosity p = (0:08 0:03) is
measured for porous H2O:CO2=2:1 ice samples.
In a laboratory environment the resulting porosity of the ice samples depends on the experimental conditions, as deposition temperature, deposition rate, growth angle and on
the abundance of the embedded molecules. The degree of porosity of a given ice sample is directly related to the surface area of the ice sample. Therefore, it can be concluded that pure H2O and porous H2O:CO2 = 10:1 and 4:1 ice samples are more reactive than the porous
H2O:CO2 = 2:1 ice samples, because of their signi cant higher degree in porosity.