  Context. The gas and ice-grain chemistry of the pre-stellar core L1544 has
been the subject of several observations and modelling studies conducted in the
past years. The chemical composition of the ice mantles reflects the
environmental physical changes along the temporal evolution. The investigation
outcome hints at a layered structure of interstellar ices with mainly H$_2$O in
the inner layers and an increasing amount of CO near the surface. The
morphology of interstellar ice analogues can be investigated experimentally.
Aims. This research presents a new approach of a three-dimensional fit where
observational results are first fitted with a gas-grain chemical model. Then,
based on the numerical results the laboratory IR spectra are recorded for
interstellar ice analogues in a layered and in a mixed morphology. These
results can then be compared with future James Webb Space Telescope (JWST)
observations. Special attention is paid to the inclusion of the IR inactive
species N$_2$ and O$_2$. Methods. Ice analogue spectra containing the most
abundant predicted molecules were recorded at a temperature of 10 K using a
Fourier transform infrared spectrometer. In the case of layered ice we
deposited a H$_2$O-CO-N$_2$-O$_2$ mixture on top of a H2O-CH$_3$OH-N$_2$ ice,
while in the case of mixed ice we examined a H$_2$O-CH$_3$OH-N$_2$-CO
composition. Results. Following the changing composition and structure of the
ice, we find differences in the absorption bands for most of the examined
vibrational modes. The extent of observed changes in the IR band profiles will
allow us to analyse the structure of ice mantles in L1544 from future
observations by the JWST. Conclusions. The comparison of our spectroscopic
measurements with upcoming JWST observations is crucial in order to put
stringent constraints on the chemical and physical structure of dust icy
mantles, and to explain surface chemistry.
